Swift/XRT counterparts to unassociated Fermi high-energy LAT sources

TL;DR

This study analyzes Swift/XRT archival data to identify X-ray counterparts of unassociated Fermi LAT sources, finding likely extragalactic blazar associations, mainly BL Lac objects, through multi-wavelength analysis.

Contribution

It provides the first systematic X-ray follow-up of unassociated Fermi sources, improving source identification and classification.

Findings

13 out of 23 regions showed X-ray emission

Most associations are compatible with Fermi positional uncertainties

Majority of identified counterparts are likely blazars, especially BL Lac types

Abstract

We report the results from our analysis of a large set of archival data acquired with the X-ray telescope (XRT) onboard Swift, covering the sky region surrounding objects from the first Fermi Large Area Telescope (LAT) catalogue of high-energy sources (1FHL), which still lack an association. Of the 23 regions analysed, ten did not show any evidence of X-ray emission, but 13 were characterised by the presence of one or more objects emitting in the 0.3-10 keV band. Only in a couple of cases is the X-ray counterpart located outside the Fermi positional uncertainty, while in all other cases the associations found are compatible with the high-energy error ellipses. All counterparts we found have been studied in detail by means of a multi-waveband approach to evaluate their nature or class; in most cases, we have been able to propose a likely or possible association except for one Fermi source whose nature remains doubtful at the moment. The majority of the likely associations are extragalactic in nature, most probably blazars of the BL Lac type.