VLA observations at 1477 MHz revealed the presence of a radio mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies. In this work we present a study of the radio emission from this cluster of galaxies at lower radio frequencies. We observed the Ophiuchus cluster at 153, 240, and 614 MHz with the GMRT. The mini-halo is clearly detected at 153 and 240 MHz while it is not detected at 610 MHz. The most prominent feature at low frequencies is a patch of diffuse steep spectrum emission located at about 5' south-east from the cluster center. By combining these images with that at 1477 MHz, we derived the spectral index of the mini-halo. Globally, the mini-halo has a low-frequency spectral index of alpha_240^153 ~1.4 +/- 0.3 and an high-frequency spectral index of alpha_1477^240 ~ 1.60 +/- 0.05. Moreover, we measure a systematic increase of the high-frequency spectral index with radius: the azimuthal radial average of alpha_1477^240 increases from about 1.3, at the cluster center, up to about 2.0 in the mini-halo outskirts. The observed radio spectral index is in agreement with that obtained by modeling the non-thermal hard X-ray emission in this cluster of galaxies. We assume that the X-ray component arises from inverse Compton scattering between the photons of the cosmic microwave background and a population of non-thermal electrons which are isotropically distributed and whose energy spectrum is a power law with index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of gamma_min < 700 up to a maximum Lorentz factor of gamma_max =3.8 x 10^4 with an index p=3.8 +/- 0.4. The volume-averaged strength for a completely disordered intra-cluster magnetic field is B_V ~0.3 +/- 0.1 micro-G.
Galaxy clusters, the largest gravitationally bound structures in the Universe, are still forming at present epoch by merging of nearly equal-mass systems or accretion of groups and field galaxies. They are excellent laboratories to study the baryonic cosmic fraction, as well as the interplay between baryonic and dark matter in the formation and evolution process of large scale structures (e.g. Arnaud et al. 2009;Kravtsov et al. 2009). In the last twenty years important progresses have been made in the study of cluster of galaxies, of the thermal intra-cluster medium (ICM) and of their interaction (e.g. Boselli & Gavazzi 2006;Markevitch & Vikhlinin 2007). Much less is instead known about the physical properties and the origin of a non-thermal intra-cluster component (relativistic electrons with energies of ≃ 10 GeV spiralling in magnetic fields of few µGauss) that has been discovered and studied mostly through deep radio observations (see e.g. Ferrari et al. 2008 and references therein). However, it is now clear that the impact of the non-thermal component in the physics and thermo-dynamical evolution of galaxy clusters cannot be neglected anymore (e.g. Dursi & Pfrommer 2008;Parrish et al. 2009).
Intra-cluster relativistic electrons radiate through synchrotron emission in the radio domain, but also through inverse Compton scattering of cosmic microwave background (CMB) photons in the hard X-ray (HXR) band. The diffuse non-thermal component is now well detected at radio wavelengths in about 30 clusters (Giovannini et al. 2009). Only a few X-ray satellites allowed possible but controversial detection of a hard tail in the X-ray spectrum of about 10 clusters (see e.g. Fusco-Femiano et al. 2003;Nevalainen et al. 2004;Rephaeli et al. 2008). Very recent results are either in agreement with a HXR non-thermal detection (e.g. Eckert et al. 2008) or suggest a possible thermal origin of the detected HXR emission (e.g. Kawano et al. 2009).
The Ophiuchus cluster (z = 0.028, Johnston et al. 1981) is one of the brightest cluster of galaxies in the X-ray band. It is an extremely interesting target for non-thermal cluster studies, since it shows evidence of both radio and, possibly, HXR emission (Eckert et al. 2008;Govoni et al. 2009;Murgia et al. 2009;Nevalainen et al. 2009). The dynamical state of the Ophiuchus cluster has been strongly debated in the last years. A recent Chandra study (Million et al. 2009) shows evidence of a recent merger event in the central region of the cluster of 8 × 8 arcmin 2 (but see also Fujita et al. 2008 for an opposite conclusion based on Suzaku data). In addition several clusters and groups of galaxies have been detected within a distance of 8 • from the cluster centre, indicating that Ophiuchus is in a supercluster environment (Wakamatsu et al. 2005).
By analyzing Very Large Array (VLA) data of Ophiuchus at 1477 MHz, Govoni et al. (2009) recently detected a radio mini- halo surrounding the faint central point-like radio source. Radio mini-halos are diffuse steep-spectrum (α > 1; S ν ∝ ν -α ) sources, permeating the central regions of relaxed, cool-core, galaxy clusters. They usually surround a radio galaxy. These diffuse radio sources are extended on a moderate scale (typically ≃ 500 kpc) and, in common with large-scale halos observed in merging clusters of galaxies, have a steep spectrum and a very low surface brightness. As a consequence of their relatively small angular size and possibly strong radio emission of the central radio galaxy, radio mini-halos are very elusive sources and our current observational knowledge on mini-halos is limited to only a handful of well-studied clusters.
Based on current observational and theoretical analyses, radio emission from mini-halos would be due to a population of relativistic electrons ejected by the central AGN and reaccelerated by MHD turbulence, whose energy is, in turn, supplied by the cluster cooling-core (Gitti et al. 2004). Recent analysis of the most X-ray luminous cluster (RX J1347-1145) suggest that additional energy for electron re-acceleration in minihalos might be provided by sub-cluster mergers that have not been able to destroy the central cluster cooling-core (Gitti et al. 2007). Ophiuchus is the second known cluster showing a radio mini-halo, as well as a cool-core that has survived a possible recent merging event (Nevalainen et al. 2009;Million et al. 2009). Indeed, Burns et al. (2008) simulated the formation and evolution of galaxy clusters, and showed that cool-core clusters can accrete mass over time and grew slowly via hierarchical mergers; when late mergers occur, the cool-cores survive the collisions. Eckert et al. (2008) measured a high confidence level (6.4 σ) HXR excess in Ophiuchus through INTEGRAL observations. This emission may be of non-thermal origin, caused presumably by Compton scattering of cosmic microwave background radiation by the relativistic electrons responsible for the minihalo emission (see e.g., Reph
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