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The Radio Relic in Abell 2256: Overall Spectrum and Implications for Electron Acceleration

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The Radio Relic in Abell 2256: Overall Spectrum and Implications for Electron Acceleration A&A 575, A45 (2015) Astronomy DOI: 10.1051/0004-6361/201423972 & c ESO 2015 Astrophysics The radio relic in Abell 2256: overall spectrum and implications for electron acceleration M. Trasatti1,H.Akamatsu2,L.Lovisari1, U. Klein1, A. Bonafede3, M. Brüggen3, D. Dallacasa4,5,andT.Clarke6 1 Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: [email protected] 2 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 3 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany 4 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy 5 INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy 6 Naval Research Laboratory, 4555 Overlook Ave, SW, DC 20375 Washington, USA Received 10 April 2014 / Accepted 1 November 2014 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 be- 10 450 = ± tween 63 and 10 450 MHz is not inconsistent with a single power law with α63 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 ff 1369 = ± 10 450 = ± di erent power laws, with α63 0.85 0.01 and α1369 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 T2/T1 ∼ 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. Key words. galaxies: clusters: general – galaxies: clusters: individual: Abell 2256 – acceleration of particles 1. Introduction lifetime of the emitting electrons implies the need for some form of in situ production or (re-)acceleration of the electrons in all A fraction of galaxy clusters exhibit diffuse Mpc-scale syn- these sources, even though the underlying physical mechanisms chrotron emission (referred to as radio halos and radio relics) not are thought to be different for the different classes of sources. related to any particular cluster galaxy (for reviews see Feretti Moreover, these diffuse radio emitting regions are mostly found et al. 2012; Brüggen et al. 2012). This emission manifests itself in unrelaxed clusters, suggesting that cluster mergers play a key by the presence of relativistic electrons (∼GeV) and weak mag- role in producing them. ∼ netic fields ( μG) in the intracluster medium (ICM), together Radio gischt are large, extended arc-like sources, believed with the hot thermal plasma emitting X-rays. Radio halos per- 3 to be synchrotron emission from electrons accelerated or re- meate the central Mpc of galaxy clusters and the radio emis- accelerated in merger or accretion shocks through diffusive sion usually follows the roundish X-ray emission from the ther- shock acceleration (DSA, Fermi-I process; see Ensslin et al. mal gas. Radio relics are more irregularly shaped and are lo- 1998; Kang & Ryu 2011). A textbook example of such gi- cated at the clusters periphery. They are usually further subdi- ant radio relic has been observed in the galaxy cluster CIZA vided into three classes: radio gischt, radio phoenices, and AGN J2242.8+5301 (van Weeren et al. 2010; Stroe et al. 2013). Radio relics (see Kempner et al. 2004), depending on their characteris- phoenices are believed to be the result of the re-energization tics and proposed origin (as described below). The combination via adiabatic compression, triggered by shocks, of fossil plasma of the Mpc size of such sources and the relatively short radiative from switched-off AGN radio galaxies (Enßlin & Gopal-Krishna 2001; Enßlin & Brüggen 2002). The relativistic plasma of AGN Appendix A is available in electronic form at origin had the time to age and without the re-energization http://www.aanda.org would not longer be visible at the currently observable radio Article published by EDP Sciences A45, page 1 of 18 A&A 575, A45 (2015) frequencies. An example of a radio phoenix has been found in spectrum at high energies upward, without modifying the spec- the galaxy cluster A2443 (Cohen & Clarke 2011). AGN relics tral slope. Even steeper spectra are expected in case of AGN are indeed such fossil radio galaxies where the AGN switched relics. The average spectral indices reported in Table 4 of Feretti off more recently and no re-energization occurred. The plasma et al. (2012) for integrated spectra with measured steepening, is still emitting at observable radio frequencies, and it simply range from 1.7 to 2.9. evolves passively until it becomes invisible in the radio window With these ingredients, detailed studies of the integrated (Komissarov & Gubanov 1994). spectrum and of the spectral-index distribution across the sources, allow us to test the current models and study the shock properties in case of DSA. This is accomplished by observations Models and diagnostics made over a broad range of frequencies. However, an accurate The proposed formation mechanisms differ in the predictions measurement of the integrated spectra of radio relics is a difficult of the morphological and spectral characteristics of the differ- task. These sources usually contain a number of discrete sources, ent classes of relics. DSA of both thermal and pre-accelerated whose flux density needs to be carefully subtracted from the total electrons, should produce larger and more peripheral structures, diffuse emission. This requires high-resolution imaging at many with strong polarization, and pure power-law integrated syn- frequencies using radio interferometers. However, increasing the chrotron spectra (Brüggen et al. 2012). Fermi processes natu- observing frequencies, interferometers encounter the technical rally predict an injection power-law energy distribution for the problem of the missing short spacings that makes them “blind” − accelerated electron population of the form1 f (E) ∝ E δinj .From to very extended structures. On the other hand, single dishes are synchrotron theory, the emission produced by this population of optimal to catch all the emission from a field but they lack an- electrons is also described by a power law2, gular resolution. Indeed, integrated spectra over a wide range of frequencies are available in the literature only for few of these − δinj − 1 objects (see Feretti et al. 2012). S (ν) ∝ ν αinj with α = · (1) inj 2 An independent measure of the properties of shocks is pro- vided by deep X-ray observations. Through the measurements of Emitting particles are naturally subject to energy losses. These temperature and/or pressure jumps at the location of the shock, losses are governed by many physical factors such as the prop- properties such as the shock Mach number M and the shock erties of the magnetic field (see Kardashev 1962; Jaffe&Perola compression ratio C can be inferred (see review by Brüggen 1973; Komissarov & Gubanov 1994, for a description of the dif- et al. 2012). In the test particles approximation3 of DSA, if the ferent models of electron aging). The absence of any constant in- particle diffusion is specified, the shock Mach number is the pri- jection of new electrons would lead to a cutoff in the high-energy mary parameter that determines the efficiency of the acceleration region of the integrated spectrum, moving toward lower frequen- mechanism and the energy distribution of the particles at injec- cies in the course of time. The presence of constant injection tion (Kang & Ryu 2010). In this case, a simple direct relation of particles with the same energy spectrum, on the other hand, between the shock Mach number M and the injection index δinj would eventually mask the cutoff, leading instead to a break with of the energy electrons distribution exists: a change of 0.5 in the spectral index of the integrated emission (continuous injection model, Kardashev 1962): 2(M2 + 1) δ = · (3) inj (M2 − 1) αobs = αinj + 0.5. (2) However, radio relics are usually observed in the outskirts This condition translates, in case of DSA, in the assumption that of clusters where the very low density of electrons (ne < − − the properties of the shock remain unchanged (stationarity for 10 4 cm 3) make the detection of shocks in the X-ray very the shock).
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