Source identification for the Swift-BAT 150-month hard X-ray catalog using soft X-ray observations

TL;DR

The authors identify soft X-ray counterparts for 250 unassociated Swift-BAT sources in the 150-month catalog, using Chandra, XMM-Newton, Swift-XRT, and eROSITA data to achieve accurate positions and classifications. They introduce a structured association framework with flags to handle single vs. multiple candidates and eROSITA associations, and provide a 251-entry catalog detailing counterpart types, redshifts, and cross-matches to 4FGL. Their analysis reveals a higher fraction of normal galaxies (likely AGN hosts) and confirms redshift distributions comparable to prior BAT catalogs, while highlighting the importance of optical follow-up for unclassified sources. The results enhance the census of AGN, including obscured and potential Compton-thick candidates, and establish a resource for future X-ray and multiwavelength studies of the hard X-ray sky.

Abstract

We present a comprehensive catalog of 251 potential counterparts for 250 unassociated hard X-ray sources detected in the Swift Burst Alert Telescope (BAT) 150-month hard X-ray survey. Over 150 months of observation, BAT has detected 2339 sources in the 15-150 keV energy range. Among these, 344 do not have a previously identified low-energy counterpart. Our study focuses on the analysis of soft X-ray observations at energies below 10 keV, spatially overlapping with these new Swift-BAT hard X-ray sources. Such observations were taken with Chandra, Swift-XRT, eROSITA, and XMM-Newton. Within the sample of 251 potential counterparts, 94 (37 percent) are identified as active galactic nuclei and 58 (23 percent) as galaxies. The remaining 99 sources (40 percent) include pulsars, cataclysmic variables, and unclassified soft X-ray counterparts in the 0.5-10 keV band. Redshift information is available for 139 out of the 251 sources, and its distribution is in close agreement with the redshift distribution of previous BAT catalogs. We also present the results of a small optical spectroscopy campaign of 9 out of 58 galaxies. The majority of these are classified as Seyfert 2 galaxies at redshifts slightly larger than the median of the BAT AGN sample.

Figures (9)

• Figure 1: Swift-XRT observation of a single source (smaller circle) in the BAT $R_{95}$ (bigger circle) for the BAT source J1130.5+6848 in the energy band 0.3--10 keV. Only one bright soft X-ray source (whose optical counterpart is the galaxy LEDA 139586) is present within the BAT $R_{95}$, which we associate as the BAT counterpart.
• Figure 2: Swift-XRT observation for the BAT source J1937.4-4013 in the energy band 0.3--5 keV (left panel) and 5.1--10 keV (right panel). There are two soft X-ray sources within the BAT $R_{95}$, WISEA J193714.86--401014.4 and LEDA 588288 with flux 1.2 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$ and 6.5 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$ respectively in the energy band 0.3--10 keV. The spectral fitting and power-law extrapolation predicts a flux of 1.1 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$ and 6.1 $\times$ 10$^{-10}$ erg s$^{-1}$ cm$^{-2}$ for WISEA J193714.86--401014.4 and LEDA 588288 respectively in the energy band 15--150 keV, so we associated LEDA 588288 to the BAT source.
• Figure 3: Redshift distribution of 135 BAT detected sources for which a redshift information was obtained thanks to the associations provided by our analysis. The AGN sources includes Sy1/Sy2/SyG, AGN and QSO, whereas the Galaxies are galaxies in which AGN activity has not been reported until now.
• Figure 4: All-sky map showing the classification of the 344 sources from BAT 150-month survey sources. The figure uses a Hammer-Aitoff projection in Galactic coordinates.
• Figure 5: A normalized source type comparison with PBC 100 month catalog and BASS 105 month catalog. The beamed AGN includes blazars, FSRQ and QSO. The galaxies are classified as unknown AGN in the BASS 105 month catalog. Based on their X-ray luminosity, we can safely claim that all the sources optically classified as galaxies actually host an AGN.