Bow Shock and Radio Halo in the Merging Cluster A520

Abstract

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M = 2.1 +0.4 −0.3 . This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide pre-energized electrons for subsequent turbulent re-acceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of α ≃ 1.2 right behind the edge with quick steepening further away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of pre-existing relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such pre-shock emission, then the electrons are indeed accelerated (or re-accelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.

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

“…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).…”

Probing the nature of dark matter with cosmic x rays: Constraints from “dark blobs” and grating spectra of galaxy clusters

“…A unique example is the merging cluster of galaxies Abell 520 [24] containing a ''blob'' of high mass concentration with very low x-ray emission discovered recently using weak gravitational lensing; see Fig. 1 [25].…”

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

“…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.…”