An ASKAP Search for a Radio Counterpart to the First High-significance Neutron Star–Black Hole Merger LIGO/Virgo S190814bv

The 89 variable sources were filtered to remove those that were not consistent with the predicted emission of S190814bv, which should not exhibit more than a single rise and decline on these timescales (Hotokezaka et al. 2016), according to the following criteria:

• 1.  
  Sources that showed a decline between epochs 1 and 2, followed by a rise between epochs 2 and 3. 41 sources were excluded.
• 2.  
  Sources detected in RACS epoch 0 where epochs 1 and 2 had lower integrated flux values than epoch 0.3 sources were excluded.

We then searched the GLADE catalog (GLADE; Dálya et al. 2018) for galaxies in the localization volume within 20'' (or ∼20 kpc at the estimated distance of S190814bv LIGO Scientific Collaboration & Virgo Collaboration et al. 2019b) of a variable source. We found one candidate (ASKAP J005547.4–270433) that is near 2dFGRS TGS211Z177, a cataloged galaxy with z = 0.0738 (Colless et al. 2001). This source was the only strong candidate after epoch 2 and prior to the acquisition of epoch 3 we performed multiwavelength follow-up which we discuss in Section 4. We excluded two candidates that matched with a GLADE galaxy >3σ beyond the estimated distance to S190814bv (267 ± 52 Mpc LIGO Scientific Collaboration & Virgo Collaboration et al. 2019b).

We cross-matched the 42 remaining variable candidates with the Photometric Redshifts for the Legacy Surveys (PRLS) catalog (R. Zhou et al. 2019, in preparation), which is based on Data Release 8 of DESI Legacy Imaging Surveys (Dey et al. 2019). We excluded 22 variable sources that had all optical matches at distances differing by >3σ from the estimated distance to S190814bv. This left seven sources with at least one cross-match within the localization volume and 13 sources with no reliable distance estimate (see Table 2).

We carried out follow-up observations of ASKAP J005547.4–270433 (hereafter AT2019osy) with the ATCA (C3278, PI: Dobie) using two 2 GHz bands centered on 5.5 and 9 GHz at 14, 22, and 34 days post-merger. We reduced the data using the same method as Dobie et al. (2018) using PKS B1934−638 and B0118−272 as flux and phase calibrators respectively.

We also carried out VLA observations (VLA 18B-320, PI: Frail) on 2019 August 28 and September 9. Standard 8 bit WIDAR correlator setups were used for L and S bands, and 3 bit setups for C and X bands to obtain a contiguous frequency coverage between 1 and 12 GHz. 3C48 and J0118–2141 were used as the flux and phase calibrators respectively. The data were processed using the NRAO CASA pipeline and imaged using the clean task in CASA.

A summary of our observations is given in Table 3. We find a constant flux density offset²¹ of ∼40% between the initial ATCA and VLA observations across all epochs. We therefore find no evidence for radio variability beyond the initial rise observed with ASKAP.

We conducted optical imaging of AT2019osy with the Dark Energy Camera (DECam, Flaugher et al. 2015) on the 4 m Blanco telescope under NOAO program ID 2019B-0372 (PI: Soares-Santos). Images including the location of AT2019osy were taken in i and z bands nightly from 2019 August 15 to 18 and on 2019 August 21 (UT) and reduced in real-time (Goldstein et al. 2019). A detailed offline analysis of the subtraction images zooming in on the location around AT2019osy, reveals no robust point source at this location to a depth of i > 21.2 mag and z > 20.0 mag on UT 2019 August 15 (the night of the merger) increasing linearly in limiting magnitude to i > 23.5 mag and z > 23.5 mag on UT 2019 August 21 (consistent with independent analysis by Herner et al. 2019). We also analyzed the DECam images using The Tractor image modeling software (Lang et al. 2016) and found that a model with an exponential galaxy profile with a point source at the galaxy nucleus is required to fit the data, both before and after S190814bv. This suggests that there is no optical transient temporally coincident with S190814bv but possibly some underlying nuclear variability.

On 2019 August 22 UT, we observed AT2019osy in the near-infrared using the Wide-field Infrared Camera (WIRC; Wilson et al. 2003) with the 200 inch Hale telescope at Palomar Observatory for a total of 10 minute exposure time (De et al. 2019b). The WIRC data were reduced and stacked using a custom pipeline (De et al. 2019a). No counterpart to AT2019osy was detected down to an AB limiting magnitude of J > 21.5 (5σ).

We also obtained a spectrum of the host galaxy of AT2019osy using the Double Beam Spectrograph (Oke & Gunn 1982) on the Palomar 200 inch Hale Telescope (P200), which we reduced using pyraf-dbsp (Bellm & Sesar 2016). The spectrum is dominated by red continuum that is likely primarily associated with the host galaxy; no obvious broad features are evident. We identify several narrow emission lines (Hα; [N ii]λλ6548,6583, [S ii]λλ6716,6731, and marginal [O ii]λ3727) at a common redshift of 0.0733, consistent within 2σ of the LVC distance constraint. Hβ and [O iii]λ5007 are not detected in the spectrum. We measure a flux ratio of log[N iiλ6583/Hα] = 0.2, indicating at least partial contribution by an active galactic nucleus (AGN; Kauffmann et al. 2003).

We observed the field of AT2019osy, starting at 2019 September 23 10:30:48 UT for 20 ks with the Chandra ACIS-S instrument (S3 chip) and very faint data mode. The data were analyzed with CIAO (v 4.11; Fruscione et al. 2006) and calibration was carried out with CALDBv4.8.4.1. We reprocessed the primary and secondary data using the repro script and created X-ray images for the 0.3–8 keV range. No sources were visible near AT2019osy (verified with both wavdetect and celldetect), with a maximum count rate of $2.85\times {10}^{-4}\,{{\rm{s}}}^{-1}$. Assuming a neutral hydrogen column density ${N}_{{\rm{H}}}=1.8\times {10}^{20}\,{\mathrm{cm}}^{-2}$ and a power-law model with index n = 1.66 (corresponding to the observed radio spectral index of −0.4), this count rate yields a 0.3–8 keV unabsorbed flux upper limit of $3.2\times {10}^{-15}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$ (as reported in Jaodand et al. 2019) or an unabsorbed luminosity of $4.2\times {10}^{40}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$.

AT2019osy exhibits no significant radio variability beyond the initial rise and there is no evidence for a coincident optical transient. The coincident galaxy is edge-on, likely with significant dust obscuration toward the nucleus, and therefore the optical spectrum is consistent with an AGN within a star-forming galaxy. The inferred radio and X-ray luminosity of AT2019osy along with the small offset from the optical centroid of 2dFGRS TGS211Z177 suggests that the source is a variable low-luminosity AGN (Ballo et al. 2012) and unrelated to S190814bv.

We find 21 candidate counterparts to S190814bv above our detection threshold of 170 μJy, corresponding to ≲1% of observed sources. This is consistent with the expected rate of AGN variability from Radcliffe et al. (2019), who find ∼2% of μJy-level sources exhibit significant variability likely attributable to the presence of an AGN. Additionally, the expected level of compact source variability caused by refractive interstellar scintillation along this line of sight is ∼35% (Cordes & Lazio 2002), comparable to V_int for all but three sources which we discuss below.

We classify ASKAP J005434.6−280235 as a variable AGN based on follow-up observations (De et al. 2019b; Dobie et al. 2019). ASKAP J010258.6–265119 is coincident centrally between two large radio lobes and hence likely associated with core emission from a radio galaxy. ASKAP J010534.6–231604 is coincident (<1'') with WISE J010534.64–231605.5 (Cutri et al. 2012), which is likely a variable AGN at a distance of z ∼ 1 (Glowacki et al. 2017).

While we cannot conclusively rule out the sources in Table 2 as counterparts to S190814bv, they are likely AGN exhibiting a combination of intrinsic and extrinsic variability. Of course, at most one candidate can be the actual counterpart, and there is nothing yet to distinguish any of these from the others. Further observations on timescales of months to years will reveal their nature.

Our follow-up of S190814bv is the most sensitive widefield radio transients search to-date, approximately an order of magnitude more sensitive compared to previous searches with comparable areal coverage (Hobbs et al. 2016) and approximately an order of magnitude more areal coverage than previous searches at comparable sensitivities (Mooley et al. 2013).

We have found four transient candidates (i.e., sources with a prior constraining nondetection) in total; the three sources discussed in Section 5.1 and ASKAP J005104.2–230852, which was ruled out as a candidate to S190814bv based on the redshift of nearby optical sources. This source is coincident (< 06) with WISE J005104.13-230851.8, which is likely a variable AGN.

We therefore place an upper limit on the 943 MHz radio transients surface density of 0.05 deg⁻² for sources above 170 μJy at 95% confidence.

Predicted radio lightcurves from NSBH mergers span a large range of flux densities and timescales (e.g., Piran et al. 2013; Lamb & Kobayashi 2016; Bhattacharya et al. 2019). If the radio emission is dominated by the outflowing dynamical ejecta the lightcurve will peak on timescales of years, whereas if the emission is jet-dominated the lightcurve will peak at comparably lower flux densities on timescales of days to months (Hotokezaka et al. 2016). In each of these scenarios the lightcurve is also dependent on the merger energetics, circum-merger density and inclination angle, each of which can change both the peak time and flux density by an order of magnitude. The merger energetics are determined by the mass ratio, the spin of the black hole (both of which are calculable from gravitational wave strain data that is yet to be released), and the unknown neutron star equation of state (Kyutoku et al. 2011; Foucart 2012).

We place a 5σ upper limit on the 943 MHz radio emission from S190814bv of 170 μJy at ΔT = 2, 9, and 33 days post-merger. We compute off-axis afterglow lightcurves based on a top-hat jet model (see Hotokezaka & Piran 2015, for details) assuming an isotropic equivalent energy E_iso = 10⁵¹ erg (typical of short GRB afterglows; Fong et al. 2015), an initial jet opening angle of θ_j = 10° and microphysics parameters _e = 0.1, _B = 0.01, and p = 2.2. By comparing these lightcurves to the observed upper limits we can constrain the merger inclination angle, θ_obs, and circum-merger density, n. Figure 2 shows these constraints, assuming that the merger occurred within the 89% of the localization region covered by our observations. We can rule out the part of the parameter space typically occupied by short GRBs, assuming that their inclination angle is smaller than the opening of the angle of the jet (Fong et al. 2015). Under a more conservative assumption of the isotropic equivalent energy (E_iso = 10⁵⁰ erg) we can only rule out a small part of the parameter space around θ_obs = 10° and n = 1 cm⁻³.

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In comparison, if we scale the nonthermal lightcurve of GW170817 to 943 MHz based on a spectral index of α = − 0.575 (Mooley et al. 2018; Hajela et al. 2019) and place it at a distance comparable to S190814bv, we find a peak flux density of ∼5 μJy, well below our detection threshold. We note that the nonthermal emission from GW170817 did not peak until ∼150 days post-merger (Dobie et al. 2018). Further observations on timescales of months to years post-merger will enable us to place tighter constraints on the circum-merger density and inclination angle, which may be useful in improving the gravitational wave localization (Corley et al. 2019).