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

TL;DR

This paper catalogs 251 potential counterparts for unassociated Swift-BAT 150-month hard X-ray sources, using soft X-ray observations from multiple telescopes to identify and classify their nature.

Contribution

It provides the first comprehensive soft X-ray follow-up for these sources, improving identification and classification of previously unassociated hard X-ray detections.

Findings

37% are active galactic nuclei

23% are galaxies

Remaining sources include pulsars and cataclysmic variables

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.