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A&A
Volume 692, December 2024
Article Number A147
Number of page(s) 10
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/202348401
Published online 09 December 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

The hard X-ray sky, at energies greater than ~10 keV, has been systematically observed by several telescopes in the past six decades. Uhuru, launched in 1970, was the first X-ray satellite (Giacconi et al. 1971). It delivered an all-sky hard X-ray survey (Forman et al. 1978) in the 2–20 keV band consisting of 339 sources. Subsequently, Levine et al. (1984), thanks to the X-ray and Gamma-ray detector HEAO-A4 on board the HEAO-1 satellite (Rothschild et al. 1979), conducted an all-sky survey in the 13–180 keV range, providing 77 newly detected sources. The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL; Winkler et al. 2003), with its Imager on Board the INTEGRAL Satellite (IBIS; Ubertini et al. 2003), was launched in 2002, observing in the energy range from 15 keV up to 10 MeV. Finally, the Neil Gehrels Swift Observatory (Gehrels et al. 2004), launched in 2004, carried out an all-sky hard X-ray survey at 14–195 keV using the Burst Alert Telescope (BAT; Barthelmy 2004). Significant advancements in our understanding of the soft X-ray background were possible mainly thanks to NASA’s HEAO-2 Einstein observatory (Giacconi et al. 1979), the German-US-UK X-ray observatory ROSAT (Röntgen Satellite; Hasinger et al. 1999), the Chandra X-ray observatory (Weisskopf et al. 2000), and the XMM-Newton observatory (Jansen et al. 2001).

INTEGRAL and Swift have allowed for the creation of many catalogs focused on the hard X-ray sky (see, e.g., Markwardt et al. 2005; Beckmann et al. 2006; Churazov et al. 2007; Krivonos et al. 2007; Sazonov et al. 2007; Tueller et al. 2008; Cusumano et al. 2010; Bottacini et al. 2012; Bird et al. 2016; Mereminskiy et al. 2016; Krivonos et al. 2017; Oh et al. 2018; Krivonos et al. 2021, 2022) and catalogs that provide the association of hard X-ray sources with their low-energy counterparts (e.g., Malizia et al. 2010; Koss et al. 2019; Bär et al. 2019; Smith et al. 2020). They were also necessary for optical spectroscopy follow-up observations (e.g., Masetti et al. 2006a,b, 2008, 2012, 2013; Parisi et al. 2014; Rojas et al. 2017; Marchesini et al. 2019a). These missions are still operational nowadays and continue to deliver new scientific results, for example the INTEGRAL-IBIS 17-yr hard X-ray all-sky survey (Krivonos et al. 2022), the active galactic nucleus (AGN) catalog and optical spectroscopy for the second data release of the Swift-BAT AGN Spectroscopic Survey (BASS DR2; Koss et al. 2022), and the upcoming catalog based on the Swift-BAT 157-month survey (Lien et al. 2023).

Multiple catalogs based on observations by Swift-BAT exist. Koss et al. (2022) constructed BASS DR2, which provides 1449 optical spectra corresponding to 858 hard-X-ray-selected AGNs in the Swift-BAT 70-month observations. Their AGN sample is spectroscopically complete, with 857–858 AGNs reported with redshifts. Oh et al. (2018) created the 105 month Swift-BAT catalog of hard X-ray sources. This catalog covers over 90% of the sky with a sensitivity of 8.40 × 10−12 erg s−1 cm−2, and over 50% with a sensitivity of 7.24 × 10−12 erg s−1 cm−2, in the 14–195 keV band, providing 1632 hard X-ray detections above a 4.8 σ significance threshold. Cusumano et al. (2010) created the second release of the Palermo Swift-BAT hard X-ray Catalog. It was based on 54 months of observations (2PBC). Its third release, 3PBC is currently only available online1. It is based on 66 months of observations. The 3PBC is the version that we analyze in this study. It lists 1256 sources detected above a 4.8 σ level of significance in the 15–150 keV energy range. This number increases to 1593 total sources when considering a signal-to-noise ratio (S/N) threshold of 3.8. The catalog covers nearly 90% of the sky with a flux limit of 1.1 × 10−11 erg cm−2 s−1, and ~50% of the sky with a flux limit of 0.9 × 10−11 erg cm−2 s−1.

We recently conducted a refined analysis (Kosiba et al. (2023), hereinafter Paper I) of all the sources listed in the 3PBC catalog. Our refined analysis is based on the multifrequency classification scheme that we developed to analyze hard X-ray sources, mainly focusing on extragalactic source populations (Peña-Herazo et al. 2022). We also made use of the 105-month Swift-BAT catalog (Oh et al. 2018) and the 11-year INTEGRAL hard X-ray survey above 100 keV (Krivonos et al. 2015); we crossmatched these catalogs with the 3PBC sources to find counterparts and obtain luminosities and spectra when available.

Based on the Paper I analysis, we found that approximately 57% of the sources listed in the original 3PBC had an extragalactic origin, and 19% belonged to our Milky Way. The remaining 24% corresponds to the sources with no identified counterpart and were thus assigned to the unidentified category. Our final revised version of the 3PBC catalog lists 1176 classified sources, 218 unidentified ones, and 199 unclassified ones. Of the 1176 classified sources, 820 have an extragalactic origin and 356 have a Galactic origin. Compared to the original 3PBC, which has 233 unidentified and 300 unclassified sources, we have decreased the fraction of unidentified or unclassified sources from ~33% (533 sources) to ~26% (417 sources).

Our study also allowed us to discover new Seyfert galaxies included in the second Turin-SyCAT release (Paper I). In the second release of the Turin-SyCAT, there are 633 Seyfert galaxies, 282 of which are newly added thanks to our refined analysis; this corresponds to an increase of ~80% with respect to the first Turin-SyCAT release (Peña-Herazo et al. 2022).

Moreover, trends between the hard X-ray and the gamma-ray emissions of those blazars listed in the 3PBC with a counterpart in the second release of the fourth Fermi-LAT catalog (4FGL-DR2) were also found (Paper I). This was expected from emission models widely adopted to explain their broadband spectral energy distribution (e.g., Marscher & Gear 1985; Marscher & Travis 1996; Massaro et al. 2006).

In this work we examined the population of the 218 unidentified hard X-ray sources listed in our revised version of the 3PBC, that is, those lacking an assigned counterpart at lower energies than the BAT energy range. We analyzed all the soft X-ray observations (between 0.5 and 10 keV) available in the archive of the X-ray Telescope (XRT; Burrows et al. 2005) on board the Swift Observatory and found available data for 192 of the 218 3PBC sources, which is the sample that we analyzed in this study. Our aim was to find potential counterparts in the soft X-ray data of the Swift-XRT for the 192 as-of-yet-unidentified 3PBC sources. The final goal of the present analysis is to provide a catalog of all unidentified hard X-ray sources that have at least one detected candidate counterpart in the soft X-ray band. This catalog will be an excellent list of targets for follow-up spectroscopic observations to obtain their final classifications, as has successfully been carried out in recent decades (e.g., Masetti et al. 2006a,b, 2008, 2012, 2013; Parisi et al. 2014; Rojas et al. 2017; Koss et al. 2017; Landi et al. 2017; Marchesini et al. 2019a; Tomsick et al. 2020).

The paper is organized as follows. Section 2 describes the Swift-XRT data reduction and data analysis procedure. Section 3 is devoted to our results, and details on the multifrequency comparison are given in Sect. 4. Finally, Sect. 5 provides a summary, conclusions, and future perspectives. X-ray images for all the analyzed BAT sources are published on Zenodo.

As in Paper I, we have used CGS units unless stated otherwise. We also adopted Λ cold dark matter cosmology with ΩM = 0.286, and a Hubble constant of H0 = 69.6 km s−1 Mpc−1 (Bennett et al. 2014), to compute cosmological corrections.

2 Swift-XRT observations

2.1 Sample selection

The original 3PBC catalog lists 1593 hard X-ray sources, all detected with a S/N above 3.8 in the 15–150 keV energy range. Our analysis presented in Paper I identified 218 3PBC sources that lack an assigned counterpart at lower energies.

In this work, we searched the Swift-XRT archive and found that 192 of these 218 hard X-ray sources have at least one X-ray observation with an exposure time longer than 250 s in the 0.5–10 keV energy range. We found a total of 1213 such observations, which we reduced and analyzed according to the standard procedures described below. In Fig. 1, we report the distribution of the exposure time for all selected observations. The 1213 Swift-XRT observations have a mean of 1462 s and variance of ~6 × 106 s, with a total exposure time of 1.77 × 106 s. All of the observations that we reduced and analyzed in this study were performed between April 2005 and December 2022.

2.2 Data reduction

The data reduction procedures applied here to all the Swift-XRT observations are the same previously adopted for similar analyses (see, e.g., Massaro et al. 2008b,a; Paggi et al. 2013; Marchesini et al. 2019b, 2020; Massaro et al. 2023a) and procedures of the Swift-XRT X-Ray point source catalogs (D’Elia et al. 2013; Evans et al. 2014, 2020). Thus, we only describe the basic information here and refer to the above references for a more detailed description.

We downloaded raw Swift-XRT data from the archive2. Then we ran the XRTPIPELINE task, developed as part of the Swift X-Ray Telescope Data Analysis Software (XRTDAS) and distributed within the HEAsoft package (version 6.30.1; NASA High Energy Astrophysics Science Archive Research Center (Heasarc) 2014). This allowed us to obtain clean event files for all the Swift-XRT observations. The entire analysis and all X-ray images shown in the present manuscript are restricted to the 0.5–10 keV energy range unless stated otherwise.

We subsequently calibrated these cleaned event files with the usual filtering criteria and using calibration files provided in the High Energy Astrophysics Science Archive Research Center (HEASARC) calibration database (CALDB) version v. 202209073. Using the XSELECT task, we excluded all time intervals with count rates higher than 40 photons/sec as well as those with CCD temperatures exceeding −50°C, in regions located at the edges of the XRT detector (D’Elia et al. 2013). Then, the XSELECT task was used to merge all cleaned and filtered event files for those sources with multiple observations. The entire analysis was carried out using the XIMAGE software4 to merge the corresponding exposure maps (Giommi et al. 1992a).

thumbnail Fig. 1

Distribution of Texp for the 1213 Swift-XRT observations analyzed in this work. Left panel: Texp per observation with a mean of 1462 s. Right panel: total Texp per source of the 192 sources for which we found Swift-XRT observations, with a mean of 8864 s.

2.3 Data analysis

To detect sources, we used the sliding cell DETECT (DET) algorithm available in the XIMAGE software package (Giommi et al. 1992b) on all merged event files as well as on the single event files for sources with only one observation. We set a S/N threshold of 3 for claiming detection of an X-ray source in the 0.5–10 keV energy range, as done in Massaro et al. (2023b).

Then, to identify and characterize the 3PBC sources, we labeled them using three different X-ray detection flags (XDFs) on the basis of the following criteria.

  • x flag was used for 3PBC sources that have a single soft X-ray source within their BAT positional uncertainty region (see, e.g., 3PBC J1039.4-4903, shown in the left panel of Fig. 2);

  • m flag was used for 3PBC sources with more than one soft X-ray source (multiple detections) within their BAT positional uncertainty region (see, e.g., 3PBC J0819.2-2509, shown in the central panel of Fig. 2);

  • u flag was used for 3PBC sources with no soft X-ray counterparts detected in their merged event files within their BAT positional uncertainty region (see, e.g., 3PBC J1834.7-0345, shown in the right panel of Fig. 2).

We measured several parameters for all detected possible X-ray counterparts of the 3PBC hard X-ray sources in the merged event files. In particular, we obtained coordinates of the distributions of X-ray photons from each source using the XRTCENTROID task. We also measured n90, the number of photons within a circular region centered on the X-ray coordinates with a radius of 120.207″ (51 pixels), which encloses 90% of the Swift-XRT point spread function.

3 A soft X-ray perspective of the hard X-ray sky

3.1 Outline of the main goal

In our previous work (Kosiba et al. 2023), in which we revised the 3PBC catalog, we found 218 sources without a low-energy counterpart, which thus fall into the unidentified category. The main goal of the current paper is to search for candidate counterparts for these 218 unidentified sources in soft X-ray wavelengths of the Swift-XRT archival data. The final product of this analysis is a catalog of soft X-ray candidate counterparts for these as-of-yet-unidentified hard X-ray sources. We release this catalog along with this publication. This paper also describes the sources that we analyzed in this study.

3.2 Overview of results

We analyzed all available Swift-XRT data, selected according to the criteria previously described, for a total of 1213 observations, corresponding to 1923 PBC sources, with a total exposure time of 1.77 × 106 s. Considering only the Swift-XRT detections inside the BAT positional uncertainty region above the S/N = 3 threshold that we adopted, we found 93 soft X-ray sources. These 93 soft X-ray sources correspond to 73 unique 3PBC sources. Of those, 13 3PBC sources are associated with multiple soft X-ray detected sources (m flag), and the remaining 603 PBC sources are associated with a single soft X-ray detected source (x flag).

We note that all 3PBC sources with at least one soft X-ray counterpart detected within their positional uncertainty region have an integrated exposure time above 975 seconds, about four times longer than the minimum selected value. The distribution of X-ray count rates and that of their positional uncertainty in the 0.5–10 keV energy range computed using the XRTCENTROID task for the 93 Swift-XRT-detected sources are shown in Fig. 3 (left) and Fig. 3 (right), respectively. To calculate the BAT positional uncertainty region (red circles in Fig. 2), we took the values reported in the 3PBC catalog (Cusumano et al. 2010).

In this section, we focus on the m XDF flagged sources (Fig. 4). We discuss these 13 3PBC sources separately to detail their potential soft X-ray counterparts.

thumbnail Fig. 2

Examples of our XDF flags that label the 3PBC sources. Each panel shows an image from XRT-merged event files, with the red circle indicating the BAT positional uncertainty region and the black circles highlighting the position of a soft X-ray source detected in the Swift-XRT archive. The left panel is an example of a 3PBC source with a single Swift-XRT source found within the BAT positional uncertainty region (red circle), flag x. The center panel is a case of multiple detected Swift-XRT counterparts inside the BAT positional uncertainty, flag m. The right panel is an example of no Swift-XRT counterparts detected within the BAT positional uncertainty region, flag u.
thumbnail Fig. 3

Distribution of the count rate in logarithmic scale, with a mean of 0.028 photons/s (left panel) and the XRT positional uncertainty (ϑXRT), with a mean value of 4.5 arcsec (right panel). Both are in the 0.5–10 keV energy range for all 93 Swift-XRT-detected counterparts.

3.3 3PBC sources with multiple candidate counterparts

This section shows images of 3PBC sources, with a brief discussion of those for which we have found multiple XRT photon-counting (PC) counterparts consistent within the positional uncertainties (m flag). Source 3PBC1430.3+2303 deserves a more detailed description; for it, given the diffuse X-ray emission clearly detected, we only selected as potential counterparts those targets that were detected in the manner described in the previous section and that also have a mid-infrared (mid-IR) counterpart (marked with a green cross in Fig. 4). It is worth noting that one of these sources, SWXRTJ143016.094+230343.862, seems to be associated with the galaxy cluster MSPM 05080, indicating that the possible origin of this extended X-ray emission is its intracluster medium.

3.3.1 3PBC J0022.2+2539

This source has three XRT PC counterparts (Fig. 4). While sources s1 and s3 are faint, with S/Ns of 4.7 and 3.1, respectively, source s2 is much brighter, with a S/N of 37 and a count rate of 0.158 ± 0.004 s−1. In addition, source s2 is also detected by The Wide-field Infrared Survey Explorer (WISE) (Wright et al. 2010) (J002203.09+254003.2) and is in the Sloan Digital Sky Survey (SDSS; J002203.09+254003.1) with a magnitude of r = 17.0.

3.3.2 3PBC J0218.5-5005

This source has three XRT PC counterparts (Fig. 4). The brightest of these is s 1, with a S/N of 5.9, while the faintest is s2, with a S/N of 3.8. The latter was also detected by WISE (J021822.70-500557.5).

3.3.3 3PBC J0536.1-3205

This source has two XRT PC counterparts (Fig. 4). The brighter, s1, with a S/N of 4.1, was also detected by WISE (J053618.88-320533.0).

3.3.4 3PBC J0709.5-3538

This source has two XRT PC counterparts (Fig. 4), with s1 being by far the brighter, with a S/N of 28.6 and a count rate of 0.176 ± 0.006 s−1. It was also detected by WISE (J070932.05-353746.5), and a spectrum is reported in Rojas et al. (2017).

3.3.5 3PBC J0800.7-4308

This source also has two XRT PC counterparts (Fig. 4). The source s1 is the fainter, with a S/N of 5.8 and a count rate of 0.0036 ± 0.0006 s−1, while s2 has a S/N of 27.6 and a count rate of 0.065 ± 0.002 s−1. Both sources have a WISE counterpart, J080045.83-430939.3 and J080039.96-431107.2, respectively.

thumbnail Fig. 4

Images of the 133 PBC sources with more than one soft Swift-XRT source (XDF flag m) detected within the BAT positional uncertainty region (dashed red circle). The black circles indicate the positions of the soft X-ray sources, not their positional uncertainties. If the soft X-ray detection is also marked with a green cross, this indicates that it has a WISE counterpart.

3.3.6 3PBC J0819.2-2509

This source has two XRT PC counterparts (see Fig. 4), with the brighter being s1, with a S/N of 14.0, and the fainter being s2, with a S/N of 7.4. Both sources have a WISE counterpart, J081914.73-251116.6 and J081916.20-250706.4, respectively, and s1 has a redshift of 0.00557 and a spectrum reported in Strauss et al. (1992).

3.3.7 3PBC J0857.2+6703

For this source (Fig. 4), we find two XRT PC counterparts: the brighter of the two is s1, with a S/N of 17.6, and the fainter is s 2, with a S/N of 3.1. Source s1 has a WISE counterpart, namely, J085656.49+670257.3.

thumbnail Fig. 5

Comparison with the 2SXPS catalog. Left panel: comparison between the exposure of the dataset used in the present work (blue distribution) and in the 2SXPS (red distribution). Right panel: XRT PC count rates reported in the 2SXPS versus those evaluated in the present work for the 78 sources in common (see Sect. 3.4). The 2SXPS count rate has been rescaled to match the energy band used in the present analysis. The blue line indicates the linear regression to the logarithmic data, while the shaded light blue area represents the 1 − σ uncertainty around the best-fit relation. The dashed black line indicates the y = x relation. See Sect. 3.4 for more details.

3.3.8 3PBC J0905.4-1502

This source (Fig. 4) has two XRT PC counterparts, s1 and s2, with similar S/Ns, 3.8 and 3.9, respectively. Both have been detected by WISE (J090522.48-150344.6 and J090533.64-145956.2, respectively), with s2 reported in the NASA Extragalactic Database (NED) with a redshift of 0.088.

3.3.9 3PBC J1430.3+2303

For 3PBCJ1430.3+2303, due to the presence of diffuse X-ray emission, the automatic algorithm described in the previous section detected many spurious sources. We thus selected as potential soft X-ray counterparts only the sources with a mid-IR counterpart (marked with a green cross in Fig. 4). It is worth noting that one of them, SWXRTJ143016.094+230343.862, seems to be associated with the galaxy cluster MSPM 05080, indicating that the possible origin of this extended X-ray emission is its intracluster medium.

3.3.10 3PBCJ1732.0-3439

This source (Fig. 4) has two XRT PC counterparts, s3 and s4, with S/Ns of 3.3 and 4.7 and count rates of 0.0044 ± 0.0013 and 0.0087 ± 0.0018 ct s−1, respectively.

3.3.11 3PBCJ1846.1-0226

This source (Fig. 4) has three XRT PC counterparts, s7, s13, and s15, with similar S/Ns of 4.2, 4.5, and 4.9, respectively. Source s13 is also detected in SDSS (J184617.13-022753.4) with a magnitude of r = 17.6.

3.3.12 3PBCJ1856.5+2836

This source (Fig. 4) has three XRT PC counterparts, s1, s2, and s 4, with S/Ns of 3.6, 3.5, and 9.9, respectively. The sources have all been detected by WISE (J185626.89+283809.3, J185632.13+283628.8, and J185634.58+283531.3), although the faintest, s2, only has an upper limit of 8.3 mag in the W4 band.

3.3.13 3PBCJ2313.3-3402

This source (Fig. 4) has two XRT PC counterparts, s7 and s9, with the brighter being the former, which has a S/N of 8.4. Both sources have been detected by WISE (J231313.21-340056.2 and J231337.01-340302.1), although the fainter s9 only has an upper limit in the W3 and W4 bands.

3.4 Comparison with 2SXPS

Finally, we compared our results with those that can be obtained by simply crossmatching all 218 hard X-ray sources listed in the 3PBC with the latest release of the Second Swift-XRT Point Source Catalog5 (2SXPS; Evans et al. 2020). The 2SXPS has a sky coverage of 3790 deg2 and lists 206335 point sources detected by XRT in the 0.3–10 keV energy range. Here, we briefly summarize the procedure used to build the 2SXPS.

The 2SXPS was built based on all XRT observations taken between January 1, 2005, and August 1, 2018, with an exposure of at least 100 s in PC mode. Source detection was performed with the sliding-cell technique with a S/N threshold set to 1.5, in contrast to our choice of S/N = 3, yielding a final catalog of 206335 XRT PC sources. The catalog contains a “clean” subsample of 146768 sources without analysis flags (see Evans et al. 2020 for more details). In the following, we examine this clean subsample.

Due to the different dataset and analysis procedures used in the present work compared to Evans et al. (2020), we expect differences in the XRT PC source detections. In fact, crossmatching the 2SXPS clean sample with the 3PBC sources considered here (see Sect. 2.1), taking into account both the BAT and 2SXPS positional uncertainties, we find 126 2SXPS counterparts to 90 3 PBC sources, 683 PBC sources with a single 2SXPS match, and 223 PBC sources with multiple 2SXPS matches. In the left panel of Fig. 5, we compare the exposures of the XRT PC observations used in the present analysis (blue distribution) and in the 2SXPS observations for which we find the 126 2SXPS counterparts to the 3PBC sources considered in this work. We see that the 2SXPS datasets and the dataset used in our analysis span a similar exposure range.

In addition, we find XRT PC counterparts to seven 3PBC sources without 2SXPS counterparts, while in the 2SXPS there are counterparts to 223 PBC sources for which we did not find XRT PC counterparts. However, we note that the S/Ns for these 22 sources are less than 2.5, below the S/N threshold of 3 that we adopted for the present analysis. In addition, we find 78 sources that are positionally compatible between our sample of 93 XRT PC sources and the 126 2SXPS counterparts to the 3PBC sources.

In the right panel of Fig. 5, we compare the count rate evaluated in the present analysis with the count rate reported in the 2SXPS for these 78 common sources. We stress that to account for the different energy band adopted in the present analysis (0.5–10 keV) compared to the 2SXPS (0.3–10 keV), we rescaled the count rate of the 2SXPS by a factor of 0.86, evaluated via the PIMMS6 tool, assuming a power-law spectrum with a 1.8 slope. We see that the two estimates are in good agreement at low count rate values (<10−3 cps). In contrast, above 10−2 cps, the 2SXPS count rates appear systematically higher than those evaluated in the present analysis. The two count rate estimates, however, are compatible at a 2 σ level. The maximum deviation below 10−3 cps is 0.34 σ, while above 10−2 cps it reaches 1.95 σ. In addition, below 1.8−4 cps, the best-fit line is systematically below the y = x relation, while above 1.8−4 cps, the best-fit line is systematically above the y = x relation.

4 Multifrequency comparison

To search for additional information regarding all detected soft X-ray candidate counterparts, we crossmatched their positions derived with the XRTCENTROID task – taking into account their positional uncertainties – with three main catalogs or surveys: (i) NED7; (ii) the SIMBAD Astronomical Database8; (iii) the ALLWISE catalog (Cutri et al. 2021), which is based on the allsky survey performed with the WISE telescope (Wright et al. 2010); and (iv) the spectroscopic catalog of SDSS data release 16 (DR16; Blanton et al. 2017). To search for and claim an association with a WISE counterpart, we used the Swift-XRT positional uncertainty region for all the soft X-ray detected sources.

Our crossmatching analysis reveals that 84 of the 93 soft X-ray potential counterparts have an identification reported in NED. In addition, 74 of the 93 have photometry available in WISE, and 10 of the 93 have spectra in SDSS DR 16 (Fig. 6).

4.1 A mid-infrared perspective

Of the 93 detected soft X-ray possible counterparts corresponding to the 73 unique 3 PBC sources, 74 have been detected in at least one WISE band. There are 74 sources detected in both the W1 and W2 bands, 66 in W3, and 52 in W4. Figure 6 shows the distribution of the angular separation between the Swift-XRT centroid and the WISE centroid of the 74 counterparts that have a WISE detection. We took all sources that have been detected in the W1, W2, and W3 bands (i.e., 66 sources) and plotted them on a color-color diagram (Fig. 7). The mid-IR colors of this sample of 66 sources are not in good agreement with the mid-IR colors of stars; they are more consistent with AGNs, mainly Seyfert galaxies and quasi-stellar objects (QSOs).

thumbnail Fig. 6

Distribution of the angular separation between the Swift-XRT centroid and the WISE centroid (θXRT–WISE) of the 74 soft X-ray detections with a WISE counterpart.
thumbnail Fig. 7

[3.4]–[4.6]–[12] μm color-color diagram of WISE thermal sources and blazars. The gray dots represent a sample of 3000 randomly selected mid-IR sources in a region of 0.5 deg radius around the Galactic coordinates (50.411113,−45.668864) and (50.411113, 45.668864). The 66 Swift-XRT-detected sources with available luminosities in the first three WISE bands are marked as black dots. The mid-IR colors of this sample of 66 sources do not agree with the mid-IR colors of stars. Instead, they are more consistent with AGNs, mainly Seyfert galaxies and QSOs. The sources with [4.6]–[12] mag >2 are AGNs and QSOs, while the concentration of sources around 0 [4.6]–[12] mag is mostly made up of normal elliptical galaxies and stars.
Table 1

Candidate counterparts for which we found spectra in the literature.

thumbnail Fig. 8

Ten spectra collected from the SDSS archive for the soft X-ray Swift-XRT sources detected in the BAT positional uncertainty region of the 3PBC sources. The upper left spectrum (J0040.5+2542) and the upper middle spectrum (J0838.7+2613) correspond to quasars. We classified the upper right spectrum (J1041.2+0451) as a type 2 AGN. The second row shows spectra of type 2 AGNs (left and middle) and a quasar (right). The third row shows the spectra of a quasar (left), a type 2 AGN (middle), and a type 1 AGN (right). The last spectrum image on the bottom corresponds to a star-forming galaxy. The main spectral emission and/or absorption features are marked in each figure. We are reporting the SDSS spectra with the same redshift precision, since none of the fittings had warnings.

4.2 Archival optical spectra

According to previous analyses carried out during past follow-up spectroscopic campaigns (see, e.g., Massaro & D’Abrusco 2016; Peña-Herazo et al. 2020, Peña-Herazo et al. 2022; Kosiba et al. 2023), we adopted a conservative criterion to provide spectroscopic identification of selected X-ray counterparts. We adopted the same classification scheme as in Paper I and only considered reliable redshift measurements, those for which we could verify the presence of a published image of the optical spectrum or a description of the published spectrum with emission and/or absorption lines clearly reported in table format or a published article. We found spectra in SDSS DR 16 for ten soft X-ray sources. We show the spectra in Fig. 8 and our classification of the sources in Table 1. The most prominent spectral lines among the spectra were the Hα + [N II], [O III], and [O II]. We classified four sources as type 2 AGNs, one as a type 1 AGN, four as QSOs, and one as a star-forming galaxy, which agrees with all the WISE colors and previous results.

Additionally, we computed the Baldwin, Phillips, and Terlevich (BPT) diagrams (Baldwin et al. 1981) for sources with narrow lines, with the objective of classifying them as either Seyfert 2 or star-forming galaxies. We present the BPT diagram in Fig. 9. Our findings indicate that all narrow-line sources, except SDSS J143010.96+230134.7, fall outside the region corresponding to star-forming galaxies on the plot. This region is delineated by the theoretical line of Kewley et al. (2001, 2013). Consequently, we classify these sources as Seyfert 2 galaxies. For 3PBCJ1504.1-6019 and 3PBCJ0800.7-4308, our results are in agreement with those of Landi et al. (2017); we found the same soft X-ray sources lying within the positional uncertainty region of these BAT-unidentified objects. Inspecting the NED and SIMBAD databases, we found the spectra for the soft X-ray counterparts of the unidentified 3PBC sources 3PBCJ1329.7-1052 and 3PBCJ1854.4-3436. They are MCG-02-34-058 and ESO 396-G 007, which are at z=0.021648 and z=0.019483 (Jones et al. 2009), respectively. The Swift-XRT counterparts we assigned for the sources 3PBCJ2136.1+2002, 3PBCJ2155.3+6204, and 3PBCJ2238.8+4050 are the same as reported in the literature; optical spectroscopic follow-up observations revealed them to be three active galaxies at z=0.081 (Sy1), z=0.058 (Sy1), and z=0.055 (LINER) (Rojas et al. 2017). The as-of-yet-unidentified source 3PBCJ0024.1-6823 that has the Swift-XRT counterpart SWXRTJ002406.457-682052.549 may also be associated with the radio source PKS 0021-686, a gamma-ray blazar candidate selected based on its mid-IR colors (D’Abrusco et al. 2012, 2014, 2019).

thumbnail Fig. 9

BPT diagram for distinguishing type 2 and star-forming galaxies. The error bars are all less than 0.009 and not visible in the plot. Note that all of the sources, except for SDSS J143010.96+230134.7, are above the theoretical line of Kewley et al. (2013) (right part of the graph separated by the blue line), which serves to discriminate between starburst regions and harder ionization sources.

5 Summary and conclusions

The main goal of the present analysis was to prepare a catalog of candidate soft X-ray Swift-XRT counterparts detected in the 0.5–10 keV energy range. These counterparts are potential targets for a future optical spectroscopic campaign that will be carried out to classify the as-of-yet-unknown sources in the hard X-ray sky so that we can obtain a more complete overview of it. We found archival Swift-XRT observations for 192 of the 218 3PBC sources marked as unidentified in our previous analyses, hard X-ray sources lacking an assigned low-energy counterpart. In this work, we searched for possible counterparts at soft X-ray energies for those 192 3PBC sources. If found, we carried out a literature search and multiwavelength analyses, as done in Paper I.

We have found that in only 172 out of the 192 sources is there at least one soft X-ray detected source above our S/N threshold of 3 present in the cleaned and merged event file, and only in 73 of the 3PBC sources do we find at least one soft X-ray candidate counterpart detected within the BAT positional uncertainty region. In particular, for 13 3PBC sources, there are multiple detected soft X-ray objects, while the remaining 60 3PBC sources have only a single Swift-XRT-detected object. Thus, including multiple matches, the total number of Swift-XRT-detected possible counterparts within the BAT positional uncertainty region listed in our final catalog is 93, corresponding to 73 3PBC hard X-ray sources.

Our X-ray results are in agreement with those achieved simply by crossmatching the sample of unidentified 3PBC sources (created in Paper I) with the 2SXPS, with only marginal differences (see Sect. 3) mainly due to (i) longer exposure times and new observations collected after the 2SXPS release that were used in our analysis and (ii) a small difference in the detection thresholds of the two analyses.

We have found available spectra in the literature for ten detected counterparts. For those, we carried out the same multifrequency analyses as in Paper I. We find four sources to be quasars, four sources to be type 2 AGNs, one source to be a type 1 AGN, and one a star-forming galaxy. We thus decrease the number of unidentified 3PBC hard X-ray sources without a low-frequency counterpart from 218 to 143. This corresponds to a decrease of ~34%. Of the 733 PBC sources for which we found at least one assigned candidate counterpart, 10 were classified according to our multifrequency criteria and were thus identified. The other 63 sources remain unclassified, as they lack spectroscopic information, and are thus excellent candidates for future spectroscopic follow-up observations.

Data availability

Along with this publication, we provide a catalog of all 93 soft X-ray detections and a short table with our classification of the sources with spectra. These tables are available online via the Strasbourg Astronomical Data Centre (CDS). We have also published the images of the 73 3PBC sources for which we found at least one Swift-BAT soft X-ray counterpart on Zenodo9. The soft X-ray sources we found from this analysis can be targets of future spectroscopic campaigns aimed at classifying them, determining their redshifts, and confirming that most of the ones detected in the mid-IR are AGNs, as is expected according to the mid-IR plot in Fig. 7.

Two catalogs are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/692/A147

Acknowledgements

M. K. and N. W. are supported by the GACR grant 21-13491X. E. B. acknowledges NASA grant 80NSSC21K0653. M. K. was supported by the Italian Government Scholarship issued by the Italian MAECI. This work was partially supported by CONACyT (National Council of Science and Technology) research grants 280789 (Mexico) Funding for the Sloan Digital Sky Survey V has been provided by the Alfred P. Sloan Foundation, the Heising-Simons Foundation, the National Science Foundation, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, Center for Astrophysics I Harvard & Smithsonian (CfA), the Chilean Participation Group, the French Participation Group, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional/MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

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All Tables

Table 1

Candidate counterparts for which we found spectra in the literature.

In the text

All Figures

thumbnail Fig. 1

Distribution of Texp for the 1213 Swift-XRT observations analyzed in this work. Left panel: Texp per observation with a mean of 1462 s. Right panel: total Texp per source of the 192 sources for which we found Swift-XRT observations, with a mean of 8864 s.
In the text
thumbnail Fig. 2

Examples of our XDF flags that label the 3PBC sources. Each panel shows an image from XRT-merged event files, with the red circle indicating the BAT positional uncertainty region and the black circles highlighting the position of a soft X-ray source detected in the Swift-XRT archive. The left panel is an example of a 3PBC source with a single Swift-XRT source found within the BAT positional uncertainty region (red circle), flag x. The center panel is a case of multiple detected Swift-XRT counterparts inside the BAT positional uncertainty, flag m. The right panel is an example of no Swift-XRT counterparts detected within the BAT positional uncertainty region, flag u.
In the text
thumbnail Fig. 3

Distribution of the count rate in logarithmic scale, with a mean of 0.028 photons/s (left panel) and the XRT positional uncertainty (ϑXRT), with a mean value of 4.5 arcsec (right panel). Both are in the 0.5–10 keV energy range for all 93 Swift-XRT-detected counterparts.
In the text
thumbnail Fig. 4

Images of the 133 PBC sources with more than one soft Swift-XRT source (XDF flag m) detected within the BAT positional uncertainty region (dashed red circle). The black circles indicate the positions of the soft X-ray sources, not their positional uncertainties. If the soft X-ray detection is also marked with a green cross, this indicates that it has a WISE counterpart.
In the text
thumbnail Fig. 5

Comparison with the 2SXPS catalog. Left panel: comparison between the exposure of the dataset used in the present work (blue distribution) and in the 2SXPS (red distribution). Right panel: XRT PC count rates reported in the 2SXPS versus those evaluated in the present work for the 78 sources in common (see Sect. 3.4). The 2SXPS count rate has been rescaled to match the energy band used in the present analysis. The blue line indicates the linear regression to the logarithmic data, while the shaded light blue area represents the 1 − σ uncertainty around the best-fit relation. The dashed black line indicates the y = x relation. See Sect. 3.4 for more details.
In the text
thumbnail Fig. 6

Distribution of the angular separation between the Swift-XRT centroid and the WISE centroid (θXRT–WISE) of the 74 soft X-ray detections with a WISE counterpart.
In the text
thumbnail Fig. 7

[3.4]–[4.6]–[12] μm color-color diagram of WISE thermal sources and blazars. The gray dots represent a sample of 3000 randomly selected mid-IR sources in a region of 0.5 deg radius around the Galactic coordinates (50.411113,−45.668864) and (50.411113, 45.668864). The 66 Swift-XRT-detected sources with available luminosities in the first three WISE bands are marked as black dots. The mid-IR colors of this sample of 66 sources do not agree with the mid-IR colors of stars. Instead, they are more consistent with AGNs, mainly Seyfert galaxies and QSOs. The sources with [4.6]–[12] mag >2 are AGNs and QSOs, while the concentration of sources around 0 [4.6]–[12] mag is mostly made up of normal elliptical galaxies and stars.
In the text
thumbnail Fig. 8

Ten spectra collected from the SDSS archive for the soft X-ray Swift-XRT sources detected in the BAT positional uncertainty region of the 3PBC sources. The upper left spectrum (J0040.5+2542) and the upper middle spectrum (J0838.7+2613) correspond to quasars. We classified the upper right spectrum (J1041.2+0451) as a type 2 AGN. The second row shows spectra of type 2 AGNs (left and middle) and a quasar (right). The third row shows the spectra of a quasar (left), a type 2 AGN (middle), and a type 1 AGN (right). The last spectrum image on the bottom corresponds to a star-forming galaxy. The main spectral emission and/or absorption features are marked in each figure. We are reporting the SDSS spectra with the same redshift precision, since none of the fittings had warnings.
In the text
thumbnail Fig. 9

BPT diagram for distinguishing type 2 and star-forming galaxies. The error bars are all less than 0.009 and not visible in the plot. Note that all of the sources, except for SDSS J143010.96+230134.7, are above the theoretical line of Kewley et al. (2013) (right part of the graph separated by the blue line), which serves to discriminate between starburst regions and harder ionization sources.
In the text

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