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

TL;DR

This study conducts a comprehensive follow-up analysis of 218 unidentified hard X-ray sources from the Swift-BAT catalog, using archival Swift-XRT data to identify potential soft X-ray counterparts and facilitate future classification.

Contribution

It provides the first systematic search for soft X-ray counterparts of previously unidentified hard X-ray sources, creating a catalog for future spectroscopic follow-up.

Findings

93 potential counterparts identified within BAT positional uncertainties

73 sources have possible Swift-XRT counterparts, with some having multiple detections

A catalog of counterparts and classifications is provided for future studies

Abstract

It is currently established that the sources contributing to the cosmic X-ray background (CXB) emission are mainly nearby active galactic nuclei (AGN), 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. We present a follow-up analysis of all the 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 lack an assigned low-energy counterpart. We searched for soft X-ray counterparts of these 218 3PBC sources in archival Swift-XRT observations obtained between 2005 January 1st and 2018 August 1st. In particular, we found 1213 archival Swift-XRT observations for 192 of the 218 unidentified sources. We found 93 possible Swift-XRT counterparts inside of the Swift-BAT positional uncertainty regions. These correspond to 73 3PBC sources, where 60 have only a single Swift-XRT detection, and 13 sources have multiple detections. We present a catalog of all the detected possible counterparts of the 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 we found available spectra.