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

Abstract

Cluster mergers leave distinct signatures in the intracluster medium (ICM) in the form of shocks and diffuse cluster radio sources that provide evidence for the acceleration of relativistic particles. However, the physics of particle acceleration in the ICM is still not fully understood. Here we present new 1-4 GHz Jansky Very Large Array (VLA) and archival Chandra observations of the HST Frontier Fields Cluster Abell2744. In our new VLA images, we detect the previously known ∼2.1Mpc radio halo and ∼1.5Mpc radio relic. We carry out a radio spectral analysis from which we determine the relic's injection spectral index to be a = - 1.12 0. 0.31 under the assumption of diffusive shock acceleration. We also find evidence for spectral steepening in the post-shock region. We do not find evidence for a significant correlation between the radio halo's spectral index and ICM temperature. In addition, we observe three new polarized diffuse sources and determine two of these to be newly discovered giant radio relics. These two relics are located in the southeastern and northwestern outskirts of the cluster. The corresponding integrated spectral indices measure −1.81±0.26 and −0.63±0.21 for the SE and NW relics, respectively. From an X-ray surface brightness profile we also detect a possible density jump of = -+ R 1.39 0.22 0.34 co-located with the newly discovered SE relic. This density jump would correspond to a shock front Mach number of  = -+ 1.26 0.15 0.25 .

Polarization of radio relics in galaxy clusters

“…The relic population shows a vast range of the orientation of the polarisation vectors. In the cluster CIZA J2242.8+5301, the polarisation E-vectors align perfectly with the shock normal across the entire length of the relic, ∼ 2 Mpc, (see van Weeren et al 2010) and other relics, such as Abell 2744 (see Pearce et al 2017) or ZwCl 0008.8+5215 (see van Weeren et al 2011b;Golovich et al 2017) show a similar behaviour. For a sufficiently strong shock, the magnetic field compression at the shock front could explain the alignment of the shock normal and the polarisation E-vector.…”

“…Footnotes: a: taken fromGolovich et al (2018); b: we cite the Mach number derived from the integrated spectral index, while Hoang et al (2018) also use the injection spectral index to compute the Mach number. Using the injection spectral index, they obtain Mach numbers of ∼ 2.4 and ∼ 2.3 for Abell 1240-1 and Abell 1240-2 respectively; c: BothGolovich et al (2017) andPearce et al (2017) only provide a frequency range, 2000 − 4000 GHz, hence, we assume the mean of 3000 GHz; d: we assume this value, following the caption of…”

The discovery of radio halos in the frontier fields clusters Abell S1063 and Abell 370

“…In this paper, we present new 325 MHz Giant Metrewave Radio Telescope (GMRT) and 1-4 GHz Jansky Very Large Array (VLA) observations of two Frontier Fields clusters, Abell S1063 and Abell 370. Deep VLA observations of the Frontier Fields cluster MACS J0717.5+3745 and Abell 2744 were already presented in van Weeren et al (2016bvan Weeren et al ( , 2017 and Pearce et al (2017). The observations and data reduction are described in Section 2.…”

A massive cluster at z = 0.288 caught in the process of formation: The case of Abell 959

“…All of the processes important to the formation of clusters dissipate energy into the intra-cluster medium (ICM) through shocks: e.g., accretion shocks, merger shocks, AGN related shocks, or ICM bulk motion shocks (see the reviews of Brüggen et al 2012;Brunetti & Jones 2014). Observationally, merger shocks have been detected in Chandra Xray and XMM-Newton observations of a small number of merging clusters (e.g., Bullet Cluster, A520, A521, A2146, A3667, A754, El Gordo, A665, A2219, and A2744; Marke Botteon et al 2016;Dasadia et al 2016;Canning et al 2017;Eckert et al 2016;Pearce et al 2017) with modest Mach numbers of M = 1.5 − 3 and, in radio images, in the form of large-scale, diffuse emission associated with the shocks (e.g., Shimwell et al 2014;Botteon et al 2016;Vacca et al 2014;Giacintucci et al 2008;Golovich et al 2018). Such radio structures, known as radio relics (or radio shocks, see the review of van Weeren et al 2019), have polarized emission resulting from ordered magnetic fields aligned by the shock.…”