XMM-Newton Observations of the Southeastern Radio Relic in Abell 3667

TL;DR

This study uses XMM-Newton observations to confirm a weak shock in the southeastern radio relic of galaxy cluster Abell 3667, providing insights into the mechanisms behind radio relic formation and challenging existing acceleration models.

Contribution

First XMM-Newton measurement of the southeastern relic in Abell 3667 confirming a weak shock, advancing understanding of radio relic origins in galaxy clusters.

Findings

Confirmed a shock with Mach number ~1.8 in A3667's southeastern relic.

Detected temperature jump consistent with previous Suzaku observations.

Discussed implications for diffusive shock acceleration and alternative scenarios.

Abstract

Radio relics, elongated, non-thermal, structures located at the edges of galaxy clusters, are the result of synchrotron radiation from cosmic-ray electrons accelerated by merger-driven shocks at the cluster outskirts. However, X-ray observations of such shocks in some clusters suggest that they are too weak to efficiently accelerate electrons via diffusive shock acceleration to energies required to produce the observed radio power. We examine this issue in the merging galaxy cluster Abell 3667 (A3667), which hosts a pair of radio relics. While the Northwest relic in A3667 has been well studied in the radio and X-ray by multiple instruments, the Southeast relic region has only been observed so far by Suzaku, which detected a temperature jump across the relic, suggesting the presence of a weak shock. We present observations of the Southeastern region of A3667 with XMM-Newton centered on the radio relic. We confirm the existence of an X-ray shock with Mach number of about 1.8 from a clear detection of temperature jump and a tentative detection of a density jump, consistent with previous measurements by Suzaku. We discuss the implications of this measurement for diffusive shock acceleration as the main mechanism for explaining the origin of radio relics. We then speculate on the plausibility of alternative scenarios, including re-acceleration and variations in the Mach number along shock fronts.