Identifying the 3FHL Catalog. IV. Swift Observations of Unassociated Fermi-LAT 3FHL Sources

TL;DR

This study uses X-ray observations and machine learning to identify and classify unassociated high-energy sources in the 3FHL catalog, primarily finding likely BL Lac objects, enhancing the catalog's completeness.

Contribution

It provides the first X-ray analysis of 38 3FHL sources and applies machine learning to classify potential blazars, improving source identification.

Findings

Most X-ray detected sources are extragalactic.

15 sources are likely BL Lac objects.

BL Lacs dominate the >10 GeV source population.

Abstract

The Fermi Large Area Telescope (Fermi-LAT) 3FHL catalog is the latest catalog of >10 GeV sources and will remain an important resource for the high-energy community for the foreseeable future. Therefore, it is crucial that this catalog is made complete by providing associations for most sources. In this paper, we present the results of the X-ray analysis of 38 3FHL sources. We found a single bright X-ray source in 20 fields, two sources each in two fields and none for the remaining 16. The analysis of the properties of the 22 3FHL fields with X-ray sources led us to believe that most (19/22) are of extra-galactic origin. A machine-learning algorithm was used to determine the source type and we find that 15 potential blazars are likely BL Lacertae objects (BL Lacs). This is consistent with the fact that BL Lacs are by far the most numerous population detected above >10 GeV in the 3FHL.