Shock acceleration as origin of the radio relic in A 521?

Abstract

Aims. We present new high sensitivity observations of the radio relic in A 521 carried out using the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. Methods. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327−610 MHz. In our present analysis we use our new GMRT observations in addition to proprietary and archival data. We search for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. Results. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is supported by our detection of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is probably a shock front.

Chandra and JVLA Observations of HST Frontier Fields Cluster MACS J0717.5+3745

“…Radio relics often display spectral index gradients, with the spectral index steepening in the direction of the cluster center (e.g., Clarke & Enßlin 2006;Giacintucci et al 2008;van Weeren et al 2010;Bonafede et al 2012;Kale et al 2012;Stroe et al 2013). This spectral steepening is explained by synchrotron and Inverse Compton (IC) losses in the post-shock region of an outward traveling shock front.…”

“…Another model to solve the low acceleration efficiency of standard DSA is that of re-acceleration of fossil electrons (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Kang et al 2012;Pinzke et al 2013). These fossil electrons could, for example, originate from the (old) lobes of radio galaxies.…”

“…A problem with this interpretation is that shock Mach numbers in clusters are typically low (  3), in which case DSA is thought to be inefficient. For that reason, several alternative models have been proposed including shock re-acceleration (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Kang et al 2012;Pinzke et al 2013) and turbulent re-acceleration (Fujita et al 2015). Recent work from particle-in-cell (PIC) simulations indicates that cluster shocks can inject electrons from the thermal pool (Guo et al 2014a(Guo et al , 2014band that these electrons gain energy via the shock drift acceleration (SDA) mechanism.…”

Lofar, Vla, and Chandra Observations of the Toothbrush Galaxy Cluster

“…There is a considerable amount of evidence that radio relics trace shock fronts where particles are being (re-)accelerated, as was proposed initially in Enßlin et al (1998). The key observational facts that support the shock-radio relic connection are (i) the high polarization of some relics, with the apparent magnetic field lines being parallel to the major axis of the relics (e.g., van Weeren et al 2010;Bonafede et al 2012;Kale et al 2012;de Gasperin et al 2014), indicating that the ICM and associated magnetic fields are compressed, (ii) the presence of spectral index gradients, indicating electron cooling in the post-shock region of an outward traveling shock wave (e.g., Giacintucci et al 2008;van Weeren et al 2010van Weeren et al , 2011Stroe et al 2013;Hindson et al 2014), (iii) a change from power-law radio spectra at the outer edge of relics toward curved spectra in the post-shock region, indicating a site of acceleration and electron cooling (van Weeren et al 2012a;Stroe et al 2013), and (iv) the presence of ICM density and/or temperature jumps at the location of relics (e.g., Finoguenov et al 2010;Macario et al 2011;Akamatsu & Kawahara 2013;Bourdin et al 2013;Ogrean et al 2014a). X-ray observations indicate that the shock Mach numbers () are low, typically 3  .…”

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

“…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).…”