Identification of the Early Fermi LAT Gamma-Ray Bright Objects with Extragalactic VLBI sources

TL;DR

This study confirms that most gamma-ray bright extragalactic objects detected by Fermi LAT are radio loud blazars with strong jets, and demonstrates the effectiveness of using VLBI catalogs for their identification.

Contribution

It provides a robust method for identifying gamma-ray sources by cross-matching Fermi LAT data with VLBI radio source catalogs, confirming previous associations and suggesting new ones.

Findings

Most gamma-ray bright sources are radio loud blazars.

The fraction of chance associations is less than 5%.

VLBI catalogs are effective for identifying gamma-ray sources.

Abstract

A list of 205 gamma-ray strong objects was reported recently as a result of a 3-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the gamma-ray positions with VLBI positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed and six new identifications are suggested. A Monte-Carlo analysis shows that the fraction of chance associations in our analysis is less than 5 per cent, and confirms that the vast majority of gamma-ray bright extragalactic sources are radio loud blazars with strong parsec-scale jets. A correlation between the parsec-scale radio and gamma-ray flux is supported by our analysis of a complete VLBI flux-density-limited sample of extragalactic jets. The effectiveness of using a VLBI catalog to find associations between gamma-ray detections and compact extragalactic radio sources, especially near the Galactic plane, is demonstrated. It is suggested that VLBI catalogs should be used for future identification of Fermi LAT objects.