Detection of a New GeV Source in the Outer Region of the Coma Cluster: A Signature of External Accretion Shock?

Abstract The supersonic flow motions associated with infall of baryonic gas toward sheets and filaments, as well as cluster mergers, produces large-scale shock waves. The shocks associated with galaxy clusters can be classified mainly into two categories: internal shocks appearing in the hot intracluster medium within the viral radius and external accretion shocks forming in the outer cold region well outside of the virial radius. Cosmic-ray (CR) electrons and/or protons accelerated by these shocks are expected to produce gamma rays through inverse-Compton scattering (ICS) or inelastic pp collisions, respectively. Recent studies have found a spatially extended GeV source within the virial radius, consistent with the internal shock origin. Here, we report the detection of a new GeV source at a distance of about 2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 8 from the center of the Coma cluster through the analysis of 16.2 yr of Fermi-LAT data. The hard spectrum of the source, in agreement with the ICS origin, and its location in a large-scale filament of galaxies points to the external accretion shock origin. The gamma-ray (0.1−10 3 GeV) luminosity of the source, 1.4 × 10 42 erg s −1 , suggests that a fraction ~10 −3 of the kinetic energy flux through the shock surface is transferred to relativistic CR electrons.