Chandra X-Ray Observatory Observations of 13 Fermi LAT Sources

3FGL J1306.4–6043 (which later became 4FGL J1306.3–6043) has been associated with the radio pulsar PSR J1306−6043 discovered in the MPIfR-MeerKAT Galactic Plane Survey (MMGPS; Padmanabh et al. 2023), which has also been detected in γ-rays from the 3PC (Smith et al. 2023). It is an MSP with a spin period of 5.67 ms, which lives in a potential MSP helium white dwarf system with an orbital period of ≈ 86 d (Padmanabh et al. 2023). No X-ray source is seen in the ACIS-I image within $10^{\prime\prime}$ from the position of the radio pulsar.

Within the 95% PU of the LAT source, only a single source, CXO J130621.8–604328 (A9), is detected in the ACIS image at ≈3σ significance (see panel 1 in Figure 1). Because A9 is only ∼23'' away from PSR J1306–6043, it might be associated with the MSP if the (unpublished) uncertainty of the pulsar position is large enough. This X-ray source likely has an optical, NIR, or IR counterpart (Gaia DR3 6055821050370180608, 2MASS 13062177-6043281, and CatWISE2020 J130621.76-604328.1), which we refer to as A9-1 when considering it as an association. The MUWCLASS pipeline has classified A9 as an NS with a classification confidence threshold (hereafter CT ¹⁶ ) = 1.1 without the optical-IR counterpart, while A9-1 is classified as a YSO with CT = 2.2.

Given that the positional offset of the Gaia source (012) is much smaller than the PU of the X-ray source (11), we estimate the association probability to be high (P_i = 83%, based on matching to the Gaia DR3 catalog), and hence, the latter classification is more likely.

In the 4FGL-DR4 catalog, the observed 3FGL J1906.6+0720 source is resolved into two distinct γ-ray sources: 4FGL J1906.4+0723 and 4FGL J1906.9+0712. The former, 4FGL J1906.4+0723, is associated with the γ-ray pulsar PSR J1906+0722, whose pulsations were found by the 3PC (Clark et al. 2015; Smith et al. 2023). Observations with the Swift XRT provided an upper limit for the unabsorbed X-ray (0.5–10 keV) flux of 2 × 10⁻¹³ erg cm⁻² s⁻¹ (Clark et al. 2015).

There is no compelling evidence that PSR J1906+0722 is associated with any of the four known SNRs located within 1° of the pulsar timing position. The closest SNR, G41.1–0.3, is likely a Type Ia SNR (Clark et al. 2015). This makes it an unlikely birthplace for a pulsar. Due to their proximity, the association between the G41.1–0.3 and the second 4FGL source, 4FGL J1906.9+0712 has a much higher probability than the associations with G41.1–0.3 and 4FGL J1906.4+0723.

Although wavdetect did not report any significant (S/N > 3) X-ray sources within the 95% PU ellipse of 4FGL J1906.4+0723, a faint and possibly slightly extended emission is discernible at the pulsar location determined from the γ-ray timing (see panel 2 in Figure 1). Because the observed X-ray excess is precisely coincident with the accurately known γ-ray timing position, we consider it unlikely to be a noise fluctuation. The somewhat extended nature of the emission hints at the possibility that this is a PWN. Running srcflux at this position yields a flux of $({3.4}_{-1.4}^{+1.9})\times {10}^{-14}$ erg cm⁻² s⁻¹ within the 90% encircled PSF aperture (r = 27).

We did not detect any significant (≥3σ) X-ray sources in the ACIS-I image within the 4FGL J1906.9+0712 PU.

4FGL J1104.9–6037 has been associated with the γ-ray pulsar PSR J1105–6037, identified by the 3PC (Clark et al. 2017; Smith et al. 2023). A potential X-ray counterpart, 1SXPS J110500.3–603713, was reported in Wu et al. (2018), with an observed flux of ${2.5}_{-1.1}^{+1.5}\times {10}^{-13}$ erg cm⁻² s⁻¹ in the 0.3–10 keV, while the LSXPS has only provided a 3σ upper limit on the observed flux of ∼2 × 10⁻¹³ erg cm⁻² s⁻¹.

In the ACIS-I image, a faint source (∼1.7σ), CXO J110500.5–603715 (C6), coincides with the position of PSR J1105–6037 (see panel 3 in Figure 1). The source does not pass the significance threshold and lies near the ACIS-I chip gap. ¹⁷ Because the observed X-ray excess is precisely coincident with the accurately known γ-ray timing position, we consider it unlikely that this is a noise fluctuation. The observed broadband flux of ∼2.5 × 10⁻¹⁵ erg cm⁻² s⁻¹ (in 0.5–7 keV) primarily comes from the medium band (1.2–2.0 keV). The flux difference between the CXO and Swift observations suggests that the X-ray source might be variable, which is unexpected for an isolated pulsar. Alternatively, the flux measurement in Wu et al. (2018) may be inaccurate, provided that LSXPS only gives an upper limit. The difference may also be explained by the substantial absorption and the contamination of the ACIS detector at low energies and/or the fact that the source lies near the chip gap. No optical-IR counterpart is found within the 12 PU of the X-ray source. Without any matching counterpart, the X-ray source is classified as an NS with CT = 1.7.

4FGL J1736.1–3422 has been associated with the γ-ray pulsar PSR J1736–3422, identified by the 3PC (Smith et al. 2023). No X-ray source is seen in the ACIS-I image within $10^{\prime\prime}$ from the position of the γ-ray pulsar.

Only one significant X-ray source, CXO J173609.9–342419 (D7), is detected (with S/N = 3.8) within the 95% PU of J1736.1–3422 (see panel 4 in Figure 1). This source has an association (D7-1) with an optical source, Gaia DR3 4053560004787094528, but with a fairly low association probability, P_i = 0.45, given the X-ray PU of 14 and the high density of Gaia sources in this field. D7 is classified as an LMXB without the optical counterpart, albeit with a very low confidence (CT = 0.7). With the optical counterpart, D7-1 is very confidently classified as a CV (with CT = 10.4).

Running srcflux at the position of PSR J1736–3422 yields an upper limit (for the observed/absorbed flux) of 6.3 × 10⁻¹⁵ erg cm⁻² s⁻¹ at the 95% confidence level.

The Giant Metrewave Radio Telescope (GMRT) study by Frail et al. (2016) revealed a bright (174.9 ± 16.3 mJy at 150 MHz) and compact radio source, TGSSADR J163923.8–514634 (with R.A. = 16:39:23.83(0.03) and decl. = –51:46:34.1(2.8), which is also listed in the GMRT all-sky survey TGSS ADR; Intema et al. 2017), within the LAT source PU. A potential optical counterpart with a B-band magnitude of 18.8, cataloged in the USNO-B1.0 survey (Monet et al. 2003), is located 21 from the TGSSADR source. The radio source landed within the chip gap. No statistically significant X-ray sources are detected in the ACIS-I image within the 95% PU of 4FGL J1639.3–5146. We note that an X-ray source, CXO J163932.2–514620 (E8), was detected outside the PU of 4FGL J1639.3–5146, but inside the PU of 3FGL J1639.4–5146. This X-ray source has no associations at lower frequencies. It is classified as an LMXB (with CT = 2.3).

4FGL J1203.9–6242 has been associated with the γ-ray pulsar PSR J1203–6242, discovered by the 3PC (Smith et al. 2023). The pulsar is not significantly detected by wavdetect at the preliminary localization from the 4FGL-DR4 catalog within an assumed PU of $10^{\prime\prime}$, with only a slight hint of enhanced emission seen in the ACIS-I image. Running srcflux at the position of PSR J1203–6242 yields an upper limit for the flux of 1.0 × 10⁻¹⁴ erg cm⁻² s⁻¹ at the 95% confidence level within the 90% encircled PSF aperture.

Only one X-ray source, CXO J120355.5–624231 (F2), is significantly (S/N = 5.1) detected within the 95% γ-ray PU (see panel 5 in Figure 1). Because F2 is only ∼13'' away from PSR J1203–6242, it might be associated with PSR J1203–6242 if the (unpublished) uncertainty of the pulsar position is large enough. This X-ray source has a potential association (F2-1) with an optical source, Gaia DR3 6057424069589180032. Without the optical association, MUWCLASS classifies F2 as an NS with low confidence (CT = 1.3). With the association, F2-1 is classified as a CV with high confidence (CT = 8.5). Given that the positional offset of the Gaia source (012) is much smaller than the PU of the X-ray source (08), we estimate the association probability, P_i, to be high (81%), and hence, the latter classification is more likely. A CV is unlikely to contribute to the γ-ray emission from this region.

Using an ML approach, 3FGL J0854.8–4503 (which later became 4FGL J0854.8–4504) has been classified as a young pulsar based solely on its γ-ray properties (Saz Parkinson et al. 2016). However, no γ-ray or radio pulsations were reported from this region. LSXPS detected three marginal X-ray sources, LSXPS J085451.1-450418, LSXPS J085448.8-450524, and LSXPS J085456.8-450348, within the γ-ray PU.

Only one X-ray source, CXO J085457.4–450350 (G1), is significantly detected (S/N = 3.8) in the ACIS-I image within the 95% PU of 4FGL J0854.8–4504. The X-ray source also coincides with LSXPS J085456.8-450348 with the observed fluxes 2.6 ± 1.2 × 10⁻¹⁴ erg cm⁻² s⁻¹ (from CXO) and ${3.7}_{-1.7}^{+1.8}\times {10}^{-13}$ erg cm⁻² s⁻¹ (from Swift XRT), suggesting that the source is variable. It is located near the edge of the ellipse and has an association (G1-1) with an optical (and IR) counterpart, Gaia DR3 5331365758050152704 (2MASS 08545736-4503504 and CatWISE2020 J085457.36-450350.4). Without the optical-IR association, G1 is classified as an NS with fairly high confidence (CT = 4.8). However, when the optical-IR association is included, G1-1 is classified as an LM-STAR with CT = 2.3. Given that the positional offset of the Gaia source (007) is much smaller than the PU of the X-ray source (097), we estimate the association probability to be high (96%), and hence, the LM-STAR classification is more likely. However, at the distance of 2100 ± 100 pc (Bailer-Jones et al. 2021), the X-ray luminosity would be in the (1–30) × 10³¹ erg s⁻¹ range (corresponding to the CXO and Swift flux measurements), which is unusually high for a solitary LM-STAR outside a major coronal flare. We do not see any evidence of such a flare during the CXO observation. LSXPS does not detect a significant variability in this source either. Moreover, given that the stellar spectral type is ∼A (this is within our LM-STAR class definition) according to Gaia DR3, significant coronal activity is not expected (see, however, Chen et al. 2023, who reported several X-ray bright A stars).

We note that 4FGL J0854.8–4504 is located within the Vela SNR extent (r ≈ 4°), and it could be that some part of the SNR shell is detected by LAT.

4FGL J2035.0+3632 has been associated with the γ-ray pulsar PSR J2034+3632, discovered by the 3PC (Smith et al. 2023). This is an MSP with a rather low spin-down power $\dot{E}\approx {10}^{33}$ erg s⁻¹. Hence, its nondetection in X-rays is not surprising, especially if the distance to the pulsar is substantial (it cannot be reliably determined from γ-rays alone). No X-ray sources were detected by wavdetect above the 3σ significance threshold within the 95% PU ellipse of 4FGL J2035.0+3632. Running srcflux at the position of PSR J2034+3632 yields an upper limit for the observed (absorbed) flux of 3.9 × 10⁻¹⁵ erg cm⁻² s⁻¹ at the 95% confidence level.

3FGL J2041.1+4736 (which later became 4FGL J2041.1+4736) has been classified as a young pulsar based solely on its γ-ray properties, using an ML approach (Saz Parkinson et al. 2016).

Only one X-ray source, CXO J204116.7+473658 (I2), is significantly detected (≈6σ) in the ACIS-I image within the 95% PU ellipse of 4FGL J2041.1+4736 (see panel 7 in Figure 1). The X-ray source has an association (I2-1) with an IR counterpart, CatWISE2020 J204116.56+473659.8, with a relatively high P_i = 0.82. Without the association, I2 is classified as an NS with low confidence (CT = 1.1). If the IR association is included, the classification changes to a confident AGN (CT = 8.8). The detection of a radio source, MSC J204116.66+473659.2, with a flux of 0.65 ± 0.12 mJy at 5.9 GHz and spectral index α = 0.35 ± 0.25 (Bruzewski et al. 2023), coincident with the X-ray source, supports the AGN classification for CXO J204116.7+473658. The identification of the only X-ray counterpart as an AGN suggests that the 4FGL source may be an AGN.

No X-ray source is detected in the ACIS-I image for 4FGL J1317.5–6316 above the wavdetect 3σ threshold within its 95% PU ellipse.

3FGL J1329.8–6109 (which later became 4FGL J1329.9–6108) was classified as an MSP using an ML-based classification based only on its γ-ray properties (Saz Parkinson et al. 2016). The source has been observed by Swift XRT, and was associated with SwF3 J132939.6–610735, detected within the 95% PU of 3FGL J1329.8–6109 (Kaur et al. 2019; Kerby et al. 2021), but outside the 95% PU of 4FGL J1329.9–6108. SwF3 J132939.6–610735 was classified as a pulsar with a probability of 0.94 based on its X-ray and γ-ray properties using an ML approach (Kaur et al. 2019). However, the detection of SwF3 J132939.6–610735 is likely severely affected by optical loading ¹⁸ (Hare et al. 2021; Kerby et al. 2021) from the nearby (offset by 10''), bright (G = 9 mag) optical star, Gaia DR3 5868669568445358336, given the very soft X-ray spectrum of the XRT source with a high flux. The origin of the 10'' between the star and SwF3 J132939.6–610735 is unclear because the coordinates and the uncertainties of the XRT source position are not listed in Kaur et al. (2019).

We note that LSXPS does not detect SwF3 J132939.6–610735, but does detect an X-ray source, LSXPS J132938.8–610745, which is ≈10'' offset from SwF3 J132939.6–610735. The origin of this offset is unclear, but LSXPS J132938.8–610745 lies only 18 offset from the bright star, suggesting that its X-ray properties are impacted by optical loading. LSXPS J132938.8–610745 has an observed (absorbed) flux of ${1.2}_{-0.2}^{+0.3}\times {10}^{-13}$ erg cm⁻² s⁻¹ in the 0.3–10 keV energy band. It is coincident with one of the X-ray sources, CXO J132939.1–610744 (K4), detected in our CXO observations, which has a consistent flux of ∼1.5 × 10⁻¹³ erg cm⁻² s⁻¹ in the 0.5–7 keV energy band. The positions of both of these X-ray sources are compatible with the bright star. Because the ACIS-I spectrum should not be affected by the optical loading, ¹⁹ we conclude that X-rays must be coming from the star and likely have a coronal origin, given the soft spectrum (see Table 3) and the relatively low luminosity of 1.3 × 10²⁹ erg s⁻¹ at the Gaia-inferred distance of 84 pc (Bailer-Jones et al. 2021).

No X-ray sources were detected by wavdetect above the 3σ threshold in the ACIS-I image within the 95% error ellipse of 4FGL J1329.9–6108. Six X-ray sources were detected within the PU of 3FGL J1329.8–6109, but outside the PU of 4FGL J1329.9–6108, which we discuss in Section 4.3 and Table 3.

4FGL J0744.0–2525 has been associated with the γ-ray pulsar PSR J0744–2525, discovered in LAT data by the 3PC (Smith et al. 2023). This pulsar is old, with a characteristic age (τ_c) of about 1.6 Myr, and it has a fairly low $\dot{E}=4.7\times {10}^{34}$ erg s⁻¹. No X-ray sources were detected by wavdetect above the 3σ significance threshold within the 95% PU ellipse of 4FGL J0744.0–2525. Because the pulsar is old, its nondetection in X-rays is not surprising. There are no photons in the 0.5–7 keV energy band within the 3'' radius circle around the position of PSR J0744–2525. We have used a circular annulus with a radius of 30'' centered on PSR J0744–2525 to measure the local background and obtained a mean background surface brightness of 0.005 ± 0.001 counts arcsec⁻². Thus, within the r = 1'' extraction aperture (corresponding to ≈90% encircled PSF at 1.5 keV), the background is very small (≈0.016 counts). For a Poissonian distribution, the nondetection with zero counts translates into an upper limit $N\lt -\mathrm{ln}(1-\mathrm{CL})$ counts at the confidence level CL, which gives the 95% count rate upper limit of 3 × 10⁻⁴ counts s⁻¹. For an absorbed PL with Γ = 2 and N_H = 10²² cm⁻², this corresponds to an absorbed flux of 6 × 10⁻¹⁵ erg s⁻¹ cm⁻².

4FGL J1358.3–6026 has been associated with the γ-ray pulsar PSR J1358–6025, discovered in the LAT data by the 3PC (Smith et al. 2023). Although wavdetect does not detect the pulsar counterpart in the ACIS-I image above the 3σ threshold, extended X-ray emission is seen in the ACIS-I image, with its brightest part covering the γ-ray pulsar position. We find that this brightest part is detected by wavdetect as CXO J135825.9–602555 (M8) with a lower significance of 2.7σ. We note that the wavdetect search is more sensitive for point sources than for extended sources. We describe the properties of this extended source in more detail below. CXO J135825.9–602555 is classified as an NS with CT = 2.2, and it lacks any lower-frequency counterparts.

Two other X-ray sources, CXO J135834.6–602800 (M4) and CXO J135817.2–602504 (M6), have been detected by wavdetect above the 3σ significance threshold in the ACIS-I image within the 95% PU ellipse of the 4FGL source (see panel 8 in Figure 1). Both sources are situated near the periphery of the γ-ray error ellipse and are tentatively classified as AGNs without any matching counterparts, albeit with very low confidence (CT = 0.3 and CT = 0.2).