No Shock Across Part of a Radio Relic in the Merging Galaxy Cluster ZwCl 2341.1+0000?

TL;DR

This study investigates the galaxy cluster ZwCl 2341.1+0000, revealing unexpected low Mach number shocks associated with radio relics, challenging existing theories of shock acceleration in merging galaxy clusters.

Contribution

The paper presents combined X-ray and radio observations of ZwCl 2341.1+0000, highlighting discrepancies between observed shock properties and standard models of radio relic formation.

Findings

Detected density discontinuities near radio relics with low Mach numbers.

Found that the extent of density discontinuities does not match the radio relics' arcs.

Results challenge current understanding of shock acceleration in galaxy clusters.

Abstract

The galaxy cluster ZwCl 2341.1+0000 is a merging system at z=0.27, which hosts two radio relics and a central, faint, filamentary radio structure. The two radio relics have unusually flat integrated spectral indices of -0.49 +/- 0.18 and -0.76 +/- 0.17, values that cannot be easily reconciled with the theory of standard diffusive shock acceleration of thermal particles at weak merger shocks. We present imaging results from XMM-Newton and Chandra observations of the cluster, aimed to detect and characterise density discontinuities in the ICM. As expected, we detect a density discontinuity near each of the radio relics. However, if these discontinuities are the shock fronts that fuelled the radio emission, then their Mach numbers are surprisingly low, both <=2. We studied the aperture of the density discontinuities, and found that while the NW discontinuity spans the whole length of the NW radio relic, the arc spanned by the SE discontinuity is shorter than the arc spanned by the SE relic. This startling result is in apparent contradiction with our current understanding of the origin of radio relics. Deeper X-ray data are required to confirm our results and to determine the nature of the density discontinuities.