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

¹ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
² Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA
³ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
⁶ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
⁷ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
⁸ INFN, Sezione di Catania, Catania, Italy
⁹ Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy
¹⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 June 2025
Accepted: 22 January 2026

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. During 150 months of observation, BAT detected 2339 sources in the 15-150 keV range. Among these, 344 do not have a low-energy counterpart. Our study focuses on the analysis of soft X-ray observations (i.e., at energies <10 keV) spatially overlapping with these Swift-BAT new hard X-ray sources. These observations were taken with Chandra, Swift-XRT, eROSITA, and XMM-Newton. Within the sample of 251 potential counterparts, 94 (37%) are identified as active galactic nuclei (AGN) and 58 (23%) as galaxies. The rest of the 99 sources (40%) include pulsars, cataclysmic variables, and unclassified soft X-ray (0.5-10 keV) counterparts. Redshift data is available for 139 out of the 251 sources, and its distribution are in close agreement with the redshift distribution of previous BAT catalogs. We also present the results of a small optical spectroscopy campaign of 9 of the 58 galaxies. The majority of these turned out to be Seyfert 2 galaxies at a redshift slightly higher than the median of the BAT AGN sample.