LOFAR, VLA, and Chandra observations of the Toothbrush galaxy cluster

TL;DR

This study uses deep LOFAR, VLA, GMRT, and Chandra data to analyze the Toothbrush galaxy cluster, revealing detailed spectral properties of radio relics and halos, and highlighting discrepancies in shock Mach number estimates.

Contribution

It introduces a novel calibration scheme enabling deeper low-frequency observations and provides new insights into shock acceleration mechanisms in galaxy clusters.

Findings

Spectral index of the radio relic varies from -0.8 to -2 across the structure.

Radio halo exhibits a remarkably uniform spectral index of -1.16.

Discrepancy between radio-derived and X-ray-derived Mach numbers suggests complex shock structures.

Abstract

We present deep LOFAR observations between 120-181 MHz of the "Toothbrush" (RX J0603.3+4214), a cluster that contains one of the brightest radio relic sources known. Our LOFAR observations exploit a new and novel calibration scheme to probe 10 times deeper than any previous study in this relatively unexplored part of the spectrum. The LOFAR observations, when combined with VLA, GMRT, and Chandra X-ray data, provide new information about the nature of cluster merger shocks and their role in re-accelerating relativistic particles. We derive a spectral index of $\alpha = -0.8 \pm 0.1$ at the northern edge of the main radio relic, steepening towards the south to $\alpha \approx - 2$. The spectral index of the radio halo is remarkably uniform ($\alpha = -1.16$, with an intrinsic scatter of $\leq 0.04$). The observed radio relic spectral index gives a Mach number of $\mathcal{M} = 2.8^{+0.5}_{-0.3}$, assuming diffusive shock acceleration (DSA). However, the gas density jump at the northern edge of the large radio relic implies a much weaker shock ($\mathcal{M} \approx 1.2$, with an upper limit of $\mathcal{M} \approx 1.5$). The discrepancy between the Mach numbers calculated from the radio and X-rays can be explained if either (i) the relic traces a complex shock surface along the line of sight, or (ii) if the radio relic emission is produced by a re-accelerated population of fossil particles from a radio galaxy. Our results highlight the need for additional theoretical work and numerical simulations of particle acceleration and re-acceleration at cluster merger shocks.