SRGA J181414.6-225604: A new Galactic symbiotic X-ray binary outburst triggered by an intense mass loss episode of a heavily obscured Mira variable

We present the discovery and multi-wavelength characterization of SRGA J181414.6-225604, a Galactic hard X-ray transient discovered during the ongoing SRG/ART-XC sky survey. Using data from the Palomar Gattini-IR survey, we identify a spatially and temporally coincident variable infrared (IR) source, IRAS 18111-2257, and classify it as a very late-type (M7-M8), long period ($1502 \pm 24$ days) and luminous ($M_K\approx -9.9 \pm 0.2$) O-rich Mira donor star located at a distance of $\approx 14.6^{+2.9}_{-2.3}$ kpc. Combining multi-color photometric data over the last $\approx 25$ years, we show that the IR counterpart underwent a recent (starting $\approx 800$ days before the X-ray flare) enhanced mass loss (reaching $\approx 2.1 \times 10^{-5}$ M$_\odot$ yr$^{-1}$) episode resulting in an expanding dust shell obscuring the underlying star. Multi-epoch follow-up from Swift, NICER and NuSTAR reveal a $\approx 200$ day long X-ray outburst reaching a peak luminosity of $L_X \approx 2.5 \times 10^{36}$ erg s$^{-1}$, characterized by a heavily absorbed ($N_{\rm H} \approx 6\times 10^{22}$ cm$^{-2}$) X-ray spectrum consistent with an optically thick Comptonized plasma. The X-ray spectral and timing behavior suggest the presence of clumpy wind accretion together with a dense ionized nebula overabundant in silicate material surrounding the compact object. Together, we show that SRGA J181414.6-225604 is a new symbiotic X-ray binary in outburst, triggered by an intense dust formation episode of a highly evolved donor. Our results offer the first direct confirmation for the speculated connection between enhanced late-stage donor mass loss and active lifetimes of the symbiotic X-ray binaries.


ABSTRACT
We present the discovery and multi-wavelength characterization of SRGA J181414.6-225604, a Galactic hard X-ray transient discovered during the ongoing SRG/ART-XC sky survey. Using data from the Palomar Gattini-IR survey, we identify a spatially and temporally coincident variable infrared (IR) source, IRAS 18111-2257, and classify it as a very late-type (M7-M8), long period (1502 ± 24 days) and luminous (M K ≈ −9.9±0.2) O-rich Mira donor star located at a distance of ≈ 14.6 +2.9 −2.3 kpc. Combining multi-color photometric data over the last ≈ 25 years, we show that the IR counterpart underwent a recent (starting ≈ 800 days before the X-ray flare) enhanced mass loss (reaching ≈ 2.1 × 10 −5 M yr −1 ) episode resulting in an expanding dust shell obscuring the underlying star. Multi-epoch follow-up from Swift, NICER and NuSTAR reveal a ≈ 200 day long X-ray outburst reaching a peak luminosity of L X ≈ 2.5 × 10 36 erg s −1 , characterized by a heavily absorbed (N H ≈ 6 × 10 22 cm −2 ) X-ray spectrum consistent with an optically thick Comptonized plasma. The X-ray spectral and timing behavior suggest the presence of clumpy wind accretion together with a dense ionized nebula overabundant in silicate material surrounding the compact object. Together, we show that SRGA J181414.6-225604 is a new symbiotic X-ray binary in outburst, triggered by an intense dust formation episode of a highly evolved donor. Our results offer the first direct confirmation for the speculated connection between enhanced late-stage donor mass loss and active lifetimes of the symbiotic X-ray binaries.

INTRODUCTION
Symbiotic binaries consist of a compact object (white dwarf, neutron star or black hole) accreting from the wind of a late-type giant companion (Kenyon 1986;Belczyński et al. 2000). They are broadly classified into two groups based on their near-infrared (NIR) spectral energy distributions (SEDs): i) the S-type systems exhibiting SEDs characteristic of a cool photosphere with temperature T eff ≈ 3000 − 4000 K and ii) the D-type systems with SEDs characteristic of warm dust with T eff ≈ 1000 K (Allen & Glass 1974;Webster & Allen 1975). The recent compilation of Akras et al. (2019) suggests that only ≈ 15% of known symbiotic stars are identified with D-type SEDs. Spectral classification of symbiotics (Mürset & Schmid 1999), together with light curves from photometric surveys (Whitelock 1987;Gromadzki et al. 2009) suggest that D-type systems contain late type Mira variables as donor stars (Phillips 2007;Angeloni et al. 2010). Mira symbiotics are known to exhibit long pulsation periods (Whitelock 1987) and thick dust shells that give rise to their dust dominated SEDs (Muerset et al. 1996;Angeloni et al. 2010).
Symbiotic X-ray binaries (SyXRBs; Masetti et al. 2006, see Kuranov et al. 2015 for a review) form a sub-class of the symbiotic binary population, exhibiting X-ray emission extending up to high energies (E 2.4 keV) produced by optically thick Comptonizing plasma (Masetti et al. 2007b;Luna et al. 2013;Lutovinov et al. 2020) from accretion on to a neutron star (NS) or black hole (BH). SyXRBs are relatively rare among Galactic XRBs, with only about a dozen systems known (Yungelson et al. 2019), all of them believed to contain NSs. The accretion of the companion wind onto the compact object produces relatively low average Xray luminosity (L X ∼ 10 32−36 erg s −1 ), rendering them difficult to detect in shallow X-ray surveys. Unlike their white dwarf (WD) counterparts, the survival of a wide orbit companion during the supernova (SN) that created the compact object requires fine-tuned mass loss and kick parameters (Iben et al. 1995), making SyXRBs unique probes of extremes in binary evolution physics.
Binary population synthesis models predict that there are ≈ 50 − 1000 SyXRBs in the Galaxy (Lü et al. 2012;Kuranov et al. 2015;Yungelson et al. 2019) compared * NASA Einstein Fellow to roughly dozen known candidate and confirmed systems. It is generally understood that SyXRBs become conspicuous in the X-ray sky only for a brief fraction of the binary lifetime when the donor produces a dense wind, a fraction of which is captured by the compact object (Hinkle et al. 2006). The most well-known SyXRB GX 1+4 (L X ∼ 10 37−38 erg s −1 ) indeed has a very late type (M6-III) donor at the tip of the Red Giant Branch (Chakrabarty & Roche 1997). While the first SyXRBs were identified as persistent and highly variable X-ray sources, repeated scans of the hard X-ray sky from allsky surveys are offering new avenues to finding outbursting sources where the persistent emission is too faint to be detected. Although an understanding of these transient flares (lasting ∼weeks to ∼years) is crucial to establishing their active lifetimes and Galactic demographics, their nature remains poorly constrained.
Previous speculations attribute the accretion variability to the motion of the compact object in an eccentric orbit (Pereira et al. 1999;Masetti et al. 2002;I lkiewicz et al. 2017), to changes in the donor star's mass loss properties (Masetti et al. 2002;Galloway et al. 2002;Bozzo et al. 2018) or to changes in the angular momentum of the accreted material near the NS magnetosphere (Postnov et al. 2010;Marcu et al. 2011;Shakura et al. 2012). While temporal characterization of the donor stars offers a promising pathway to directly test these scenarios, the intrinsically red donor SEDs together with heavy dust reddening preclude detailed optical investigations of the donor variability and its impact on the accretion rate on to the compact object. The timely emergence of deep all-sky hard X-ray surveys together with near-infrared (NIR) time domain surveys offers a new opportunity to understand this population.
In this paper, we present the discovery of SRGA J181414.6-225604, a hard X-ray transient discovered by the Mikhail Pavlinsky ART-XC instrument onboard the SRG satellite. Combining archival photometry from time domain surveys and extensive follow-up in the X-ray, optical and infrared bands, we establish the source as a new D-type Galactic SyXRB triggered by a recent intense mass loss episode of a highly evolved Mira-like variable. Section 2 presents details of the discovery, counterpart identification and follow-up observations. Section 3 presents an analysis of the spectral and temporal behavior of the X-ray transient. In Section 4, we constrain the nature of the donor star with optical/infrared photometry and spectroscopy, while we present evidence for a recent massive dust obscuration episode in Section 5. We end with a discussion on the nature of the binary system in Section 6 and summarize our findings in Section 7.
Following the discovery of the X-ray transient, we identified a highly variable infrared counterpart within the eROSITA localization region at a magnitude of J ≈ 10.2 − 12.5 mag in data from the Palomar Gattini-IR (PGIR) NIR time domain survey (Moore & Kasliwal 2019;De et al. 2020a). The variable NIR source (also known as 2MASS J18141475-2256195) was spatially coincident with the bright mid-IR source IRAS 18111-2257 listed in the SIMBAD database 1 . The PGIR data revealed a long-term NIR brightening coincident with the X-ray flare, suggesting a likely association (De et al. 2021). The association was subsequently supported by a follow-up observation with the Neil Gehrels Swift Observatory (Swift), which localized the X-ray transient to within 0.8 of the IR source ).
On UT 2021-04-07, we obtained a very low resolution optical spectrum of the source with the SED Machine Spectrograph (Blagorodnova et al. 2018) on the Palomar 60-inch telescope. The spectrum was reduced with the pysedm pipeline (Rigault et al. 2019), and ex- The image shows the 90% confidence localization regions for SRGA J181414.6-225604 reported by ART-XC and eROSITA at the time of discovery, as well as the position obtained from the Swift XRT follow-up campaign. The image is centered on IRAS 18111-2257 (shown with a red cross), which is consistent within 0.4 of the best-fit location from the Swift XRT observations. The green circle shows the 1 σ localization region of a coincident X-ray source reported in the XMM Newton Slew Survey catalog (see text).
hibited a steep red continuum with broad absorption features consistent with VO and TiO absorption in latetype stars, suggesting that the source was a likely new Galactic symbiotic X-ray binary in outburst. Figure 1 shows a NIR image of the localization region of the Xray transient, together with best derived positions from the X-ray discovery and follow-up observations.

Optical/IR Photometry
The highly reddened NIR counterpart of SRGA J181414.6-225604, IRAS 18111-2257, was monitored as a part of regular survey operations of PGIR, and we derived a J-band light curve of the source as described in Appendix A. In addition, we searched all publicly available data from time domain surveys to derive multi-color light curves of the counterpart. As described in Appendix A, the variable source is detected in the o-filter of the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018) (Wright et al. 2010;Mainzer et al. 2011). Figure 2 shows the optical/IR photometric evolution of the counterpart during 2019-2021, in data from PGIR and ATLAS which have the highest cadence observations around the X-ray brightening of the source.  On UT 2021-04-09, we obtained additional gri followup photometry with the rainbow camera on the Palomar 60-inch telescope. The source is only detected in the i filter. On UT 2021-07-02, we obtained multi-color NIR photometry of the source in the J, H and Ks band using the Wide-field Infrared Camera (WIRC; Wilson et al. 2003) on the Palomar 200-inch telescope. We obtained a series of dithered exposures of ≈ 0.9 s each, which were reduced, stacked and photometrically calibrated using the pipeline described in De et al. (2020a). Figure 1 shows the multi-color composite NIR image of the source obtained from the WIRC data, and Table 1 provides the complete ground-based photometric dataset. Table  2 provides the photometric dataset derived from spacebased archival/follow-up observations.
The source was also observed by several surveys prior to 2010, including the original identification of the bright mid-infrared source by the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984;Abrahamyan et al. 2015), the Midcourse Space Experiment (MSX; Egan et al. 2003), the Two Micron All-sky Survey (2MASS; Skrutskie et al. 2006), the Deep Near-Infrared Survey of the Southern Sky (DENIS 2 ), AKARI (Murakami et al. 2007) and the Spitzer Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE; Benjamin et al. 2003). In Table 3, we provide the reported archival fluxes of the source along with the approximate date of observation.
2.3. Optical/IR spectroscopy 2 http://cdsweb.u-strasbg.fr/denis.html We obtained three epochs of follow-up optical spectroscopy of the source using the Wide-field Spectrograph (WiFeS; Dopita et al. 2010) on the ANU 2.3 m telescope at Siding Spring Observatory and the Double Spectrograph (DBSP; Oke & Gunn 1982) on the Palomar 200-inch telescope. The WiFeS data were reduced using the PyWiFeS pipeline (Childress et al. 2014) and the DBSP data were reduced using the pyraf-dbsp pipeline (Bellm & Sesar 2016). We obtained a total of eight epochs of NIR spectroscopy using the Triple-Spec spectrograph on the Palomar 200-inch telescope (Herter et al. 2008), SpeX on the NASA Infrared Telescope Facility Program 2021A083, PI: De) and the Folded-port Infrared Echellete (FIRE; Simcoe et al. 2008Simcoe et al. , 2010) spectrograph on the Magellan 6.5 m telescope. The TripleSpec and SpeX data were reduced using the spextool pipeline (Cushing et al. 2004), while telluric and flux calibration were performed using the xtellcor package (Vacca et al. 2003). The FIRE data were reduced, flux calibrated and corrected for telluric absorption using the pypeit package (Prochaska et al. 2020). Table 4 summarizes all the epochs of spectroscopy of the source, while they are shown by vertical lines in Figure 2 along the photometric evolution.

X-ray coverage from surveys
As noted in Mereminskiy et al. (2021), an X-ray source is reported in the XMM Newton Slew Survey catalog (Saxton et al. 2008) within the ART-XC localization, as XMMSL2 J181414.6-225613. The reported location of the XMM Slew Survey source along with its uncertainty is shown in the NIR image in Figure  1. The spatial coincidence (within ≈ 2σ ≈ 7 ) suggests that XMMSL2 J181414.6-225613 is likely associated with SRGA J181414.6-225604. To derive the com- Table 5. Long-term X-ray flux constraints on SRGA J181414.6-225604 from wide-area surveys. Count rates were converted to observed X-ray fluxes in the same energy range assuming a power law spectrum with photon index Γ = 1.5 and column density of NH = 6 × 10 22 cm −2 . plete history of X-ray data at this position, we used the High Energy Light Curve Generator 3 (Saxton et al. 2021) at the location of the IR counterpart. The source position was covered over four epochs in the XMM Slew Surveys, including two epochs of X-ray detection in 2010 and 2015 respectively. The source was also covered by the ROSAT (Voges et al. 1999) and INTEGRAL sky surveys (Krivonos et al. 2021) but not detected. We summarize the archival X-ray coverage in Table 5. During the all-sky survey, ART-XC covers the same sky position nearly each six months. We reprocessed all data, covering the SRGA J181414.6-225604 position using the ART-XC pipeline artproducts v0.9 and CALDB version 20200401. The source is clearly detected in the 4-12 keV energy band during third and fourth surveys (April, 01-02 and October, 02-03, 2021). Although initially undetected, forced photometry of the stacked data from the first two surveys also provides a low significance (≈3 σ) detection of the source at a flux level of ≈ 6 +2 −3 × 10 −12 erg cm −2 s −1 . We are unable to assess the source variability during the first two surveys due to its faint flux. Due to the small number photons detected from the source, we adopt a simple spectral model and calculate all fluxes assuming that the source spectrum is similar to that of the Crab nebula (for which we used the absorbed power law model from Madsen et al. 2017). The source flux and its uncertainty intervals are estimated using Bayesian inference for the observed count rate per pixel given the measured background count rate and best estimates for the instrument PSF and vignetting.   (Gehrels et al. 2004). We generated the X-ray light curve of the source using the automated online tool 4 (Evans et al. 2009). The event data were binned over ≈ 2 days to track the long-term X-ray evolution of the source, and the light curve is shown in Figure 2. The observation log and derived count rates are given in Table 6. In addition, we examined images from the Ultraviolet Op-tical Telescope (UVOT; Roming et al. 2005) obtained during the XRT observations. The source is undetected in all of the UVOT images, as expected given its highly reddened nature. Upper limits are listed in Table 1. 2.6. NICER Follow-up SRGA J181414.6-225604 was observed by the X-ray Timing Instrument (XTI) on board the Neutron Star Interior Composition Explorer (NICER; Gendreau et al. 2016) shortly after discovery (Bult & Gendreau 2021). Following the brightening of the X-ray transient detected in Swift, we requested two additional epochs of ToO follow-up around the peak of the X-ray flare via DDT requests. Table 6 provides an overview of the NICER observations. We started our data reduction with the unfiltered events lists (ufa files), which we processed using the nicerl2 task as per the guidelines given in the NICER data analysis threads 5 . We used HEASoft version 6.29c with NICER calibration data (xti20210707). For nicerl2, we used default values for all the parameters except for underonly range, overonly range, and overonly expr which were set to '-', '-', and NONE, respectively. This retains all Good Time Intervals (GTIs) that could have high optical light leak and/or particle background but are later screened based on the criteria that hbgcut < 0.05 and s0cut < 2.0 as recommended by Remillard et al. (2021). Here, hbgcut and s0cut represent the net count rates in the 13−15 keV and 0.0 − 0.2 keV bands, respectively, after subtracting the background contribution.
Allowing data with all possible values of undershoots and overshoots initially and screening later based on hbgcut and s0cut values would ensure that we are not inadvertently excluding any good exposures. This procedure yielded a total exposure of roughly 26 ks. NICER consists of 52 co-aligned detectors known as Focal Plane Modules (FPMs). During a given GTI, some detectors can be 'hot/bad' in the sense that they behave anomalously compared to the rest of the FPMs. This can happen temporarily due to optical light leakage into these detectors or due to other factors. A given detector is marked as hot/bad for a given GTI if: 1) its observed low energy noise peak, i.e., 0.0 − 0.2 keV count rate, is more than four standard deviations above the median 0.0 − 0.2 keV value or 2) the observed in-band 0.4 − 1.0 keV count rate is more than two standard deviations above the median 0.4 − 1.0 keV of all detectors. We then generate combined responses for the GTI excluding these hot/bad detectors. We use the ni3C50 5 https://heasarc.gsfc.nasa.gov/docs/nicer/analysis threads/ background model (Remillard et al. 2021) to estimate the background. The final background subtracted light curve normalized to 50 FPMs is shown in Figure 2.

NuSTAR Follow-up
We obtained one epoch of DDT ToO follow-up (PI: K. De; Sequence ID 90701314002) of SRGA J181414.6-225604 using the NuSTAR hard X-ray telescope (Harrison et al. 2013). Table 6 provides a summary of the NuSTAR observation. The data from the two focal plane modules were processed using the NuSTAR Data Analysis Software (nustardas) V2.1.1 and the 20211202 CALDB. The nupipeline command was used to produce cleaned event files using the default parameters. Barycentric correction was performed on the event files using the barycorr task and the clock offset correction file version V128 . Light curves and spectra for the source were generated by extracting events within a radius of 60 centered at the position of the source using nuproducts. Background events were selected from a region of 100 located away from the source in the same part of the detector. Light curves were generated with the Stingray package V0.3 (Bachetti et al. 2021) using the barycentered event lists.

Broadband spectral analysis
We begin the discussion on the X-ray properties of SRGA J181414.6-225604 with the broadband spectral analysis. We use the NICER and NuSTAR observations near the first detection of the outburst in April 2021 (see Table 6) to perform a joint fit. Although not strictly simultaneous, there is little spectral variability between the NICER and NuSTAR observations and we fit them together to well constrain the broadband continuum and absorption column density. The fitting was carried out using the xspec package (Arnaud 1996) version 12.12.0. For NuSTAR, the individual FPMA and FPMB spectra produced by nuproducts were grouped with ftgrouppha to have at least 20 counts per bin in order to fit them with χ 2 statistics. We included data in the 3.0 − 30.0 keV energy range since the spectrum/observation becomes background dominated at higher energies. The NICER data were similarly binned to include at least 20 counts per bin. We include NICER data in the 0.7-8.0 keV range since the source is heavily absorbed at lower energies and comparable to the background at higher energies. Uncertainties of model parameters are presented at the 68% confidence level. The derived parameters for the models are discussed below and given in Table 7.

Component Parameter
Value Unit Model 3: edge * Tbabs * (pcfabs * (compTT + gauss) + apec) Table 7. Comparison of the best-fit parameters used to model the broadband X-ray spectrum of SRGA J181414.6-225604. For each model, the NuSTAR FPMA constant was set to 1 while the scaling constants for the NICER and NuS-TAR FPMB spectra were allowed to vary. We show the best-fit χ 2 values and null hypothesis probability for each model above the parameter table. CF denotes the covering fraction for the pcfabs model. The apec model norm is related to the emission measure (EM ) of the ionized region as norm = 10 −14 4πD 2 EM , where D is the distance to the source.
Preliminary analysis of the X-ray spectra suggested a heavily absorbed power law spectrum for SRGA J181414.6-225604 (Mereminskiy et al. 2021;Bult & Gendreau 2021;Heinke et al. 2021). Bult & Gendreau (2021) also noted a low energy excess in the NICER spectra which was attributed to a possible unrelated source in the field of view. However, the Swift XRT images do not show any other source within the NICER field. We thus first attempted to fit the 0.7 − 30.0 keV spectrum using a simple absorbed power-law model (Tbabs * powerlaw) using the wilms interstellar abundances (Wilms et al. 2000). We allowed for a vary-ing normalization between NICER and NuSTAR, and the standard cross-calibration constant between the two NuSTAR modules. The resulting model is constant * Tbabs * powerlaw (Model 1). We performed spectral fitting with χ 2 minimization, and the resulting residuals are shown in Figure 3. The best-fit model suggests a high column density of N H ≈ 7.2 × 10 22 cm −2 consistent with preliminary reports, and a photon index of Γ ≈ 2.2. However, the power-law fit is overall poor with χ 2 /d.o.f ≈ 2097.84/239, and leaves clear positive residuals below ≈ 2 keV and between ≈ 5 − 10 keV, and negative residuals above ≈ 15 keV. The residuals also reveal a feature at 6.4 keV. We do not discuss this model further.
We then attempted a more complex phenomenological model consisting of a cuttoff power-law implemented using highecut * powerlaw for the high energy emission component, where the high energy roll over (E cut ) for the power law takes effect at a variable cut-off (E f old ) energy (Model 2). We used an apec (Smith et al. 2001) component to model the low energy excess 6 . Corroborating evidence for the presence of an optically thin plasma component is clearly detected in subsequent NICER spectra, and is discussed in Section 3.2. We fixed the apec abundance to 1.0, thus assuming a Wilms et al. (2000) composition. We found improved fits by adding a partial absorber (pcfabs) to the high energy component in addition to Tbabs 7 .
A clear 6.4 keV excess is also seen in the residuals from the simple power-law fit, which we model using a gauss component in the fit. Due to limitations of the F-test for line emission features (Protassov et al. 2002), we simulate its significance using the simftest command in xspec, which suggests a null-hypothesis probability of < 0.01. We further note a systematic feature at ≈ 1.8 − 1.9 keV, which is clearly detected as an absorption edge in subsequent NICER spectra at higher fluxes (see Section 3.2). We identify this feature as the Si K-edge at 1.844 keV, which is detected in many highly absorbed X-ray binaries (Schulz et al. 2016). We include this feature in the fit using the edge component in xspec, keeping the energy fixed and the optical depth as a variable parameter. The addition of the edge indeed results in an improved fit; simulations for the F-test suggest a null-hypothesis probability of < 0.03. The result-6 Due to the very high absorption column, alternative attempts at fitting the low energy component with blackbody (bbody) or disk (diskbb) emission produced fits that were poorer or produced unreasonably large luminosity/radii for optically thick components. 7 Placing the apec and high energy component behind the same absorbing column produces unphysically high luminosity for the apec component. We therefore suggest that the high energy emission component exhibits a higher absorption column.

Spectral evolution
We attempted to fit the two subsequent epochs of NICER spectra using the same Comptonization model discussed in the joint analysis and were able to get acceptable fits. We list the derived parameters in Table 8, noting that the high energy comptt component is relatively poorly constrained due to the absence of higher energy coverage. In Figure 4, we show the NICER spectra in the 0.9 − 9.0 keV range for the three epochs. The low energy apec component is present in all the spectra, while we see a clear hardening of spectrum with increasing X-ray flux. Similar hardening of the X-ray spectrum with increasing flux have been previously observed in other SyXRBs (Masetti et al. 2002;Enoto et al. 2014).
The subsequent NICER observations also show evidence of unresolved emission and absorption features that are well detected in the higher signal-to-noise ratio spectra. In the data taken around UT 2021-05-25, we find an unresolved emission line around 1.34 keV, which coincides with a known feature of Mg XI. The feature is strongest at this epoch ( Figure 4), but is also visible in the other two epochs at lower significance. The variability argues against a possible origin in background Galactic ridge X-ray emission (Revnivtsev et al. 2006). NICER Epoch 2 (2021-05-25):  Table 8. Evolution of the best-fit model parameters for SRGA J181414.6-225604 as observed with NICER. The parameters correspond to the adopted Model 3 in Table 7.
For each model, we show the best fit parameters, their uncertainties, the resulting χ 2 as well as estimated absorbed 0.8 − 8.0 keV fluxes (as FX ).
This emission line suggests the presence of an optically thin plasma emission component, and is well fit by the adopted apec component. The NICER spectrum near the peak of the 2021 outburst (around UT 2021-06-09) shows a strong absorption feature near the Si K-edge, while a lower significance feature is detected in the earlier epochs. The derived optical depth of the Si edge is variable across the epochs, although we cannot rule out unresolved contamination with the nearby Si Kα emission line in these low resolution spectra. There is clear variability in the narrow Fe Kα line. The line is detected in all epochs as confirmed with the F-test, which suggests a null-hypothesis probability of ≈ 3 × 10 −3 and ≈ 10 −11 in the second and third epochs respectively. The line center is consistent with  Table 7. Model 1 is a simple absorbed power law model which does not provide a good description of the data (see text). rest wavelength and is unresolved (Sigma< 140 eV; the spectral resolution of NICER). This suggests that it does not arise from reflection from the inner part of the disk (García et al. 2011); it may come from reflection off colder gas at larger distances, or from the extended thermally-emitting plasma. Based on the best-fit parameters, we find the broadband hardening to be associated with increasing optical depth for the Comptonizing plasma towards the peak of the outburst. We also note a variable covering fraction for the partial absorber, while the Tbabs absorber remains roughly constant.

Temporal variability
SRGA J181414.6-225604 exhibits dramatic X-ray variability on timescales of minutes to weeks. As in Figure  2 and Section 3.2, the X-ray transient was observed to brighten and harden for ≈ 100 days after the initial discovery, followed by a drop to its near-quiescent flux level over the next ≈ 100 days. The individual NICER observations obtained during the brightening provide strong constraints on the short-term variability. Figure 5 shows the X-ray light curve of the source in the soft (1−3 keV) and hard (3 − 10 keV) bands during the three epochs of NICER observations. All epochs are characterized by brief periods of rapid X-ray activity where the source flux varies by 5× over timescales of ∼minutes. The short timescale variability is nearly identical in the hard and soft bands, as indicated by the relatively constant hardness ratio as the count rate changes by factor of ≈ 10×. Similar 'colorless' variability is known in several SyXRBs (Masetti et al. 2002;Enoto et al. 2014). We also searched for the presence of X-ray bursts by binning the NICER light curve at 1 s time resolution but did not find any bursts.
To search for the presence of pulsed emission, we used the complete NICER event list (corrected to barycentric time relative to the JPLEPH.DE200 ephemeris as per the default in the FK5 reference frame) and created a mean power spectral density (PSD) from 10 −3 Hz to 10 Hz. We averaged power spectra of segments of 1000 s each, with a time resolution of 0.1 s. Figure 6 shows that the source exhibits a strong red noise dominated PSD. We searched for the presence of a pulsation by fitting the red noise component with a power law and searching for excess power after normalizing the PSD by the red noise component (van der Klis 1989;Israel & Stella 1996). The magenta dotted line in Figure 6 shows the 3σ threshold for the detection of a signal accounting for the expected noise distribution in the averaged PSD. No pulsations are detected at the 3σ level, corresponding to an upper limit on the fractional pulsation amplitude of ∼ 1% at ∼ 10 Hz and ∼ 20% at ∼ 10 −3 Hz. A similar pulsation analysis on the NuSTAR data also did not re-  discovery, we used the online XRT products tool 8 tool (Evans et al. 2009) to derive an enhanced position using stars in the UVOT field of view (Goad et al. 2007). The best-fit position is α = 18:14:14.74, δ = -22:56:19.1 with an error-radius of ≈ 2.4 . Figure 1 shows a NIR image of the field overlaid with the X-ray localization regions from ART-XC, eROSITA and Swift. The bestfit position is squarely consistent with the position of IRAS 18111-2257. IRAS 18111-2257 is reported in the 2MASS Point Source Catalog (Skrutskie et al. 2006) as 2MASS J18141475-2256195, with NIR magnitudes of J = 8.32 ± 0.07, H = 6.93 ± 0.04 and Ks = 6.15 ± 0.02, and is coincident within ≈ 0.4 of the position of the X-ray source. The IR source is one of the brightest and reddest sources in the nearby field, making the spatial association with SRGA J181414.6-225604 very probable. Following Kaplan et al. (2007), we use the 2MASS catalog to estimate a spatial density of ≈ 7 × 10 −6 −2 for sources brighter than K s = 6.5 in a 10 × 10 arcmin 2 region around the source. This suggests a random probability of ≈ 3 × 10 −6 for the X-ray transient to be spatially coincident within ≈ 0.4 of the bright IR source. Similarly, using the PGIR archival data, we derive a chance coincidence probability of ≈ 4 × 10 −5 8 https://www.swift.ac.uk/user objects/ for the X-ray transient to be coincident within 0.4 of a variable 2MASS source brighter than J = 10.0 mag. Together with the temporal coincidence of the IR and X-ray brightening, we conclude that the IR source is very likely the infrared counterpart of SRGA J181414.6-225604. Hereafter, we refer to the IR counterpart of SRGA J181414.6-225604 as IRAS 18111-2257.

Spectral Type
In Figure 7, we show the two epochs of low resolution (R ≈ 1000) optical spectra of IRAS 18111-2257 obtained with DBSP. The most prominent absorption features are identified with sharp TiO and VO absorption band-heads typically seen in late M-type stars (Lançon et al. 2007). The inset in Figure 7 shows the higher resolution spectrum obtained with WiFeS, zoomed in around 8200Å. The source does not exhibit strong Na absorption around 8200Å, which is characteristic of late M-type dwarfs (Cushing et al. 2005), suggesting that the counterpart is a luminous giant or supergiant.
It is well established that the TiO band strengths correlate well with the spectral type of late-type giants, with deeper bands corresponding to later and cooler spectral types (e.g. Ramsey 1981;Kenyon & Fernandez-Castro 1987). To identify the spectral type, we compare our spectra to that of late type (M5-M7) optical spectra of cool giants in the recent VLT X-Shooter Spectral Library DR2 (Gonneau et al. 2020) in Figure 7. The steep red continuum of the source is inconsistent with the typical colors of cool giants, and hence an additional dust extinction was applied to the library spectra for easier comparison. As shown, the deep TiO absorption features suggest the spectral type to be > M6-M7. Figure 8 shows the NIR spectrum of IRAS 18111-2257 compared to a star of the same spectral type (as inferred from the optical spectrum) in the IRTF spectral library (Rayner et al. 2009). The spectrum shows clear signatures of broad water absorption features around ≈ 1.4 and ≈ 2.0 µm, which are only seen in the very extended atmospheres of large amplitude pulsating Mira variables (amplitude δV > 1.7 mag; Matsuura et al. 1999;Lançon & Wood 2000). We measure the H 2 O absorption index defined in Blum et al. (2003) to be ≈ 8%, suggesting that the source is a Mira AGB star since supergiants show absorption indices of < 6% (Messineo et al. 2021). Compared to the reddened spectrum of the comparison star, IRAS 18111-2257 exhibits a large excess in K-band, suggestive of the presence of warm circumstellar dust as seen in D-type symbiotic binaries and consistent with the steep reddened optical spectrum. Figure 9 shows the NIR spectum of IRAS 18111-2257 zoomed into the individual JHK bands. The J-band  WiFeS 2021-04-08 Figure 7. Comparison of the low resolution optical spectra of IRAS 18111-2257 with spectral standards from the VLT X-Shooter Spectral Library (Gonneau et al. 2020). The standard spectra were artificially extinguished with additional reddening (using the Cardelli et al. 1989 extinction law with RV = 3.1) as indicated in the legend to match the spectral slope of IRAS 18111-2257 for easier comparison. The inset shows a higher resolution contemporaneous optical spectrum from WiFeS, zoomed in around the region of the Na 8200Å feature. The deep TiO and VO absorption band-heads suggest a spectral type of at least M6-M7, while the absence of strong Na 8200Å absorption (Cushing et al. 2005) constrains the luminosity class to a giant.
spectrum is dominated by absorption bandheads of TiO at 0.97 µm, 1.1 µm and 1.25µm, and VO bandheads at 1.046 µm and 1.325 µm. The TiO bandheads near 1.25 µm become discernible at spectral types M7 or later (Wright et al. 2009) for cool giants, while the VO features are known to become prominent at very low effective temperatures of T eff 3200 K (spectral type M6 or later; Hinkle et al. 1989;Joyce et al. 1998). The strong TiO band head at 1.1 µm detected here is not seen in the spectra of known red supergiants (RSGs) while we also do not detect strong CN features around 1.09 µm characteristics of RSGs (Messineo et al. 2021), further arguing against a supergiant classification. The presence of VO instead of the characteristic double absorption bands of ZrO in J-band confidently identifies the source as a Mtype star instead of a S or C-type AGB star (Hinkle et al. 1989). We identify a strong emission line of He I λ10833Å on top of the stellar continuum in J-band. We discuss the properties of this feature in Section 5.4.
In H-band, we identify Si I 1.59 µm, numerous absorption features of the 12 CO second overtone series as well as OH molecular features blended with the CO series. The strong CO second overtone features corroborate the classification of the source as a luminous giant since these features are not seen in M-type dwarfs (Rayner et al. 2009). The most prominent features in K-band are the first overtone absorption series of 12 CO and 13 CO. These features are known to be particularly strong in giants and supergiants (Rayner et al. 2009). In addition, we identify strong absorption features from Na I, Si I and Ca I in the K-band spectra. While the equivalent widths of these atomic absorption features can be used to constrain the spectral type and luminosity class of the star (e.g. Ramirez et al. 1997), the large K-band dust excess and water absorption makes this measurement unreliable (Blum et al. 2003;Messineo et al. 2021). Overall, the spectroscopic signatures of i) strong water absorption, ii) deep TiO, VO and CO features and iii) weak/absent CN lines suggest the IR counterpart to be a very late-type (M7-M8) O-rich Mira AGB star, akin to the class of D-type symbiotic binaries. . The NIR spectrum of IRAS 18111-2257 compared with a star of the same spectral type (HD 108849; as inferred from the optical spectrum) from the IRTF Library (Rayner et al. 2009). The comparison star spectrum has been reddened with extinction (indicated in legend) as in the case of the optical spectra. The reddening was applied to match the slope of the J and H-band spectra, but consistently leaves a large excess in K-band (while a larger value of extinction is inconsistent with the J and H band continuum slope), likely suggesting the presence of warm dust. Prominent water absorption features are visible and marked. Figure 10 shows the long-term multi-wavelength light curve of IRAS 18111-2257 over the last ≈ 12 years. The source exhibits clear variability in from the shortest (gband) to longest (W 2-band) wavelengths. The W 1 light curve shows variability with amplitude of ≈ 0.7 mag, while the W 2 light curve exhibits lower variability of 0.5 mag. The J-band data from PGIR data between 2019 and 2021 shows the largest amplitude variability, with steep drop of ≈ 2 mag over ≈ 100 days in 2019 followed by a gradual brightening in 2020 and 2021. The source is also systematically fainter in J-band (by at least ≈ 2 mags) compared to the 2MASS reported magnitudes (from 1998) even accounting for the full variability amplitude observed in PGIR data. The source is also fainter in H and K band when comparing the WIRC and 2MASS magntudes. The ATLAS o-band and ZTF r-band exhibit smaller amplitude ( 0.7 mag) drops in 2019 compared to J-band.

Variability
The PS1  57000 and 58600 (prior to the onset of the abrupt fading episode in 2019) with the PS1 i-band light curve, we fit the photometry with a sinusoidal function allowing for a different amplitude between the two filters but requiring a constant period and phase 9 . We thus derive a period of 1502 ± 24 days. The amplitude is 1 mag in i-band, suggesting that the variability is associated with fundamental mode pulsations seen in Mira variables (Wood & Sebo 1996). However, the likely period lies amongst the longest known in Galactic Mira variables (Soszyński Optical/IR Flux (Mag) . We note that the recent optical data in r and i bands from ZTF also show evidence of the same periodicity but are systematically fainter than the corresponding PS1 magnitudes detected ≈ 10 years ago, even after accounting for the observed variability amplitudes.

Distance and Foreground Extinction
The spectroscopic diagnostics as well as the likely detection of a ≈ 1500 day photometric periodicity suggest that IRAS 18111-2257 is a long-period pulsating Mira variable. However, the presence of a thick circumstellar dust shell dominating the NIR SED makes the estimation of its luminosity non-trivial from spectroscopic techniques (e.g. Ramirez et al. 1997), which is commonly the case for D-type symbiotics (Hinkle et al. 2013). Gaia EDR3 (Gaia Collaboration et al. 2021) reports a parallax of −0.3236 ± 0.1498 mas, while the corresponding distance estimate in the Bailer-Jones catalog (Bailer-Jones et al. 2021) is 6.0 − 11.7 kpc (16 − 84 percentile confidence). However, Gaia EDR3 also reports a astrometric gof al ≈ 12 and astrometric excess noise sig ≈ 77, which together with the > 2σ negative parallax, suggests a very poor (and likely unreliable) astrometric solution.
Next, Mira variables are known to exhibit a periodluminosity (PL) relation (Feast et al. 1989;Wood 2000) which has been previously used to estimate distances to symbiotic Miras (Whitelock 1987;Gromadzki et al. 2009). Using the Whitelock et al. (2008) periodluminosity relation for O-rich variables suggests an intrinsic absolute magnitude of M K = −9.9 ± 0.2 mag. However, we caution that the PL relation at such long periods remains debated (Ita & Matsunaga 2011;Yuan et al. 2017) and we do not discuss it further here, but note that many 'normal' long period (> 1000 days) Orich Miras are found to lie on the extrapolation from shorter periods (Whitelock et al. 2003). Using the recent calibration of bolometric luminosity as a function of pulsation period for very long-period O-rich Miras from Groenewegen et al. (2020), we estimate a bolometric luminosity of M bol = −6.85 ± 0.40 (L = 4.7 +2.1 −1.5 × 10 4 L ). We then use the period-color relation from Whitelock et al. (2000) to estimate the total (interstellar + circumstellar) extinction, and derive E(J − K) = 0.31 ± 0.25 mag (E(B − V ) ≈ 0.50 ± 0.40 mag) based on the 2MASS magnitudes. Based on recent three dimensional dust maps (Bovy et al. 2016;Green et al. 2019), the Galactic dust extinction in this direction is estimated to be E(B − V ) ≈ 0.9 − 1.3 mag for sources farther than 3 kpc away. The 2MASS colors are then roughly consistent with the expected interstellar extinction and intrinsic colors of Miras. We thus take the 2MASS measurements to represent the star prior to its current obscured state (see Section 5). We adopt the foreground extinction to be E(B − V ) = 0.7 ± 0.6 mag, corresponding to A K = 0.22 ± 0.17 mag, placing this source at 14.6 +2.9 −2.3 kpc based on the Mira period-luminositycolor relations. The estimated distance and sky location places the source in the Norma arm on the other side of the Galactic center. The total extinction along this line of sight is A K ≈ 0.5 mag (Schlafly & Finkbeiner 2011), consistent with the source being located within the Galaxy.

Long term SED evolution
IRAS 18111-2257 exhibits variability on timescales of ∼days to ∼years (Figure 10). In this section, we analyze the variability of the source and provide evidence for massive dust obscuration episodes over the last ≈ 20 years that have progressively obscured the underlying Mira variable. We begin by showing the broadband photometric SEDs 10 of the counterpart at three different epochs separated by ≈ 10 years in Figure 11. The NIR and MIR data for the first epoch capture the peak and Rayleigh-Jeans tail of the SED, which can be well described by a blackbody (BB) of temperature T ≈ 2000 K. The apparent BB temperature is consistent with the typical effective temperatures of late type Mira variables at similar J − K color and spectral type (Haniff et al. 1995;Feast 1996). Since the 2MASS colors also suggest relatively small circumstellar reddening (Section 4.4), we adopt 2000 K as the average effective temperature of the underlying unobscured star as observed around the year 1998.
The 2010 SED is characterized by a fainter and colder (T ≈ 1500 K) broadband continuum compared to 1998. There are noticable deviations from a simple BB model, which we analyze in Section 5.2. The 2020 epoch exhibits major differences when compared against both the 1998 and 2010 epochs, characterized by a red SED peaking at 5 µm. In addition, the 2020 SED deviates significantly from a single BB model, and we do not attempt to derive a corresponding BB temperature. The source flux has faded > 10× in J-band compared to the 2MASS epoch. Comparing against the long term light curve in Figure 10, the dramatic fading was clearly associated with a sudden large amplitude ( 2 mag) drop in the J-band flux in late 2019. Such a sudden large dimming is inconsistent with the smooth pulsations in O-rich Miras; however, the observed drop and reddening is strikingly similar to the sporadic dust formation episodes seen in some symbiotic and C-rich Miras (e.g. Jurkic & Kotnik-Karuza 2012a; Ita et al. 2021).

DUSTY modeling
With evidence for recent dust obscuration in IRAS 18111-2257 based on both the temporal and spectral characteristics, we turn to detailed multi-epoch modeling of the source SED using the radiative transfer code DUSTY (Ivezic & Elitzur 1997). DUSTY solves the For each epoch, we also show the best-fit black body curve and the corresponding best-fit temperature. The 2020 SED is not well described by a BB curve and hence we nominally show the best-fit curve without a corresponding temperature.
radiative transport through a dust shell including dust absorption, scattering and emission assuming spherical symmetry. We model the SEDs at four different epochs listed in Table 9 to track the evolution of the mass loss rate and dust parameters of the underlying Mira star. The four epochs were selected to ensure that the wavelength coverage extends from the BB Wien tail of the unobscured star (to constrain the extinction law) to the MIR bands ( 4 µm; to constrain the dust emission). While DUSTY uses self-similarity and scale invariance to reduce the input parameters, we applied additional constraints based on the inferred stellar parameters. We model the central star as a BB with T = 2000 K as derived in the previous section, and adopt a distance of 14.6 kpc. We assume the dust grains to be composed of warm silicates as in O-rich AGB stars (Ossenkopf et al. 1992) with a MRN grain size distribution (∝ a −3.5 ; Mathis et al. 1977). We set the minimum grain size to a min = 0.005 µm and allow for a variable maximum grain size a max . In all cases, we first attempt to fit the SED first with a max = 0.25 µm as in standard interstellar grains (Draine 2003), but change a max if no acceptable fits are found. The radial density profile is adopted to be a r −2 power law corrected for radiative acceleration in AGB winds (Elitzur & Ivezić 2001). The resulting free parameters of the model are the maximum grain size (a max ), dust optical depth at 0.55 µm (τ V ), the shell thickness (Y ) as a multiplicative factor of the inner radius (r in ), the dust temperature at the inner edge of the shell (T d ) and the luminosity (L).
We attempted several different values for the thickness of the shell Y (in the range ≈ 2 − 1000) but did not find it to significantly affect the derived parameters, and fixed it at Y = 5. This is expected since we primarily model the NIR and MIR data (λ 10 µm) where the emission is dominated by the hottest and innermost part of the shell (Kotnik-Karuza et al. 2007). Due to limited historical data, we do not attempt to include the effects of the Mira pulsations in the multi-epoch modeling for simplicity, but note that the stellar effective temperature can change by ≈ 300 K (Reid & Goldston 2002); however the large SED changes modeled here cannot be explained by temperature variations.
We then used a python wrapper on the DUSTY code to perform least-squares fitting using the Markov Chain Monte Carlo library emcee (Foreman-Mackey et al. 2013). The fitting was performed via minimization of the likelihood function defined using the χ 2 of the model fit and allowing for underestimated uncertainties 11 . We assumed flat priors for the fitted parameters and convergence of the posterior sampling chains was ensured (Foreman-Mackey et al. 2013). The best-fit parameters were derived using the median of the posterior sample  Table 9. Derived dust parameters from the multi-epoch DUSTY modeling of IRAS 18111-2257. We list The parametersṀ (the mass loss rate) and ve (the terminal outflow velocity) are calculated by DUSTY as part of the radiative wind acceleration solution, while other parameters were varied to minimize the least squared residuals from the observed data. The mass loss rates are derived assuming a gas-to-dust mass ratio of 200 and dust grain bulk density of 3 g cm −3 .
distribution while their confidence intervals are derived from the 16th-84th percentile (68% confidence) interval of the distributions. The derived parameters and their uncertainties are listed in Table 9 and discussed here.

1998: An optically thin, distant and cool dust shell
Our best-fit model for the observed SED in 1998 is shown in Figure 12. The SED can be well described by a largely unobscured (τ ≈ 0.6) stellar BB spectrum with a maximum grain size corresponding to interstellar grains (Mathis et al. 1977). The MSX MIR data show clear evidence for an excess around ≈ 9 − 11 µm, corresponding to silicate emission features, consistent with our assumption of the dust composition. The relatively small excess of the feature over the continuum constrains the optical depth to be small, consistent with our assumption of a an unobscured star observed in the NIR bands. The silicate emission feature suggests a cool dust temperature of ≈ 260 K located at a distance of ≈ 300 AU from the star. The inferred shell parameters are strikingly similar to the cool dust shells detected in D-type symbiotic stars (Angeloni et al. 2010;Hinkle et al. 2013).

2010: Detection of a warm dust shell
The 2010 SED shows clear deviations from the bestfit model derived for 1998, but can still be explained with nearly interstellar grain size distribution. In particular, the steep drop in flux from NIR to optical wavelengths suggests significant circumstellar extinction and a higher dust optical depth of τ ≈ 7.6. The WISE MIR photometry beyond ≈ 10 um exhibits clear excess over a simple BB curve (e.g. Figure 11) coincident with the silicate and continuum emission from a dust shell. However, the large dust optical depth (from the optical-NIR SED) together with the flux drop from W3 to W4 bands constrains the dust shell to be warm with a temperature of ≈ 1600 K, consistent with typical silicate dust sublimation temperatures. Unlike the cool distant dust shell detected in 1998, the warm dust shell is located close to the star at ≈ 10 AU suggesting that it may have been recently formed. The slow formation of such warm dust shells is consistent with that seen in many D-type symbiotic stars (Whitelock et al. 1983;Shenavrin et al. 2011), and also suggested to be a unique feature of symbiotic Miras (Hinkle et al. 2013).

2019: Evidence for progressive obscuration
The 2019 SED is derived from combining the earliest detections of the source in the PGIR and ZTF surveys together with MIR data from NEOWISE. Compared to the 2010 epoch, the source faded further in the optical r-band ( Figure 10) while the MIR WISE emission remained at a similar level. In particular, combining the ZTF optical data and PGIR NIR data, the models required a change in the maximum dust grain size to simultaneously describe the shallower extinction law and the relatively stable MIR emission. While the optical depth remained similar, we derive a higher mass loss rate suggesting that the star likely underwent increased dust production and mass loss between 2010 and 2019, obscuring the stellar optical-NIR flux as seen in the long term light curves. Evidence for such grain growth under increased mass loss and dust production has also been detected in other symbiotic Miras (e.g. Jurkic & Kotnik-Karuza 2010a).

2020: A rapid dust obscuration episode
The 2020 SED data was obtained after the dramatic fading observed in 2019. The observed 2019 fading had the largest amplitude in J-band compared to the optical bands while the W 1 band also exhibited a minor fading. The shallower optical-NIR extinction law uniquely constrains the grain size distribution, requiring it to extend to a larger size of a max ≈ 2.0 µm. Such large grain sizes were also inferred in DUSTY modeling of the sudden obscuration episodes of the symbiotic Mira RR Tel (Jurkic & Kotnik-Karuza 2012b). The rising flux in the MIR up to the W 2 band requires the dust to be cooler (≈ 750 K) than earlier epochs, and located farther away from the star at ≈ 40 AU. The change in the dust properties are accompanied by a large increase in the mass loss rate, confirming our suggestion of a dust obscuration episode. The inferred source bolometric luminosity is ≈ 25% fainter than that in 2019, which is likely due to the dust formation happening near the minimum of the long term pulsation cycle, as typically seen in other types of pulsating stars (Willson 2000).

Grain growth and the peculiar color evolution
The J-band dimming in IRAS 18111-2257 during the 2019 dust obscuration episode exhibits a peculiar color evolution. The dimming has a larger amplitude in the NIR J-band compared to the optical r and o bands while static dust extinction laws universally suggest larger extinction at shorter wavelengths (Draine 2003). As noted from the DUSTY modeling, the post obscuration SED also require much larger dust grains. The growth of grains during dust formation changes the optical properties of the grains over the duration of the event, suggesting a likely connection with the peculiar color evolution. In order to understand this connection, we show in Figure  13, the color magnitude diagram of the 2019-2021 dimming in IRAS 18111-2257 using PGIR and ATLAS data (which have the highest cadence).
IRAS 18111-2257 initially becomes bluer while fading, turns around in a loop during the obscuration minimum and becomes redder while brightening from the dust dip. Similar color peculiarities have also been discussed in dust formation episodes of the R Coronae Borealis variables (Efimov 1988a;Pugach 1991), C-rich Miras (Ita et al. 2021) and even noted for the dimming in the O-rich symbiotic nova PU Vul (Efimov 1988b). To understand this phenomenon, we created a grid of DUSTY models to simulate the effect of dust growth on the source colors. Although we assume a power law grain size distribution for the DUSTY modeling discussed thus far for the individual epochs, the grain extinction evolution may be dominated by specific size ranges during these transient episodes. For simplicity and motivated by the observed evolution of the dust properties between 2019 and 2020 (Table 9), we thus created a model grid varying only the properties of the dust (temperature fixed at T d = 1000 K) with i) single-sized grains ranging from ≈ 0.5 µm to ≈ 2.0 µm and ii) optical depth changing from τ = 7.0 to τ = 5.5.
We perform synthetic photometry on the resulting spectrum grid to simulate the expected color magnitude diagram and also show it in Figure 13. Since in our simple approximation, we do not include the contribution from the static dust distribution prior to the fading (epoch E3; DUSTY does not support multiple grain size distributions), there are differences in the absolute values of the color during the event. However, we find striking similarities in the color-magnitude 'loop' observed around the minimum of the dust obscuration event. In reality, the optical depth of the layer, the luminosity of the Mira (which is pulsating) as well as the dust temperature are changing during this event, and hence we do not expect to find perfect agreement between this model and the data. Nevertheless, as demonstrated in previous works (Efimov 1988a,b), change in dust optical properties during grain growth provides a good explanation for the observed color evolution in the dust formation episode in IRAS 18111-2257.

The He I Outflow signature
The NIR spectra of IRAS 18111-2257 exhibit a strong He I λ10833Å feature with a P-Cygni profile. Figure  14 shows the evolution of the line across our spectral sequence. From the highest resolution FIRE spectra, we find that the blue-shifted line absorption extends to velocities of ≈ 100 − 120 km s −1 . Chakrabarty et al. (1998) discovered a similar high velocity (≈ 250 km s −1 ) wind in the SyXRB GX 1+4, although it was unclear if the line arose from the donor star atmosphere or from an accretion disk wind. The He I NIR line is detected in many bright giants (Obrien & Lambert 1986), and with line profiles ranging from pure absorption to P-Cygni shapes (Dupree et al. 2009). The line is formed high in the chromosphere and shown to be a good tracer of bulk motion in the outer atmosphere where the gas flow is close to the escape speed (Dupree et al. 1992). Chakrabarty et al. (1998) pointed out that the inferred wind velocity in GX 1+4 was much larger than the typical wind speeds in red giants (≈ 10 − 30 km s −1 ; Höfner & Olofsson 2018). They suggested that the UV/X-ray radiation from the hot NS in GX 1+4 may contribute to accelerating the outflow to high speeds since the line is also detected in other symbiotic stars (e.g. Bensammar et al. 1988). Dupree et al. (2009) carried out a survey of the λ10833 line in field giants, and found velocities as high as ≈ 90 − 120 km s −1 in several objects, suggesting that fast outflows may be common. The He I λ10833 transition lower state is ≈ 20 eV above the ground state, and hence it is unlikely to be photoionized by cool photospheres. Indeed, Dupree et al. (2009) showed that the outflows are detected only in stars hotter than ≈ 4500 K. With a photospheric temperature of 2500 K in IRAS 18111-2257, the source of the He I ionization, however, remains unclear.
The hot radiation from the compact object may lead to ionization of the donor star's wind, similar to the scenario discussed in Chakrabarty et al. (1998). Alternatively, our conclusion of recent dust formation in IRAS 18111-2257 brings forward another possibility. Dust formation has been extensively monitored in the R Coronae Borealis stars (Clayton 1996), where the He I line is ubiquitously detected only around the epochs of dust formation (Clayton et al. 2013). During these phases, it has been suggested that the outflowing dust cloud shocks the surrounding gas, collisionally excites the transition and radiatively accelerates it outwards to velocities higher than the stellar escape speed (Eyres et al. 1999). The resulting velocities are expected to be ∼ 100 km s −1 for AGB stars with L ∼ 10 4 L (Castor 1981), although optical depth effects reduce the terminal velocities to ≈ 10 − 30 km s −1 in the steady state winds of AGB stars (Elitzur & Ivezić 2001).
In the case of IRAS 18111-2257, the derived terminal velocities from the DUSTY solution (Table 9) are ≈ 40 km s −1 around the epochs of dust formation, similar to the observed maximum absorption in the P-Cygni profile. Since the maximum observed absorption speed is ≈ 2 − 3× higher than the derived terminal veloci-ties, it is possible that the highly variable mass loss rates and rapid grain growth around these epochs accelerates a fraction of the material to higher speeds, as discussed in Elitzur & Ivezić (2001). Specifically, as the visual optical depth decreases during dust formation (Table 9), the longer wavelength radiation is shifted to shorter wavelengths while the rapid grain growth makes the dust grains further effective at absorbing this emerging radiation and accelerating the surrounding gas with it. We thus suggest that radiative acceleration and collisional excitation during the dust formation episode could also explain the high velocity He I outflow observed in IRAS 18111-2257. In the case of the R Coronae Borealis stars, the line is observed to persist at nearly constant strength and velocity up to ≈ 100 days after the obscuration event, since it likely takes time for the far away outer wind to decelerate (Clayton et al. 2013). For the AGB star considered here, the relevant length scales are at least ≈ 10× larger, suggesting that the He I line could persist for ∼ 1000 days. Continued monitoring of the feature as well as higher resolution spectroscopy to infer its velocity and optical depth evolution (e.g. Clayton et al. 2003;Dupree et al. 2009) can test the proposed scenario.

DISCUSSION
We have thus far discussed the identification and multi-wavelength characterization of SRGA J181414.6-225604 a Galactic hard X-ray transient which exhibits i) a heavily absorbed X-ray spectrum dominated by an optically thick Comptonized plasma at high energies (> 2 keV) and a collisionally ionized plasma at low energies, ii) X-ray variability characterized by 'colorless' flares on timescales of ∼ minutes and longer term hardening with increasing flux on timescales of ∼weeks, iii) a bright infrared counterpart consistent with a luminous pulsating ≈M8-type O-rich Mira variable and iv) evidence for an intense dust obscuration episode starting ≈ 800 days prior to the peak of the X-ray brightening. In this section, we combine the multi-wavelength properties of the source to constrain the nature of the transient and implications for the broader SyXRB population.

Origin of the X-ray emission
The broadband X-ray spectrum of SRGA J181414.6-225604 can be well described by heavily absorbed Comptonized emission with a seed photon temperature of ≈ 1 keV and a plasma temperature of ≈ 7.5 keV. The spectral shape of the Compton cloud is consistent with a very compact accreting object, likely to be a NS or a BH, surrounded by a corona of hot electrons emitting hard X-rays by Comptonization of soft seed photons.
The case for a WD accretor is very unlikely due to the hard X-ray emission extending out to 30 keV as detected with NuSTAR (Luna et al. 2013;Masetti et al. 2007b). Furthermore, the inferred broadband X-ray flux together with the estimated distance for the IR counterpart (≈ 15 kpc) suggest a peak X-ray luminosity of 2.5×10 36 erg s −1 , much more luminous than accreting WDs ( 10 34 erg s −1 ; Mukai 2017).
Although we do not detect pulsations or X-ray bursts, the derived spectral parameters are consistent with previously known SyXRBs hosting NSs (Masetti et al. 2002(Masetti et al. , 2007bEnoto et al. 2014;Kitamura et al. 2014). However, X-ray bursts among SyXRBs are rare and have been detected from only one source (IGR J17445-2747; Mereminskiy et al. 2017;Shaw et al. 2020). In case the accretor is a NS, the seed photon component may arise from a localized hotspot on the surface of the NS, possibly produced by magnetically channelled accretion onto its surface (Masetti et al. 2002;Kitamura et al. 2014). We caution that the limited duration for the X-ray follow-up would not be sensitive to the very long spin periods (∼ hours) of some known SyXRBs, and can be masked by the stochastic accretion-related variability (e.g. Enoto et al. 2014; Figure 5). The pulsed emission may also be washed out by geometric effects or by the bright Comptonizing cloud (Titarchuk et al. 2002).
The X-ray temporal and spectral variability offers additional insights into the accretion process. The transient brightened for ≈ 100 days after discovery and has nearly faded back to its quiescent level seen prior to the ART-XC discovery. The source became harder during its brightening phase as typically seen in other SyXRBs, and is consistent with accretion from a wind with a low specific angular momentum (Smith et al. 2002;Wu et al. 2002). The variable column density and covering fraction of the partial absorber in our spectral modeling across the epochs (Table 8) suggest accretion inhomogeneties affecting the spectrum on timescales of ≈weeks. On the other hand, the spherically symmetric Tbabs component exhibits relatively constant N H across all the epochs, suggesting that it is associated with a constant contribution from circumbinary material. For the estimated optical depth of the dust shell around the donor in the most recent epoch (τ V ≈ 5), the corresponding A V ≈ 5.5 mag implies a hydrogen column density of ≈ 1.2 × 10 22 cm −2 (Güver &Özel 2009), consistent with the Tbabs component. The pcfabs component exhibits a ≈ 3× larger column density, suggesting enhanced absorption close to the compact object.
We consider two alternative possibilities to explain the short-term variability (∼minutes). Since the hardness ratio remains nearly constant, the short-term vari-ability cannot be attributed to a variable covering fraction or column density, since we would expect spectral shape changes as the estimated column from our spectral modeling is too low to be opaque to X-ray photons at ≈ 1 − 10 keV. Thus, we speculate that variable accretion at constant covering fraction and column density arising from the donor wind may explain the observed variations. Alternatively, we suggest that these variations may be attributed to changing Comptonization of the thermal seed photons. Specifically, even if the thermal seed component remains roughly constant, a variable Comptonization fraction at a constant plasma temperature can produce changes in the integrated luminosity of the comptt component with only minor changes to its spectral shape. Since the spectral modeling demonstrates that the optical depth is high and the spectrum is dominated by the Comptonized emission at 1 keV, the observed achromatic variations may reflect the changing luminosity and constant spectral shape of the comptt component.

A dense ionized nebula around the compact object
The broadband spectral analysis reveals a soft flux excess (below ≈ 1 keV) that we model with an optically thin collisionally ionized plasma (apec) component. The model is motivated by the detection of strong emission lines coincident with transitions of Mg and Si in the NICER spectra. The derived fits suggest a plasma temperature of ≈ 0.6 keV and indicate that this component is located outside the dense partially absorbing cloud, and thus relatively extended compared to the central X-ray emitting region around the compact object. Collisionally ionized plasmas are commonly observed in the X-ray spectra of dense shocks in colliding wind binaries and supernovae (e.g. Oskinova et al. 2017;Chakraborti et al. 2012). Taking the estimated distance of the counterpart, the derived normalization of the apec component suggests an emission measure of ≈ 2 × 10 57 cm −3 . If we take the relevant size for the extended apec emission region to be of the order of the inferred dust shell (∼ 10 AU), we derive a required particle density of ∼ 10 7 cm −3 assuming n H ≈ n e . The effective column density arising from the apec component is then ∼ 10 AU × 10 7 cm −3 ∼ 10 21 cm −2 , and much smaller than the cold absorber column. In Section 6.4, we show that the derived particle density is consistent with an ionized nebula formed from ejected material from the donor star.
Supporting the presence of a surrounding gas cloud, the X-ray spectra show multiple narrow emission and absorption features. We detect prominent and variable narrow Fe Kα emission in all the epochs, characteristic of wind-fed systems where the line is produced by Xray fluorescence of the companion wind material. We do not find evidence for spectral residuals suggestive of cyclotron lines seen in some accreting pulsars (Staubert et al. 2019) and possibly even in a SyXRB (Bozzo et al. 2018). A striking feature noted in the NICER spectral analysis is the presence of a strong Si K-edge that we modeled using an additional edge component. The improvement in the spectral fit suggests that the gas surrounding the compact object contains an overabundance of Si 12 , which is known to be depleted into silicate dust grains in the interstellar (ISM) medium (Draine 2003;Schulz et al. 2016). The edge exhibits a variable optical depth, as visually apparent in the multi-epoch spectra (Figure 4).
Following Schulz et al. (2016), the measured (excess) optical depth of the Si edge (relative to ISM) can be related to the total Si abundance using, where τ Si is the optical depth of the edge, A z is the expected abundance (per hydrogen atom) in the ISM, x is the total Si abundance relative ISM composition and σ z is the absorption cross-section. Using the assumed Wilms et al. (2000) ISM abundances, τ Si ≈ 0.3 measured from the brightest NICER spectrum and a corresponding total N H ≈ 5 × 10 22 cm −2 (Table 8), we find that the material surrounding the accretor contains ≈ 2.5× Si abundance relative to normal ISM. However, we caution that scattering effects are known to alter the simple absorption edge profiles used in xspec , which can affect the inferred abundance (Schulz et al. 2016); such effects are only probed with higher resolution spectroscopy. Nevertheless, we conclude that multiple diagnostics in the X-ray spectra securely demonstrate the existence of a dense dust/gas cloud enshrouding the binary system.

A Mira donor star exhibiting intense mass loss
The classification of the donor star as a very late-type pulsating AGB star offers a novel addition to the growing population of Galactic SyXRBs. The majority of previously known SyXRBs (Table 1 in Yungelson et al. 2019) contain early M-type donors consistent with red giants, in addition to some sources where the donor has been suggested to be a red supergiant (Hinkle et al. 2020;Gottlieb et al. 2020). The brightest known source GX 1+4 also has a M6-III red giant donor that is nearly 12 Since interstellar Si abundances are already included in the Tbabs and pcfabs components, improvements in the fit after including additional edges suggests an overabundance overflowing its Roche Lobe and powering the luminous X-ray emission (Chakrabarty & Roche 1997). The classification of the donor in the SyXRB Scutum X-1 was recently revised from a red supergiant (Kaplan et al. 2007) to a long-period Mira variable (De et al. 2022) similar to SRGA J181414.6-225604. XTE J1743-363 was also suggested to host a >M7 type donor star, and likely a pulsating variable on the AGB (Smith et al. 2012). In the X-ray band, both Sct X-1 and XTE J1743-363 exhibit similar behavior as SRGA J181414.6-225604 including ∼ 100 day long flares (Smith et al. 2012) as well as variability on timescales of ∼hours (Sguera et al. 2006;Kaplan et al. 2007). However, the donor photometric variability in XTE J1743-363 remains unconstrained. The pulsation period of ≈ 1500 days and O-rich composition provides strong constrains on the evolutionary stage of the donor star and binary system. Such long period Miras are rare, likely because they are in a very short lived evolutionary phase (Marigo et al. 2017) or they are realized only among the rarer massive AGB stars (Feast 2009). Indeed, there are < 20 candidate Miras in the Milky Way with periods > 1000 d (Menzies et al. 2019), confirming their rarity. However, Figure  2 shows that our combined photometric dataset over the last ≈ 12 years supports the presence of a coherent ≈ 1502 day period already covering ≈ 3 cycles of the Mira pulsation. As in Groenewegen & Sloan (2018), we employ the relationship derived in Groenewegen & Jurkovic (2017) based on Cepheid models (Bono et al. 2000) between the luminosity (≈ 2 × 10 4 L ), temperature (≈ 2000 K), metallicity (assuming Z ≈ 0.015) and mass for fundamental mode pulsators to estimate a current mass of ≈ 5.2 M for the Mira variable, making it a massive AGB star. The simpler period-mass-radius relationship in Wood (1990) provides a similar estimate of ≈ 5.5 − 8.0 M depending on the adopted radius 13 .
Thanks to multi-epoch archival photometric data as well as high cadence monitoring near the X-ray flare, we present strong evidence for an intense dust obscuration episode in IRAS 18111-2257 starting ≈ 800 days prior to the observed X-ray brightening. In addition, the long ≈ 25 year baseline between 2MASS, PS1, WISE and PGIR allows us to observe a dramatic transition in the appearance of the donor star from a relatively unobscured stellar SED (i.e. an S-type system) to a heavily dust obscured Mira (i.e. a D-type system). The secular change is accompanied by an increase in the mass loss rate (indicated by the DUSTY modelling) and suggests that the donor star is possibly at the onset of a thermal pulse (Herwig 2005). The long pulsation period and high inferred mass points to a relatively young system with age 100 Myr (Marigo et al. 2017); comparing the inferred current mass to the grid of intermediate mass AGB models in Doherty et al. (2015), we estimate an initial mass of ≈ 6 M and age of ≈ 90 Myr if the donor is indeed entering the first thermal pulse.
6.4. The nature of the accretion flare and binary system The coincidence of the dust formation (Section 5.2) with the observed X-ray flare in 2021 suggests that the enhanced mass loss provides additional fuel for accretion onto the compact object, that likely caused the X-ray brightening. This allows us for the first time to constrain the binary orbit in a SyXRB together with the accretion mechanism by comparing the properties of the mass loss episode and the observed X-ray flare. The Swift monitoring shows that the X-ray outburst reached maximum luminosity at t d ≈ 800 days from the start of the dust obscuration episode in 2019. If we take the He I P-Cygni signature as an estimate of the terminal wind velocity (v t ≈ 100 km s −1 , accelerated by the dust production or X-ray emission; Section 5.4), the compact object is likely located a ∼ v t × t d ≈ 55 AU from the donor star.
Assuming the mass ratio (q) of the binary system to be ≈ 4, assuming the compact object to be a ≈ 1.3 M NS 14 and the donor to be a 5 M AGB star, the Roche Lobe radius for the donor would be ≈ 23 AU, larger than the estimated photospheric radius ( 6 AU) of the unobscured star, suggesting that the system would be nominally undergoing wind accretion. However, our DUSTY modeling shows that the intense dust obscuration episode in 2020 results in the formation of an expanding dust envelope. The densest inner region of the dust shell is observed to expand from ≈ 10 AU in 2019 to ≈ 40 AU in 2020, thus nearly reaching the location of the compact object shortly prior to the start of the X-ray brightening.
For the assumed shell thickness (Y = 5), the entire binary system is likely already engulfed in the dust shell prior to the X-ray brightening, consistent with the strong Si over-abundance observed in the X-ray spectra. We can constrain the particle density in this region by approximating the wind density profile with standard r −2 dependence at large distances from the star, as we also confirmed from the radial density profile produced from the DUSTY model. Then, the mass loss rate is re-14 The constraints on the binary configuration and mass accretion process do not change substantially if we assume a 10 M BH.
lated to the radial density profile as For a distance of ≈ 50 AU from the donor to the compact object, we estimate a gas particle number density of ≈ 1.1 × 10 7 cm −3 . This density is similar to what we estimate for the low energy apec component observed in the X-ray spectra, suggesting that ionization of the dense Mira wind near the compact object provides a good explanation for the extended collisionally ionized plasma detected around the compact object. For the low angular momentum dusty wind of the donor star, the accretion is expected to take place directly onto the compact object without the formation of a disk (Smith et al. 2002;Wu et al. 2002). Following Yungelson et al. (2019), we can estimate the expected accretion rate assuming the Bondi-Hoyle-Lyttleton formalism. The compact object captures material within a characteristic Bondi radius given by where R B is the Bondi radius, M c ≈ 1.3 M is the mass of the compact object assumed to be a NS, c s is the sound speed, v rel = v 2 orb + v 2 w , v orb is the relative orbital velocity and v w is the wind velocity. For the wide apparent orbit and cool wind considered here, we ignore the NS orbital velocity and sound speed of the gas relative to the much faster wind velocity, resulting in R B ≈ 0.25 AU. Then the accretion rate onto the compact object isṀ whereṀ B is the Bondi accretion rate andṀ d the donor mass loss rate. Taking the enhanced mass loss rate from the donor estimated from the 2020 epoch (Ṁ d ≈ 2.2 × 10 −5 M yr −1 ), we derive an accretion rate ofṀ B ≈ 1.1 × 10 −10 M yr −1 . Assuming a fraction of the rest mass energy of this material is emitted by the compact object during the X-ray flare peak (L X ≈ 2.5 × 10 36 erg s −1 ), we estimate ≈ 0.3, similar to the expected conversion efficiency for accretion onto a NS ( ∼ 0.2). A lower conversion efficiency would be required if the ejected dust shell is focused by the Roche potential into the plane of the binary system (Shagatova et al. 2021). If we assume that the wind velocity prior to dust formation was close to that estimated by DUSTY prior to the onset of dust formation (E1; 2010-11) for a steady state wind (Table 9), the ≈ 4× lower mass loss 1998 2019 2020-2021 Figure 15. Proposed model for the enhanced mass loss and X-ray activity in SRGA J181414.6-225604. We show depictions of the binary system for three epochs, as inferred from our modeling. The left configuration shows the expected state of the binary near the epoch of 2MASS, consisting of an unobscured late-type giant with a temperature Ts, radius Rs and mass loss rateṀ6 (in units of 10 −6 M yr −1 ). The donor star fits well inside its Roche lobe (shown by the dotted lines) as it orbits a distant compact object that has a faint X-ray accretion luminosity LX,35 (in units of 10 35 erg s −1 ) as constrained by INTEGRAL archival hard X-ray data. In the middle configuration, the compact object is marginally brighter in the X-ray around the epoch of the first ART-XC survey, as the donor star shows evidence for elevation in mass loss rate and the progressive formation of a warm dust shell with an inner radius R d and temperature T d . The shell has a thickness ratio of ≈ 5, and we only show the inner dense region here for clarity. In the right configuration, the donor undergoes a dramatic dust dimming episode in 2019-2020 as it ejects an expanding gas/dust shell that slowly expands beyond the Roche lobe, cools and reaches the compact object. The enhanced gas supply causes a ≈ 15× X-ray flux brightening on the compact object, while also enshrouding it in gas and silicate dust directly detected in the X-ray spectra.
rate would both reduce the mass transfer rate onto the NS as well as reduce the accretion efficiency due to the larger magnetospheric radius at low accretion rates (Kuranov et al. 2015). We summarize the proposed scenario for the X-ray flare in SRGA J181414.6-225604 in Figure  15.

SUMMARY
We have presented the discovery and detailed multiwavelength characterization of the Galactic hard Xray transient SRGA J181414.6-225604 discovered by the Mikhail Pavlinsky ART-XC instrument on board the SRG satellite. In summary, we show that • The X-ray transient exhibited a ≈ 100 day brightening to peak following the discovery (as observed with Swift), reaching a peak luminosity of ≈ 2.5×10 36 erg s −1 , followed by fading nearly to its quiescent (≈ 10× fainter) flux level. In addition, the source is observed to exhibit short timescale 'colorless' flares changing by ≈ 5× in flux over timescales of 1 hour.
• Modeling of the broadband X-ray spectroscopy from NICER and NuSTAR reveals a highly absorbed spectrum with emission extending out to 30 keV. The properties are consistent with Comptonization of low energy seed photons by a hot plasma around a NS or BH accretor. The source spectrum becomes harder as it brightens during the X-ray flare.
• We do not find any evidence for pulsations or bursts that would confidently suggest the presence of a NS accretor. We detect the presence of a low energy excess exhibiting likely emission lines of Mg and Si, which we model as an optically thin collisionally ionized plasma with a temperature of ≈ 0.6 keV.
• Imprinted on the broadband X-ray spectrum, we detect narrow and variable Fe Kα emission likely arising from X-ray fluorescence of relatively distant material. We also identify an excess of Si K-edge absorption, which we interpret as an overabundance of silicate dust around the system.
• Spatially coincident with the X-ray transient, we identify a bright and heavily obscured IR source, IRAS 18111-2257, which we demonstrate to show correlated temporal variability using data from the Palomar Gattini-IR survey.
• With optical/IR spectroscopy of the counterpart, we classify the star to be a very late-type (M7-M8), pulsating (P ≈ 1500 days) and luminous (M K ≈ −10) Mira variable on the AGB, located at a distance of ≈ 15 kpc. The X-ray properties and the IR identification suggest that SRGA J181414.6-225604 is a new Galactic symbiotic X-ray binary.
• Collating optical/IR photometric data from the last ≈ 25 years, we show that the IR source has undergone dramatic reddening likely associated with enhanced mass loss and dust production. In particular, the PGIR J-band data conclusively show evidence of an intense fading episode of the counterpart in 2019, ≈ 800 days prior to the X-ray flare.
• We model the multi-epoch optical to mid-IR SEDs of the source with DUSTY to show that the dramatic fading observed in the NIR can be explained by an intense dust obscuration episode of the donor star, characterized by the formation of large dust grains (reaching sizes of a ≈ 2.0 µm) and increased mass loss (reachingṀ ≈ 2.1 × 10 −5 M yr −1 ).
• The temporal coincidence of the enhanced mass loss with the X-ray flare suggests a causal connection, and we show that the observed timescales and peak-luminosity of the X-ray outburst is consistent with a wide-orbit (∼ 50 AU) compact object undergoing spherical Bondi accretion from the enhanced mass loss episode.
While the known SyXRBs have been suggested to be either undergoing Roche lobe overflow (GX 1+4) or steady wind-accretion (e.g. 4U 1954+31, 4U 1700+24), SRGA J181414.6-225604 presents a unique case of a system where the orbit is wide enough such that the steady wind-accretion results in only very faint X-ray emission.
The intense mass loss episode associated with dust formation episode makes it conspicuous in the X-ray sky, and presents the first unambiguous identification of the long-speculated connection between donor mass loss and X-ray activity in SyXRBs. Interestingly, the carbon star companion to the SyXRB CXOGBS J173620.2-293338 also shows evidence for dust obscuration episodes in its light curve (Hynes et al. 2014); however there are no constraints on the long term X-ray behavior of the source. SRGA J181414.6-225604 demonstrates the unique potential of combining long-term infrared and X-ray time domain surveys in revealing a holistic picture of X-ray outbursts in SyXRBs, which are now being revealed as the dominant contribution to the population of faint Xray transients in the Milky Way (Bahramian et al. 2021).
Similar to SRGA J181414.6-225604, there is likely a large population of quiescent NSs/BHs with wide-orbit companions that are otherwise unremarkable. As the donor stars evolve to later stages, X-ray activity caused by enhanced mass loss and dust formation may thus be ubiquitous in wide-orbit SyXRBs. In fact, sporadic dust formation episodes are rare among O-rich evolved stars (Bedding et al. 2002), but are known among many symbiotic O-rich Miras (Munari 1988;Munari & Whitelock 1989;Jurkic & Kotnik-Karuza 2010b). Given the typical large dust grain sizes formed during this period, NIR surveys are ideally suited to detect the dramatic fading episodes. Thus, combining deep hard X-ray surveys like ART-XC with long-term optical/NIR monitoring from deep surveys like WINTER (Lourie et al. 2020) and the Rubin observatory (Ivezić et al. 2019), it may soon be possible to 'predict' the onset of accretion outbursts of all Galactic symbiotics by searching for dust forming bright IR sources coincident with X-ray counterparts.  (Ivezic & Elitzur 1997), HEAsoft (v6.29c;HEASARC 2014) Rogantini, P. Pradhan, R. Remillard and M Guenther for valuable discussions. We thank the Swift, NICER and NuSTAR teams for approving the Target of Opportunity observations of the transient. Support for this work was provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51477.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.
PGIR is generously funded by Caltech, Australian National University, the Mt Cuba Foundation, the Heising-Simons Foundation, the Bi-national Science Foundation. PGIR is a collaborative project among Caltech, Australian National University, University of New South Wales, Columbia University, and the Weizmann Institute of Science. This work is based on data from Mikhail Pavlinsky ART-XC X-ray telescope aboard the SRG observatory. The SRG observatory was built by Roskosmos in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI) in the framework of the Russian Federal Space Program, with the participation of the Deutsches Zentrum für Luft-und Raumfahrt (DLR). The SRG spacecraft was designed, built, launched and is operated by the Lavochkin Association and its subcontractors. The science data are downlinked via the Deep Space Network Antennae in Bear Lakes, Ussurijsk, and Baykonur, funded by Roskosmos. The ART-XC team thank the Russian Space Agency, Russian Academy of Sciences and State Corporation Rosatom for the support of the SRG project and ART-XC telescope and the Lavochkin Association (NPOL) with partners for the creation and operation of the SRG spacecraft (Navigator). This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. We acknowledge the Visiting Astronomer Facility at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.
Work of IM, AL and AS was supported by the Russian Science Foundation grant 19-12-00423. RS acknowledges grant number 12073029 from the National Science Foundation of China. CP acknowledges financial support from the Swiss National Science Foundation (SNF). the bias correction. The complete WISE light curve of the source after correction of the aforementioned instrumental effects is given in Table 2.