A shock at the radio relic position in Abell 115

TL;DR

This study reports the detection of a shock coinciding with a radio relic in galaxy cluster Abell 115, supporting the idea that shocks re-accelerate electrons to produce radio relics during cluster mergers.

Contribution

First clear detection of a shock at the radio relic position in Abell 115 using deep Chandra data, linking shocks to relic formation and suggesting re-acceleration of electrons.

Findings

Shock detected with density compression factor of 2.0±0.1

Mach number of the shock estimated at 1.7±0.1 from density profile

Temperature measurements support the shock detection and Mach number estimate

Abstract

We analyzed a deep Chandra observation (334 ks) of the galaxy cluster Abell 115 and detected a shock co-spatial with the radio relic. The X-ray surface brightness profile across the shock region presents a discontinuity, corresponding to a density compression factor $\mathcal{C}=2.0\pm0.1$ leading to a Mach number $\mathcal{M}=1.7\pm0.1$ ($\mathcal{M}=1.4-2$ including systematics). Temperatures measured in the upstream and downstream regions are consistent with what expected for such a shock: $T_u=4.3^{+1.0}_{-0.6}\:\rm{keV}$ and $T_d=7.9^{+1.4}_{-1.1}\:\rm{keV}$ respectively, implying a Mach number $\mathcal{M}=1.8^{+0.5}_{-0.4}$. So far, only few other shocks discovered in galaxy clusters are consistently detected from both density and temperature jumps. The spatial coincidence between this discontinuity and the radio relic edge strongly supports the view that shocks play a crucial role in powering these synchrotron sources. We suggest that the relic is originated by shock re-acceleration of relativistic electrons rather than acceleration from the thermal pool. The position and curvature of the shock and the associated relic are consistent with an off-axis merger with unequal mass ratio where the shock is expected to bend around the core of the less massive cluster.