First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Abstract

Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of −2.3 ± 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of −0.81 ± 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last ∼0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.

On the physics of radio haloes in galaxy clusters: scaling relations and luminosity functions

“…Additionally, the classical hadronic model cannot reproduce some spectral features observed in clus-ters, such as the total spectral (convex) curvature claimed in the Coma RH or the spectral steepening observed at the boundary of some RHs. However, the recent report of spectral flattening with frequency of the RH in A2256 (van Weeren et al 2012) could easily be accommodated in the hadronic model, which naturally produces such a concave spectrum (Pinzke & Pfrommer 2010). This raises the interesting question whether such a variability among different sources that are generally classified as "radio halos" signals the presence of richer underlying physics-a question that we will address in this paper.…”

Deep VLA Observations of the Cluster 1RXS J0603.3+4214 in the Frequency Range of 1–2 GHz

“…A steepening of the halo spectrum with frequency is noticed in Abell 3562 and Abell 2256 (Giacintucci et al 2005;van Weeren et al 2012b). The turbulent acceleration model with particles emitting in a region with relatively uniform magnetic field intensity was invoked to explain the high frequency steepening in Abell 3562 while the low frequency steepening in Abell 2256 was explained with inhomogeneous turbulence.…”

“…For a few relics the injection spectral indices derived from the integrated spectral indices are even flatter than allowed by DSA test particle theory, i.e. spectra have been found flatter than −0.5, for example in A2256 (van Weeren et al 2012b;Trasatti et al 2015) and CIZA J2242.8+5301 (Kierdorf et al 2016;Hoang et al 2017). The derived integrated spectral index of relics B, D and E are consistent with the DSA approximation.…”

Lofar Facet Calibration

“…From the medium flux ratio we determine a difference in flux scale of about a factor 0.8 between LOFAR and the GMRT. Such differences are not unexpected given the uncertainties in the calibration of the low frequency fluxscale with the GMRT (which is estimated to be about 10%, Intema et al 2011) and the complex LOFAR station beams (van Weeren et al 2012b(van Weeren et al , 2014. Recently, it has been found that the normalization of the LOFAR HBA beam was not correctly implemented in the models (work is ongoing to fix this issue).…”