The GeV counterpart of VER J2019+407 in the northern shell of the supernova remnant G78.2+2.1 ($\gamma$ Cygni)

TL;DR

This study analyzes 7.2 years of Fermi-LAT gamma-ray data from supernova remnant G78.2+2.1, revealing a distinct GeV emission component associated with TeV source VER J2019+407, and explores possible origins involving hadronic and leptonic processes.

Contribution

First detailed GeV-TeV spectral analysis of G78.2+2.1 revealing a broken power-law spectrum and potential emission mechanisms.

Findings

Detection of a hard, extended GeV gamma-ray component.

Spectral fit with a broken power-law with indices 1.8 and 2.5.

Both hadronic and leptonic models can explain the observations.

Abstract

Analysis of gamma-ray emission from the supernova remnant G78.2+2.1 ($\gamma$ Cygni) with 7.2 years of cumulative data from the Fermi-LAT telescope shows a distinct hard, bright and extended component to the north of the shell coincident with the known TeV source VER J2019+407. In the GeV-TeV energy range its spectrum is best described by a broken power-law with indices 1.8 below a break energy of 71 GeV and 2.5 above the break. A broadband spectral energy distribution is assembled and different scenarios for the origin of the gamma-rays are explored. Both hadronic and leptonic mechanisms are able to account for the GeV-TeV observations. In the leptonic framework, a superposition of inverse Compton and nonthermal bremsstrahlung emissions is needed whereas the hadronic scenario requires a cosmic ray population described by a broken power-law distribution with a relatively hard spectral index of $\sim1.8$ below a break particle energy of 0.45 TeV. In addition, the neutrino flux expected from cosmic ray interactions is calculated.