Statistics of Giant Radio Halos from Electron Reacceleration Models

The most important evidence of non-thermal phenomena in galaxy clusters comes from Giant Radio Halos (GRHs), synchrotron radio sources extended over Mpc scales, detected in a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by "in situ" stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano & Brunetti (2005) have recently shown that the expected fraction of clusters with GRHs and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 % of the turbulence in clusters is in the form of compressible modes. In this work we extend these calculations by including a scaling of the magnetic field strength with cluster mass. We show that the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B \propto M^b, with b>0.5 and typical micro Gauss strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained. The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P_{1.4 GHz} 10^{24} W/Hz, and severely cut-off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters. We expect a total number of GRHs to be discovered at ~mJy radio fluxes of ~100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the fortcoming operation of low frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.Comment: 21 pages, 17 figures, accepted for publication in MNRA

An elusive radio halo in the merging cluster Abell 781?

Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610 MHz observations and to archival VLA 1.4 GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and on the basis of simple spectral considerations we do not support their claim of a radio halo with flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source.Comment: 5 pages, 4 figures, 1 table - Accepted for publication on Monthly Notices of the Royal Astronomical Society Letter

Cluster Mergers, Radio Halos and Hard X-ray Tails: A Statistical Magneto-Turbulent Model

There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidences for non-thermal phenomena in galaxy clusters comes from the diffuse Mpc-scale synchrotron radio emission (radio halos) observed in a growing number of massive clusters (Feretti 2003) and from hard X-ray (HXR) excess emission (detected in a few cases) which can be explained in terms of IC scattering of relativistic electrons off the cosmic microwave background photons (Fusco-Femiano et al. 2003). There are now growing evidences that giant radio halos may be naturally accounted for by synchrotron emission from relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events (Brunetti 2003). With the aim to investigate the connection between thermal and non-thermal properties of the ICM, we have developed a statistical magneto-turbulent model which describes the evolution of the thermal and non-thermal emission from clusters. We calculate the energy and spectrum of the magnetosonic waves generated during cluster mergers, the acceleration and evolution of relativistic electrons and thus the resulting synchrotron and inverse Compton spectra. Here we give a brief description of the main results, while a more detailed discussion will be presented in a forthcoming paper. Einstein-De Sitter cosmology, $H_o=50$ km $s^{-1}$$Mpc^{-1}$, $q_o=0.5$, is assumed.Comment: 3 pages, 2 figures. To appear in the proceedings of IAU Colloquium 195 - "Outskirts of galaxy clusters: intense life in the suburbs", Torino, Italy, March 12-16, 200

Can giant radio halos probe the merging rate of galaxy clusters?

Radio and X-ray observations of galaxy clusters probe a direct link between cluster mergers and giant radio halos (RH), suggesting that these sources can be used as probes of the cluster merging rate with cosmic time. In this paper we carry out an explorative study that combines the observed fractions of merging clusters (fm) and RH (fRH) with the merging rate predicted by cosmological simulations and attempt to infer constraints on merger properties of clusters that appear disturbed in X-rays and of clusters with RH. We use morphological parameters to identify merging systems and analyze the currently largest sample of clusters with radio and X-ray data (M500>6d14 Msun, and 0.2<z<0.33, from the Planck SZ cluster catalogue). We found that in this sample fm~62-67% while fRH~44-51%. The comparison of the theoretical f_m with the observed one allows to constrain the combination (xi_m,tau_m), where xi_m and tau_m are the minimum merger mass ratio and the timescale of merger-induced disturbance. Assuming tau_m~ 2-3 Gyr, as constrained by simulations, we find that the observed f_m matches the theoretical one for xi_m~0.1-0.18. This is consistent with optical and near-IR observations of clusters in the sample (xi_m~0.14-0.16). The fact that RH are found only in a fraction of merging clusters may suggest that merger events generating RH are characterized by larger mass ratio; this seems supported by optical/near-IR observations of RH clusters in the sample (xi_min~0.2-0.25). Alternatively, RH may be generated in all mergers but their lifetime is shorter than \tau_m (by ~ fRH/fm). This is an explorative study, however it suggests that follow up studies using the forthcoming radio surveys and adequate numerical simulations have the potential to derive quantitative constraints on the link between cluster merging rate and RH at different cosmic epochs and for different cluster masses.Comment: 10 pages, 3 figures, accepted for publication in A&

The cluster relic source in A521

We present high sensitivity radio observations of the merging cluster A521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of $\sim$1 square degree, with a sensitivity limit of $1\sigma$ = 35 $\mu$Jy b$^{-1}$. The most relevant result of these observations is the presence of a radio relic at the cluster periphery, at the edge of a region where group infalling into the main cluster is taking place. Thanks to the wealth of information available in the literature in the optical and X-ray bands, a multi--band study of the relic and its surroundings was performed. Our analysis is suggestive of a connection between this source and the complex ongoing merger in the A521 region. The relic might be ``revived' fossil radio plasma through adiabatic compression of the magnetic field or shock re--acceleration due to the merger events. We also briefly discussed the possibility that this source is the result of induced ram pressure stripping of radio lobes associated with the nearby cluster radio galaxy J0454--1016a. Allowing for the large uncertainties due to the small statistics, the number of radio emitting early--type galaxies found in A521 is consistent with the expectations from the standard radio luminosity function for local (z$\le$0.09) cluster ellipticals.Comment: 30 pages 8 figures, 5 tables, accepted by New Astronom

GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample

We present the results of the GMRT cluster radio halo survey. The main purposes of our observational project are to measure which fraction of massive galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of the non--thermal radio emission. We selected a complete sample of 50 clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues. In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for all clusters still lacking high sensitivity radio information, i.e. 16 eBCS and 7 REFLEX clusters, thus completing the radio information for the whole sample. The typical sensitivity in our images is in the range 1$\sigma \sim 35-100 \mu$Jy b$^{-1}$. We found a radio halo in A697, a diffuse peripheral source of unclear nature in A781, a core--halo source in Z7160, a candidate radio halo in A1682 and ``suspect'' central emission in Z2661. Including the literature information, a total of 10 clusters in the sample host a radio halo. A very important result of our work is that 25 out of the 34 clusters observed with the GMRT do not host extended central emission at the sensitivity level of our observations, and for 20 of them firm upper limits to the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey shows that radio halos are not common, and our findings on the fraction of giant radio halos in massive clusters are consistent with the statistical expectations based on the re--acceleration model. Our results favour primary to secondary electron models.Comment: A&A in press, 17 pages, 12 figures, 4 tables Version with high quality figures available on web at http://www.ira.inaf.it/~tventuri/pap/Venturi_web.pd

Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo ?

The spectrum of the radio halo in the Coma cluster is measured over almost two decades in frequency. The current radio data show a steepening of the spectrum at higher frequencies, which has implications for models of the radio halo origin. There is an on-going debate on the possibility that the observed steepening is not intrinsic to the emitted radiation, but is instead caused by the SZ effect. Recently, the Planck satellite measured the SZ signal and its spatial distribution in the Coma cluster allowing to test this hypothesis. Using the Planck results, we calculated the modification of the radio halo spectrum by the SZ effect in three different ways. With the first two methods we measured the SZ-decrement within the aperture radii used for flux measurements of the halo at the different frequencies. First we adopted the global compilation of data from Thierbach et al. and a reference aperture radius consistent with those used by the various authors. Second we used the available brightness profiles of the halo at different frequencies to derive the spectrum within two fixed apertures, and derived the SZ-decrement using these apertures. As a third method we used the quasi-linear correlation between the y and the radio-halo brightness at 330 MHz discovered by Planck to derive the modification of the radio spectrum by the SZ-decrement in a way that is almost independent of the adopted aperture radius. We found that the spectral modification induced by the SZ-decrement is 4-5 times smaller than that necessary to explain the observed steepening. Consequently a break or cut-off in the spectrum of the emitting electrons is necessary to explain current data. We also show that, if a steepening is absent from the emitted spectrum, future deep observations at 5 GHz with single dishes are expected to measure a halo flux in a 40 arcmin radius that would be 7-8 times higher than currently seen.Comment: 8 pages, 6 figures, accepted in Astronomy and Astrophysics (date of acceptance 19/08/2013

An unlikely radio halo in the low X-ray luminosity galaxy cluster RXC J1514.9-1523

We report the discovery of a giant radio halo in the galaxy cluster RXC J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, $L_{X \, [0.1-2.4 \rm \, kev]} \sim 7 \times 10^{44}$ erg s$^{-1}$. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. The integrated radio spectrum of the halo is quite steep, with a slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with L_X \ltsim 8 \times 10^{44} erg s$^{-1}$. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the recently discovered population of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration.Comment: 4 pages, 2 figures - Accepted for publication on A&A Research Note

Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.Comment: 11 pages, 3 figures (figure 1 available in gif format only). Requires aastex.cls - Accepted by Ap.