Chandra X-Ray Observatory Observations of 13 Fermi LAT Sources

In the latest data release from the Fermi Gamma-ray Space Telescope (the 4th Fermi LAT 14 yr Catalog, or 4FGL), more than 50% of the Galactic sources are yet to be identified. We observed 13 unidentified Fermi LAT sources with the Chandra X-Ray Observatory to explore their nature. We report the results of the classification of X-ray sources in the fields of these γ-ray sources and discuss the implications for their nature. We use multiwavelength (MW) data for a machine-learning classification, accompanied by a more detailed spectral/variability analysis for brighter sources. Eight 4FGL sources have γ-ray pulsars within their position error ellipses. We consider three of these pulsars (PSR J1906+0722, PSR J1105–6037, and PSR J1358–6025) to be detected in X-rays, while PSR J1203–6242 shows a hint of X-ray emission. Within the positional uncertainties of three of the 4FGL sources, we detect X-ray sources that may be yet unknown pulsars, depending on the MW association. In addition to point sources, we discovered two extended sources, one of which is likely to be a bow-shock pulsar-wind nebula associated with PSR J1358–6025. Finally, we classify other X-ray sources detected in these observations and report the most interesting classifications.


INTRODUCTION
During the past decade, observations with GeV γ-ray observatories, such as the Large Area Telescope (LAT) on board the NASA Fermi Gamma-ray Space Telescope (Fermi ), have revealed a large number of very high energy sources in the Galactic plane (Acero et al. 2015a).Confidently identified sources such as pulsar-wind nebulae (PWNe), supernova remnants (SNRs), active galactic nuclei (AGNs), and X-ray binaries (XRBs) make up only ≈38% of the Galactic (|b| < 5 • ) GeV γ-ray sources in the 4th Fermi LAT 14 yr Point Source Catalog (4FGL-DR4, hereafter 4FGL; Ballet et al. 2023).One of the most commonly identified types of Galactic sources are pulsars.Indeed, there has been a dramatic increase in the number of detected γ-ray pulsars since the launch of the Fermi/LAT.In the EGRET rangelov@txstate.eduera, only six γ-ray pulsars had been discovered, while since the Fermi LAT launch, the number of known γray pulsars 1 has increased to 297 (as of 2023 December 2).While many pulsars are identified through their γray and radio pulsations, imaging X-ray observations can reveal young pulsar candidates either through their spectral properties and the absence of multiwavelength (MW) associations, or by resolving extended emission from PWNe.In addition to young pulsars, GeV γ-ray sources can be associated with pulsars orbiting highmass stars (high-mass γ-ray binaries, or HMGBs) or with old recycled pulsars orbiting low-mass stars (redback and black widow systems).Finally, isolated recycled pulsars, which were either kicked out of the binary or were completely ablated by their binary companion, can be sources of GeV γ-rays.
In this paper, we report Chandra X-ray Observatory (CXO) observations of 13 Fermi LAT sources (listed in Table 1) that were unidentified when the CXO observations were proposed (except for PSR J1906+0722).To maximize the probability of finding pulsars, the sources were selected to be close to the Galactic plane (|b| < 3.5 • ) with a LAT detection significance over 15σ, a LAT position uncertainty δr < 2 ′ , and a γ-ray flux (100 MeV-100 GeV) G 100 > 10 −11 erg s −1 cm −2 .For the selected sources, we also required the variability index to be < 60 and the significance of the GeV spectral curvature to be > 5σ.The CXO observations and data reduction are described in Section 2, which is followed by the description of the automated MW classification method in Section 3. In Section 4.1 we discuss the Xray and MW content of the fields of γ-ray sources and possible classifications of X-rays sources located within the 95% confidence level positional uncertainties (PUs) of the 4FGL sources.The two discovered extended Xray sources are described and discussed in Section 4.2.Other interesting X-ray sources (i.e., capable of producing γ-rays in principle) are briefly discussed in Section 4.3.We present the conclusions in Section 5.

OBSERVATIONS AND DATA REDUCTION
We observed all 13 targets over two CXO programs (Table 1).At the time of the earlier program (which included three targets), only the 3rd Fermi LAT 4 yr Point Source Catalog (3FGL; Acero et al. 2015b) was available.All 13 targets were imaged on the ACIS-I array, which was operated in VFAINT mode.
The software Chandra Interactive Analysis of Observations (CIAO), version 4.15, and the Chandra Calibration Data Base (CALDB), version 4.10.4,were used to reduce and analyze the data 2 .We used the CIAO Mexican-hat wavelet source detection routine wavdetect (Freeman et al. 2002) to create source lists.Wavelets of 2, 4, 8, and 16 pixels and a detection threshold of 10 −6 were used.
We used an exposure-time-weighted average pointspread function (PSF) map and adhered to standard CIAO procedures3 to determine the source fluxes, spectral properties, and variability.Each observation was run through the CIAO tool srcflux with the coordinates obtained by wavdetect.The data were filtered in three energy bands, 0.5-1.2keV (soft), 1.2-2.0keV (medium), and 2.0-7.0 keV (hard), and were limited to the energy range between 0.5 and 7.0 keV. 4 Whenever specified, the uncertainties of the best-fit spectral parameters and ACIS-I fluxes are given at 68% level.
The absolute astrometry of the X-ray images was corrected for by aligning the X-ray sources to reference sources from the Gaia DR3 catalog (Gaia Collaboration et al. 2023), using the method similar to that used by the Chandra Source Catalog team 5 .The details of the astrometric correction are described in detail in Yang et al. (2024).

AUTOMATED MULTIWAVELENGTH CLASSIFICATION
The multiwavelength machine-learning classification (MUWCLASS) pipeline developed by Yang et al. (2022) was used to perform an automated classification of Xray sources detected in all 13 4FGL fields that are analyzed in this paper.The pipeline trains a random forest machine-learning (ML) model with a training data set (TD) of ∼ 3000 sources of literatureverified classes from the Chandra Source Catalog version 2.0 (CSCv2.0),presented in Yang et al. (2022) (with minor updates described in Yang et al. 2024), including AGNs, low-mass stars (LM-STARs), high-mass stars (HM-STARs), young stellar objects (YSOs), cataclysmic variables (CVs), high-mass XRBs (HMXBs), low-mass XRBs (LMXBs) 6 , and neutron stars (NSs).In addition to the X-ray properties (fluxes, hardness ratios, and X-ray variability measures), each source is characterized by its optical/near-infrared (NIR)/infrared (IR) properties from the Gaia DR3 (Gaia Collaboration et al. 2023), 2MASS (Skrutskie et al. 2006), AllWISE (Cutri et al. 2021), and CatWISE2020 (Marocco et al. 2021) catalogs using the probabilistic cross-matching algorithm NWAY (Salvato et al. 2018).The NWAY algorithm provides p any 7 , the probability of having a true counterpart among all considered associations, and p i, 4 Due to the increasing buildup of a contaminating layer on the ACIS detector and uncertainties in the ACIS quantum efficiency contamination model near the O-K edge at 0.535 keV and F-K edge at 0.688 keV (see https://cxc.cfa.harvard.edu/ciao/why/acisqecontamN0010.html),we used the 0.7-1.2keV energy range for detection and then converted the energy fluxes into 0.5-1.2keV using scaling factors calculated by assuming a power-law (PL) spectrum with a photon index Γ = 2.0. 5 https://cxc.cfa.harvard.edu/csc/memos/files/Martinez-Galarzastack astrometry translation spec doc.pdf 6 This class also includes nonaccreting XRBs, such as wide-orbit binaries with millisecond pulsars (MSPs), as well as redback and black widow systems. 7We kept the original notation from Salvato et al. (2018)   the probability of each (ith) counterpart (including the case of having no association at all) being the right counterpart.Based on p any and p i, we calculate the chance coincidence probability P c = 1 − p any and the association probability P i = p i × p any for each possible association.P i and P c are calculated per specific catalog, and they are indicated by a letter (G for Gaia, C for CatWISE2020, T for 2MASS, A for AllWISE), followed by the values of P i and P c in Tables 2 and 3.For X-ray sources for which no counterpart was the only possibility, P i and P c were set to 1 and 0, which are different from the probabilities of lacking any counterparts for the X-ray sources having some possible associations, for which we left P i and P c as empty.
We also implemented an oversampling method to produce synthetic sources by sampling (optical) extinction and (X-ray) absorption values from the distributions of those values for the TD sources and applying them to the less densely populated (excluding AGN) classes.This oversampling is more realistic (physical) than other algorithms (e.g., SMOTE; Chawla et al. 2011, which was used in Yang et al. 2022), and it better reproduces (due to the additional extinction and absorption) a fainter population of sources that are more consistent (in terms of fluxes) with the unidentified sources that we classify.Detailed descriptions of the updated TD, the crossmatching method, the oversampling method, and the evaluation of the performance of the updated MUW-CLASS pipeline are presented in Yang et al. (2024).
Below, we mostly consider classifications for sources with an X-ray detection significance ≥ 3 (according to wavdetect), leaving 91 sources in all 13 4FGL fields.We added only two sources that are below this threshold because they are coincident with the locations of discovered γ-ray pulsars (see Figure 1; Clark et al. 2015Clark et al. , 2017) ) and that are hence more likely to be real sources than an arbitrary subthreshold detection.For the classifications, we consider multiple possible associations when more than one counterpart matched the X-ray source within its PU (at the 95% confidence level), including the possibility of no counterpart.For the same X-ray source (uniquely identified by its CXO name), the cases of multiple associations are distinguished by the association names (A-name) given in the second column of Tables 2 and 3, where a dash indicates a specific association (the number after the dash indexes different associations), while the name without a dash corresponds to a case without a counterpart.When all possible associations are accounted for, 29 sources are left without any counterpart, 35 sources have one counterpart, 14 sources have 2 counterparts, and the remaining 15 sources have > 2 counterparts.The X-ray sources with many possible counterparts typically have large PUs (mostly because they were imaged at large off-axis angles) and are difficult to classify confidently.For this reason, we only discuss the classifications for sources with PUs<= 3 ′′ below and report them in Tables 2 and 3.However, the X-ray sources with larger PUs are still shown in Figure 2 and 3.

RESULTS AND DISCUSSION
Below, we first discuss each 4FGL source field individually, considering X-ray sources located within the PUs of the 4FGL sources and attempting to identify the nature of the GeV source.After this, we separately consider two extended X-ray sources and the most in-teresting sources lying outside of the PUs of the 4FGL sources.

Counterparts of Fermi LAT Sources
In the 13 4FGL fields, eight LAT sources have been associated with γ-ray pulsars (discovered after our CXO programs were approved) that were detected in the third Fermi LAT catalog of γ-ray pulsars (3PC; see Table 4; Clark et al. 2015Clark et al. , 2017;;Smith et al. 2023).Two of the γ-ray pulsars, PSR J1906+0722 and PSR J1105-6037, have been relatively well studied, and their positions are known very precisely (Clark et al. 2015(Clark et al. , 2017)).For the other six, only arcminute-scale localizations are available from the 3PC.However, the localizations of these six pulsars are listed in the 4FGL-DR4 catalog with a much higher accuracy (probably based on unpublished information from the 3PC group), and we use these locations for the rest of the paper, except for PSR J1306-6043, for which we use the original position from the radio observations (Padmanabh et al. 2023) .
We also checked whether X-ray sources were detected within the γ-ray PUs in the Swift X-ray Telescope (XRT) survey of unassociated Fermi LAT sources 8 (Stroh & Falcone 2013) 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 ′′ 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/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 8 https://www.swift.psu.edu/unassociated/classification confidence threshold (hereafter CT9 ) =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 (0.12 ′′ ) is much smaller than the PU of the X-ray source (1.1 ′′ ), 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.
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 +1.9 −1.4 ) × 10 −14 erg cm −2 s −1 within the 90% encircled PSF aperture (r = 2.7 ′′ ).
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 gap10 .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 1.2 ′′ PU of the X-ray source.Without any matching counterpart, the X-ray source is classified as an NS with CT=1.7.4.1.4.4FGL J1736.1-34224FGL 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 ′′ 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 1.4 ′′ 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.
Only one X-ray source, CXO J120355.5-624231(F2), is significantly (S/N= 5.1) detected within the 95% γray 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 (0.12 ′′ ) is much smaller than the PU of the X-ray source (0.8 ′′ ), 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.
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 Xray 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 +1.8  −1.7 × 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 (0.07 ′′ ) is much smaller than the PU of the X-ray source (0.97 ′′ ), 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) × 10 31 ergs 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.4.1.8.4FGL J2035.0+36324FGL J2035.0+3632 has been associated with the γray pulsar PSR J2034+3632, discovered by the 3PC (Smith et al. 2023).This is an MSP with a rather low spin-down power Ė ≈ 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.
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.
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 J132938.8-610745lies only 1. ′′ 8 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 +0.3 −0.2 × 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 loading12 , 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 × 10 29 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. 4.1.12.4FGL J0744.0-25254FGL J0744.0-2525 has been associated with the γray 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 Ė = 4.7 × 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 < − ln(1 − 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 = 10 22 cm −2 , this corresponds to an absorbed flux of 6 × 10 −15 erg s −1 cm −2 .4.1.13.4FGL J1358.3-60264FGL J1358.3-6026 has been associated with the γray 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).
One of the extended sources (identified above as CXO J135825.9-602555) is clearly associated with the 300 kyr old pulsar PSR J1358-6025.The X-ray emission near the pulsar location is brightest, but it does not look like a point source, suggesting that the PWN emission is predominantly detected, although on a larger scale, the emission is quite faint, and its overall morphology (emission is confined between the two dashed lines in the top panel of Figure 4) suggests that it may be a tail that is confined within the bow-shock PWN associated with the supersonically moving pulsar (Kargaltsev & Pavlov 2008).The spectrum extracted from the brightest part of the PWN can be fit by an absorbed PL model with a photon index Γ = 2.4 ± 0.8 and n H = 2.3 +1.5 −1.1 × 10 22 cm −2 .The unabsorbed flux is 2.8 +5.3  −1.6 × 10 −13 erg cm −2 s −1 in 0.5-8 keV.Because the distance to the pulsar is not constrained, the luminosity is difficult to estimate accurately.Given its location in the Galactic plane (l = 311.1,b = 1.4), we use a fiducial distance of 8 kpc to estimate the X-ray luminosity L X ≈ 2 × 10 33 erg s −1 and the X-ray PWN radiative efficiency ≈ 4 × 10 −3 .These parameters are fairly typical for a PWN (Kargaltsev & Pavlov 2008).
The other extended source has a more isotropic appearance with a gradually decreasing surface brightness, starting from the center of the source.The source spectrum (extracted from the r = 0.5 ′ circular aperture shown in Figure 4) is hard and can be described either by an absorbed PL with Γ = 0.4 +0.5   −0.3   and n H = 0.2 +0.9 −0.2 × 10 22 cm −2 or by thermal plasma emission (e.g., mekal in XSPEC) with kT> 10 keV and n H = (2 ± 1) × 10 22 cm −2 .The unabsorbed flux is 3.2 +0.6 −0.4 × 10 −13 erg cm −2 s −1 in 0.5-8 keV.Given the hard spectrum and the appearance in the X-ray image, we conclude that the source likely is a cluster of galaxies projected near the Galactic plane.It is therefore unlikely to be the source of GeV γ-rays by itself.The two X-ray sources seen in its vicinity are AGNs (based on our classifications and supported by their radio counterparts in the TGSS GMRT radio survey) and could produce some GeV emission.

X-Ray Sources outside the γ-ray Error Ellipses
Eighty-four X-ray sources were detected by wavdetect above the 3σ significance threshold in the ACIS-I images outside the 4FGL-DR4 PUs.After we removed 24 sources with X-ray PU larger than 3 ′′ , the 60 remaining X-ray sources were matched to 116 associations (including the cases without a counterpart).Below, we briefly discuss the classifications of the 60 X-ray sources with confident classifications shown in Table 3.
The sources are confidently classified as either NSs or LMXBs mostly due to the lack of their counterparts and/or because they are very faint.This is likely due to a bias caused by the attempt to classify sources that are systematically fainter than the sources in our TD.These fainter sources are underrepresented in the TD because it is more difficult to unambiguously classify them, and hence, they are not included in the TD.It is not surprising that a large fraction of faint sources does not have MW counterparts simply because of the insufficient sensitivity of the optical and IR all-sky surveys, which are currently used, exacerbated by the significant extinction in the Galactic plane.The MUWCLASS pipeline tends to interpret the lack of MW counterparts as a sign of an NS (because NSs lack any detectable counterpart in the optical, near-IR, and IR surveys).Therefore, the NS classifications of 25 sources lacking counterparts should be taken with caution.They include one relatively bright source, CXO J133035.1-611121(K2), which is more likely to be an NS than others because its spectrum shows evidence of nonthermal emission and low absorption (see details in Table 5).CXO J133035.1-611121 is not only confidently classified as an NS without a counterpart, it is also classified as an NS even with a possible Gaia counterpart (Gmag=20.4,P i = 78%) with CT= 1.7.The other relatively bright sources are likely AGN/YSO based on their large absorbing column densities and MW counterparts (see Table 5).The optical counterpart of CXO J163910.2-514555(E2-1) has a Gaia parallax (ω = 0.09 ± 0.2) consistent with 0, suggesting that it may be an AGN that is misclassified as a CV.
By individually classifying multiple associations to the same X-ray source, we have confidently identified 10 LM-STARs and 10 AGNs.Of these, 9 LM-STARs have accurate distance measurements (ranging from 84 pc to 2.1 kpc) in the Gaia eDR3 catalog (Bailer-Jones et al. 2020).The corresponding L X range from 1.2×10 29 erg s −1 to 3.0×10 31 erg s −1 , which is typical for LM-STARs (Núñez & Agüeros 2016).Of the 10 AGNs, none has accurate distance measurements in Gaia DR3, consistent with their extragalactic na-ture.We also cross-correlated all X-ray sources to radio catalogs and find that CXO J204023.6+473315(I1) matches an extended radio source that is reported in multiple radio catalogs, including the Very Large Array Sky Survey Quicklook catalog (Gordon et al. 2021) and the SPECFIND V3.0 catalog (Stein et al. 2021).The source shows a flat spectrum between 73 and 150 MHz, but a steep/broken spectrum above 232 MHz (up to 4.85 GHz), as shown in its radio spectral energy distribution from Stein et al. (2021), which is consistent with a radio AGN scenario.The other source, CXO J204119.1+473854(I3), also matches an unresolved radio source from Stein et al. (2021), showing a steep spectrum from 150 MHz up to 4.85 GHz.I3 is classified as an NS with low confidence and lacks a matching counterpart.It is therefore not shown in Table 3.

SUMMARY AND CONCLUSIONS
• Eight of the 4FGL sources contain γ-ray pulsars within their position uncertainty ellipses, three of which are detected in X-rays, and one shows a hint of X-ray emission.
• We have detected two extended X-ray sources, for one of which (associated with 4FGL J1358.3-6026/PSRJ1358-6025), the properties are consistent with a PWN of a bow-shock type.The other extended X-ray source is likely to be a galaxy cluster located outside the 4FGL J2041.1+4736PU.
• We detected 93 X-ray sources above the 3σ significance threshold in the CXO ACIS-I observations of 13 GeV γ-ray sources.Nine of the CXO sources are located within the error ellipses of seven 4FGL sources.Of these seven, only two 4FGL sources (J0854.8-4504,and J2041.1+4736)lack a γ-ray pulsar association.In each of these, only one X-ray source is confidently detected with the 4FGL 95% PU ellipse (G1 and I2).G1 is confidently classified as an NS without the counterpart.With the counterpart (the more likely case), it is a stellar type source, but given the surprisingly large luminosity, it may be a binary.An optical spectroscopic observation might be useful to explore the putative binary nature, and alternatively, a deeper X-ray and/or radio observation can be used to search for pulsations from the possible NS scenario that might be associated with the GeV emission.I2 is likely to be an AGN given its MW classification and the presence of the radio counterpart.
• For X-ray sources detected outside of the LAT source PUs, we provide classifications, some of which are consistent with isolated or binary compact objects, depending on the confidence with which they are associated with sources at lower frequencies.
We have selected a sample of bright γ-ray sources (i.e., only 17% of GeV sources in the 4FGL-DR4 catalog have G 100 > 10 −11 c.g.s.) for this study.Thus, it is not surprising that blind pulsation searches have been successful in establishing the pulsar nature for most of them.However, for fainter γ-ray sources, blind pulsation searches are progressively more difficult, and identifying a counterpart at lower energies becomes crucial.We demonstrate that the ML approach can be used to identify promising X-ray candidates in some cases at least.Our analysis also shows that partly due to the degrading sensitivity of ACIS caused by the accumulating contamination, 10 ks observations can no longer provide enough photons for reliable classifications or even for significant detections, especially if the exact location of the source is not known a priori.On the other hand, the very low ACIS background and subarcsecond resolution of CXO can resolve the faint extended emission, which facilitates the identifications.Finally, for three pulsars detected in γ-rays, we were able to estimate the X-ray fluxes, which can be used to explore the connection between the γ-ray and X-ray emission in pulsars.This work was supported by the National Aeronautics and Space Administration through Chandra Awards GO8-19058X, GO1-22072A, GO1-22072B, and AR0-21007X issued by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for the National Aeronautics Space Administration.J.H. acknowledges support from NASA under award number 80GSFC21M0002.
F2-1 5.1 0.83 5.45 0.32 18.59 Note-a The X-ray detection significance from wavdetect.b The X-ray positional uncertainty of the 95% confidence level.c The observed broadband X-ray flux in the 0.5-7 keV energy band.d Hardness ratio (F h − Fm − Fs)/(F h + Fm + Fs), where F h , Fm, and Fs are the energy fluxes in the hard (2.0-7.0 keV), medium (1.2-2.0 keV), and soft (0.5-1.2 keV) energy bands.e Gaia DR3 G-band magnitude.f 2MASS J-band magnitude.g WISE W1-band magnitude.h The distances for sources with Gaia DR3 associations are taken from Bailer-Jones et al. (2021) and are only listed when the ratio of the parallax to its standard error is ≥ 3; i Pc is the chance coincidence probability, and Pi is the association probability calculated for a specific catalog as indicated by a letter (G for Gaia, C for CatWISE2020, T for 2MASS, and A for AllWISE).j The predicted classification.The column definitions are the same for the same columns in Tables 3, 4, and 5.All X-ray sources listed in this table are not significantly variable (the variability probability of each X-ray observation calculated from the Kuiper test is < 95%).X-ray sources whose names are marked by † are located within the PU of 4FGL sources currently lacking γ-ray pulsar associations (see Figure 1).
Note-Variable sources (variability probability > 95%) are marked by an asterisk next to their CXO names.
Note-a Spin periods of γ-ray pulsars.b When the X-ray source is not detected, we report the upper limit of the observed (absorbed) X-ray flux in the 0.5-7 keV energy band at the 95% confidence level.References:    4).

Figure 1 .
Figure 1.Error ellipses (in blue) of eight 4FGL sources with X-ray sources and/or γ-ray pulsars detected inside.The red circles indicate the detected X-ray sources.The magenta diamonds indicate the accurately (with subarcsecond precision) known positions of γ-ray pulsars from γ-ray timing solutions.The purple and black squares indicate the locations of the γ-ray pulsars from the 4FGL-DR4 catalog and MMGPS, for which the uncertainties are not yet published.

Figure 2 .
Figure 2. First 6 of 13 Fermi fields with 4FGL-DR4 error ellipses shown in blue, and the 3FGL error ellipses are shown in green (if available).The red circles, magenta diamonds, and purple and black squares are the same as in Figure 1.

Figure 3 .
Figure 3.The following 7 of 13 Fermi fields.The markers are the same as in Figure 2. One extended source is marked as galaxy cluster in panel (I), and the other extended source is marked as PSR J1358-6025 (M8) in panel (M).

Figure 4 .
Figure 4. Extended sources in the ACIS-I images from ObsIDs 23595 (top) and ObsIDs 23591 (bottom).The ellipse and circle show spectral extraction regions (see text).In the top panel, the cross marks the position of the pulsar (see Table4).

Table 2 .
Summary of the MW Properties and Classifications of CXO Sources within the 4FGL 95% PU Ellipses

Table 3 .
Summary of the MW Properties and Confident Classifications of X-Ray Sources outside the Fermi Error Ellipses with X-Ray PU≤ 3 ′′

Table 4 .
Summary of the Properties for Eight γ-Ray Pulsars Coincident with 4FGL Sources

Table 5 .
Summary of the Confident Classifications and X-Ray Properties (Including Spectral Fitting) of X-Ray Sources with F b ≥ 10 −13 erg s −1 cm −2 and the Number of Net Counts Nc > 60.Note-All X-ray sources listed in this table are not significantly variable (the variability probability is < 95%).