A Shock Front in the Merging Galaxy Cluster A754: X-Ray and Radio Observations

Abstract

We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster Abell 754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas temperature jump across the X-ray brightness edge previously seen in the imaging data. A 754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number M= 1.57 +0.16 −0.12 . In our new GMRT observation at 330 MHz, we find that the previously-known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74 MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of preexisting relativistic electrons is more plausible.

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

“…A similar situation is observed in the Bullet cluster (Shimwell et al 2014) and in Abell520 Markevitch 2010;Vacca et al 2014), the Coma Cluster (Planck Collaboration et al 2013; Uchida et al 2015), and Abell754 (Macario et al 2011). Therefore, this seems to be a common phenomenon.…”

“…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

“…Macario et al 2011). Therefore this appears to be a common phenomenon and it does suggest that some relation exists between cluster shocks and radio halos.…”

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

“…A problem with this scenario is that shocks in clusters generally have Mach number of   3and the acceleration of electrons from the thermal pool is thought to be very inefficient for these low Mach numbers, in apparent conflict with the presence of bright radio relics. In this case, an unrealistic fraction of the energy flux through the shock surface (Macario et al 2011;Eckert et al 2016;van Weeren et al 2016a) needs to be converted to the non-thermal electron population.…”