Bow shock in merging cluster A520: the edge of the radio halo and the electron-ion equilibration timescale

TL;DR

This study investigates the bow shock in galaxy cluster A520 using X-ray and radio data, revealing rapid electron-ion equilibration and the shock's impact on the radio halo structure.

Contribution

It provides new measurements of shock Mach numbers, electron temperature equilibration timescales, and the correlation between shock fronts and radio halo edges in A520.

Findings

Electron temperature behind the shock matches Rankine-Hugoniot predictions.

The shock shows signs of faster-than-Coulomb electron-ion equilibration.

The radio halo edge coincides with the shock front, with no pre-shock emission detected.

Abstract

We studied the prominent bow shock in the merging galaxy cluster A520 using a deep Chandra X-ray observation and archival VLA radio data. This shock is a useful diagnostic tool, owing to its clear geometry and relatively high Mach number. At the "nose" of the shock, we measure a Mach number of $M=2.4_{-0.2}^{+0.4}$. The shock becomes oblique away from the merger axis, with the Mach number falling to $\simeq$1.6 around 30$^{\circ}$ from the nose. The electron temperature immediately behind the shock nose is consistent with that from the Rankine-Hugoniot adiabat, and is higher (at a 95% confidence) than expected for adiabatic compression of electrons followed by Coulomb electron-proton equilibration, indicating the presence of equilibration mechanisms faster than Coulomb collisions. This is similar to an earlier finding for the Bullet cluster. We also combined four archival VLA datasets to obtain a better image of the cluster's giant radio halo at 1.4 GHz. An abrupt edge of the radio halo traces the shock front, and no emission is detected in the pre-shock region. If the radio edge were due only to adiabatic compression of relativistic electrons in pre-shock plasma, we would expect a pre-shock radio emission detectable in this radio dataset; however, an interferometric artifact dominates the uncertainty, so we cannot rule this model out. Other interesting features of the radio halo include a peak at the remnant of the cool core, suggesting that the core used to have a radio minihalo, and a peak marking a possible region of high turbulence.