A search for VHE counterparts of Galactic Fermi bright sources and MeV to TeV spectral characterization

TL;DR

This study searches for VHE gamma-ray counterparts to Galactic Fermi bright sources, compares their spectra from MeV to TeV energies, and finds correlations and spectral complexities that suggest multiple emission components.

Contribution

It provides the first comprehensive analysis of spatially coincident VHE and Fermi sources, revealing spectral differences and supporting the pulsar nebula hypothesis.

Findings

Bright LAT GeV sources are correlated with TeV sources.

Many sources exhibit spectra that cannot be described by a single component.

VHE observations can test the pulsar nebula hypothesis.

Abstract

Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide range of astronomical objects, such as pulsar wind nebulae (PWNe), supernova remnants (SNRs), giant molecular clouds, gamma-ray binaries, the Galactic Center, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and possibly star-forming regions as well. At lower energies, observations using the Large Area Telescope (LAT) onboard Fermi provide a rich set of data which can be used to study the behavior of cosmic accelerators in the MeV to TeV energy bands. In particular, the improved angular resolution of current telescopes in both bands compared to previous instruments significantly reduces source confusion and facilitates the identification of associated counterparts at lower energies. In this paper, a comprehensive search for VHE gamma-ray sources which are spatially coincident with Galactic Fermi/LAT bright sources is performed, and the available MeV to TeV spectra of coincident sources are compared. It is found that bright LAT GeV sources are correlated with TeV sources, in contrast to previous studies using EGRET data. Moreover, a single spectral component seems unable to describe the MeV to TeV spectra of many coincident GeV/TeV sources. It has been suggested that gamma-ray pulsars may be accompanied by VHE gamma-ray emitting nebulae, a hypothesis that can be tested with VHE observations of these pulsars.