The radio relic in Abell 2256: overall spectrum and implications for electron acceleration

Abstract

Context. Radio relics are extended synchrotron sources thought to be produced by shocks in the outskirts of merging galaxy clusters. The cluster Abell 2256 hosts one of the most intriguing examples in this class of sources. It has been found that this radio relic has a rather flat integrated spectrum at low frequencies that would imply an injection spectral index for the electrons that is inconsistent with the flattest allowed by the test particle diffusive shock acceleration (DSA). Aims. We aim at testing the origins of the radio relic in Abell 2256. Methods. We performed new high-frequency observations at 2273, 2640, and 4850 MHz. Combining these new observations with images available in the literature, we constrain the radio-integrated spectrum of the radio relic in Abell 2256 over the widest sampled frequency range collected so far for this class of objects (63−10 450 MHz). Moreover, we used X-ray observations of the cluster to check the temperature structure in the regions around the radio relic. Results. We find that the relic keeps an unusually flat behavior up to high frequencies. Although the relic integrated spectrum between 63 and 10 450 MHz is not inconsistent with a single power law with α 10 450 63 = 0.92 ± 0.02, we find hints of a steepening at frequencies >1400 MHz. The two frequency ranges 63−1369 MHz and 1369−10 450 MHz are, indeed, best represented by two different power laws, with α 1369 63 = 0.85 ± 0.01 and α 10 450 1369 = 1.00 ± 0.02. This broken power law would require special conditions to be explained in terms of test-particle DSA, e.g., non-stationarity of the spectrum, which would make the relic in A2256 a rather young system, and/or non-stationarity of the shock. On the other hand, the single power law would make of this relic the one with the flattest integrated spectrum known so far, even flatter than what is allowed in the test-particle approach to DSA. We find a rather low temperature ratio of T 2 /T 1 ∼ 1.7 across the G region of the radio relic and no temperature jump across the H region. However, in both regions projection effects might have affected the measurements, thereby reducing the contrast.

Tracing low-mass galaxy clusters using radio relics: the discovery of Abell 3527-bis

“…Also the integrated spectral index (extracted from uvharmonised images at 323 and 608 MHz) appears quite flat (α = −0.70 ± 0.04) with respect to the average spectral index of radio relics (α = −1; Feretti et al 2012). As already noted for another radio relic with a flat integrated spectral index (Abell 2256;van Weeren et al 2012b;Trasatti et al 2015), if α > −1 the stationary shock conditions do not hold. As a consequence, a possible way to reconcile the flat spectrum with shock acceleration is that the relic must have been produced very recently (∼ 0.1 Gyr ago).…”

JVLA S- and X-band polarimetry of the merging cluster Abell 2256

“…Radio observations have discovered a radio relic and a halo in the central part of Abell 2256 (Bridle & Fomalont 1976;Bridle et al 1979;Rottgering et al 1994;Miller et al 2003;Clarke & Ensslin 2006;Brentjens 2008;van Weeren et al 2009;Kale & Dwarakanath 2010;van Weeren et al 2012;Owen et al 2014;Trasatti et al 2015). The radio relic located in the north-west of the cluster is ∼ 440 kpc away from the X-ray center.…”

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

“…index is rather flat, giving an unphysical result for a inj , i.e., a > -0.5 inj (van Weeren et al 2012b;Trasatti et al 2015). Therefore, the usage of Equation (8) should be limited to those cases where a inj cannot be directly estimated from the data, and any Mach numbers derived from such a inj values should be treated with caution.…”

“…Based on an injection value of a = - 0.63 0.21 inj the shock would have a Mach number measuring  =  4.04 1.4. It is possible we are observing the integrated spectrum well before the break frequency, whilst the emission is determined simply by the injection spectrum, n µ n a S inj , in which case this relic is "young," and stationary shock conditions do not (yet) apply (e.g., van Weeren et al 2012b;Trasatti et al 2015). Alternatively, the flat spectrum reflects the slope of the fossil distribution in the case of reacceleration, or a combination of the two above (reacceleration + nonstationary shock conditions, Kang & Ryu 2015;Kang 2015).…”