Discovery of spectral curvature in the shock downstream region: CIZA J2242.8+5301

Abstract

Context. Giant cluster radio relics are thought to form at shock fronts in the course of collisions between galaxy clusters. Via processes that are still poorly understood, these shocks accelerate or re-accelerate cosmic-ray electrons and might amplify magnetic fields. The best object to study this phenomenon is the galaxy cluster CIZA J2242.8+5301 as it shows the most undisturbed relic. By means of Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT) data at seven frequencies spanning from 153 MHz to 2272 MHz, we study the synchrotron emission in this cluster. Aims. We aim at distinguishing between theoretical injection and acceleration models proposed for the formation of radio relics. We also study the head-tail radio sources to reveal the interplay between the merger and the cluster galaxies. Methods. We produced spectral index, curvature maps, and radio colour-colour plots and compared our data with predictions from models.Results. We present one of the deepest 153 MHz maps of a cluster ever produced, reaching a noise level of 1.5 mJy beam −1 . We derive integrated spectra for four relics in the cluster, discovering extremely steep spectrum diffuse emission concentrated in multiple patches. We find a possible radio phoenix embedded in the relic to the south of the cluster. The spectral index of the northern relic retains signs of steepening from the front towards the back of the shock also at the radio frequencies below 600 MHz. The spectral curvature in the same relic also increases in the downstream area. The data is consistent with the Komissarov-Gubanov injection models, meaning that the emission we observe is produced by a single burst of spectrally-aged accelerated radio electrons.

The rise and fall of star formation in z ∼ 0.2 merging galaxy clusters

“…1). The bright radio relics show clear signs of steepening and curving radio spectrum from the shock front into the downstream area, suggesting a scenario where the synchrotron electrons are shock accelerated and subsequently cool (van Weeren et al 2010;Stroe et al 2013). Nevertheless, the high-frequency, 16 GHz observations of the 'Sausage' relic suggests a more complicated scenario in which the electrons are injected and accelerated also in the downstream area (Stroe et al 2014b).…”

“…In the case of the 'Sausage' cluster, a binary merger lead to the formation of two unequal relics along the merger axis (van Weeren et al 2011). The northern relic has a stronger Mach number than the southern one by a factor of ∼ 2 (Stroe et al 2013). In the 'Toothbrush' cluster, simulations and X-ray data indicate a more complicated merger scenario with a main binary merger, possibly followed by a smaller mass interloper participating in the merger towards the end (Brüggen et al 2012;Ogrean et al 2013b).…”

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.…”

The First Murchison Widefield Array low-frequency radio observations of cluster scale non-thermal emission: the case of Abell 3667

“…The NW relic has a length and width of approximately 1.9 by 0.9 Mpc, respectively. If we were viewing the NW relic edge-on, we would expect a much larger ratio between the length and width such as is seen in the toothbrush relic and the relics in CIZA J2242.8+5301 (van Weeren et al 2012a;Stroe et al 2013). If we assume that the relic traces a planar shock located in the xy plane (i.e.…”