Spectral index image of the radio halo in the cluster Abell 520, which hosts the famous bow shock

Abstract

Context. Synchrotron-radio emission has been detected from an increasing number of galaxy clusters. Spectral index images are a powerful tool for investigating the origin and nature of these sources, and their connection with the dynamical state of the cluster. Aims. The aim of this work is to investigate the spectral index distribution of the radio halo in the galaxy cluster A520, known to be a complex system from an optical, radio, and X-ray point of view. Methods. We present deep Very Large Array observations in total intensity at 325 and 1400 MHz. We produced and analyzed spectral index images of the radio halo in this frequency range at a resolution of 39 and 60 and determined possible correlations with the thermal properties of the cluster. Results. We found an integrated radio-halo spectral index α 1400 325 ∼ 1.12. No strong radial steepening is present and the spectral index distribution is intrinsically complex with fluctuations only partially due to measurement errors. The radio-halo-integrated spectral index and the cluster temperature follow the general trend observed in other galaxy clusters, although a strong point-to-point correlation between the spectral index and the thermal gas temperature has not been observed. Conclusions. The complex morphology in the spectral index image of the radio halo in A520 agrees with the primary models for radio-halo formation. The flatness of the radial profile suggests that the merger is still ongoing and is uniformly and continuously (re-) accelerating the population of relativistic electrons responsible of the radio emission even at large (∼1 Mpc) distances from the cluster center.

VLA Radio Observations of the HST Frontier Fields Cluster Abell 2744: The Discovery of New Radio Relics

“…We note that Orrú et al (2007) did not provide a statistical measure of the significance of the correlation. Our results are also consistent with recent work by Vacca et al (2014) and van Weeren et al (2016). We therefore conclude that currently there is no convincing evidence of the existence of a spatial correlation between X-ray temperature and spectral index for individual radio halos.…”

“…This is confirmed by comparing the median error value of the distributions with the corresponding standard deviations. As described in Vacca et al (2014), if the variations in spectral index are the result of measurement errors then we expect our median error value to be of comparable size to the standard deviation. At 15″, since the median error is almost equal in magnitude to the standard deviation, we conclude that the fluctuations in the halo spectral index are not significant.…”

“…In the turbulent reacceleration model, electrons are reaccelerated via magnetohydrodynamical turbulence (e.g., Schlickeiser et al 1987;Brunetti et al 2001;Petrosian 2001). In this case, spectral index variations across radio halos should be related to underlying spatial variations in the turbulent energy and magnetic field strength (Feretti et al 2004;Orrú et al 2007;Vacca et al 2014). The most reliable spectral index map of a radio halo that has been made so far, using LOFAR and VLA data, is the one for the Toothbrush cluster (van Weeren et al 2016).…”

“…The leading theory for relics is that they trace particles (re) accelerated at shocks (e.g., Enßlin et al 1998;Markevitch et al 2005;Kang & Ryu 2011;van Weeren et al 2017a). The radio spectral index is related to the slope of the underlying electron…”

“…Giant radio relics are arc-like sources of synchrotron radiation of megaparsec size. The leading theory behind their formation is that of shock acceleration (Enßlin et al 1998;Drury 1983) of either thermal or preaccelerated fossil electrons from AGNs or other radio galaxy activity (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Pinzke et al 2013;van Weeren et al 2017a), although other models exist (Fujita et al 2015).…”