Swift-XRT follow-up analysis of unidentified hard X-ray sources

Searching for soft X-ray counterparts of unidentified hard X-ray sources

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno 611 37, Czech Republic
² Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
³ East Asian Observatory, 660 North A’ohōkū Place, Hilo, HI 96720, USA
⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
⁵ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 101, 40129 Bologna, Italy
⁷ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁸ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla 72840, Mexico
⁹ Dipartimento di Fisica e Astronomia G. Galilei, Univeristà di Padova, Padova, Italy
¹⁰ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
¹¹ W. W. Hansen Experimental Physics Laboratory & Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA

^★ Corresponding author; matej.kosiba@gmail.com

Received: 27 October 2023
Accepted: 30 July 2024

Abstract

Context. It has been established that the sources contributing to the cosmic X-ray background (CXB) emission are mainly nearby active galactic nuclei (AGNs), in particular those that are obscured. Thus, it is important to fully identify the hard X-ray sky source population to accurately characterize the individual contribution of different AGNs to the overall CXB emission.

Aims. We present a follow-up analysis of all 218 sources marked as unidentified in our previous revision of the third release of the Palermo Swift-BAT hard X-ray catalog (3PBC) based on our multifrequency classification scheme. These 218 sources were classified as unidentified in our previous analyses because they lacked an assigned low-energy counterpart.

Methods. We searched for soft X-ray counterparts of these 218 3PBC sources in archival Swift-XRT observations obtained between January 1, 2005, and August 1, 2018. In particular, we found 1213 archival Swift-XRT observations for 192 of the 218 unidentified sources.

Results. We find 93 possible Swift-XRT counterparts within the Swift-BAT positional uncertainty regions. They correspond to 73 3PBC sources, 60 of which have only a single Swift-XRT detection; the rest have multiple detections. We present all the detected possible counterparts of the as-of-yet-unidentified hard X-ray sources to the community as a catalog for future spectroscopic follow-up targets, together with a short catalog of our classification of the ten sources for which there were available spectra.