Hard X-ray radiation has been detected for the first time in the Coma cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ~80 keV. There is clear evidence (4.5 sigma) for non-thermal emission in excess of thermal above ~25 keV. The hard excess is very unlikely due to X Comae, the Seyfert 1 galaxy present in the field of view of the PDS. A hard spectral tail due to inverse Compton on CMB photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ~0.15 micro G is derived, while the electron energy density of the emitting electrons is ~7x10**-14 erg/cm**3.Comment: 9 LaTex pages, 3 Postscript figures, to appear in the Astrophysical Journal Letter

Abstract. New radio data is presented for the rich cluster Abell 2163. The cluster radio emission is characterized by the presence of a radio halo, which is one of the most powerful and extended halos known so far. In the NE peripheral cluster region, we also detect diffuse elongated emission, which we classify as a cluster relic. The cluster A2163 is very hot and luminous in X-ray. Its central region is probably in a highly non relaxed state, suggesting that this cluster is likely to be a recent merger. The existence of a radio halo in this cluster confirms that halos are associated with hot massive clusters, and confirms the connection between radio halos and cluster merger processes. The comparison between the radio emission of the halo and the cluster X-ray emission shows a close structural similarity. A power law correlation is found between the radio and X-ray brightness, with index =0.64. We also report the upper limit to the hard X-ray emission, obtained from a BeppoSAX observation. We discuss the implications of our results.

We report the hard X-ray spectrum of the Coma cluster obtained using the PDS data of two independent BeppoSAX observations performed with a time interval of about three years. In both the spectra a non thermal excess with respect to the thermal emission is present at a confidence level of ∼ 3.4σ. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ∼ 4.8σ at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised non-thermal X-ray flux which is slightly lower than that previously estimated (Fusco-Femiano et al. 1999) and in agreement with that measured by two RXTE observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

After the positive detection by BeppoSAX of hard X-ray radiation up to approximately 80 keV in the Coma Cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6 sigma confidence level. In addition to this power-law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, which is not surprising given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the Phoswich Detection System on board BeppoSAX is marginally consistent with that expected for the inverse Compton model. A value of approximately 0.05 µG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of approximately 0.5 µG could be present in the central radio halo, which is likely related to the hard tail detected by ASCA.

In galaxy clusters the entropy distribution of the intracluster plasma (ICP) modulates the latter's equilibrium within the dark matter gravitational wells, as rendered by our supermodel. We argue the entropy production at the boundary shocks to be reduced or terminated as the accretion rates of DM and intergalactic gas peter out; this behavior is enforced by the slowdown in the outskirt development at late times, when the dark energy dominates the cosmology while the outer wings of the initial perturbation drive the growth. For these conditions, we predict the ICP temperature profiles to steepen into the cluster outskirts. The detailed expectations from our simple formalism agree with the X-ray data concerning five clusters whose temperature profiles have been recently measured out to the virial radius. We predict steep temperature declines to prevail in clusters at low z, tempered only by rich environs including adjacent filamentary structures.

We take up from a library of 12 galaxy clusters featuring extended X-ray observations of their Intra Cluster Plasma (ICP), analyzed with our entropy-based Supermodel. Its few intrinsic parameters − basically, the central level and the outer slope of the entropy profile − enable us to uniformly derive not only robust snapshots of the ICP thermal state, but also the 'concentration' parameter marking the age of the host dark matter halo. We test these profiles for consistency with numerical simulations and observations. We find the central and the outer entropy to correlate, so that these clusters split into two main classes defined on the basis of low (LE) or high (HE) entropy conditions prevailing throughout the ICP. We also find inverse correlations between the central/outer entropy and the halo concentration. We interpret these in terms of mapping the ICP progress on timescales around 5 Gyr toward higher concentrations, under the drive of the dark matter halo development. The progress proceeds from HEs to LEs, toward states of deeper entropy erosion by radiative cooling in the inner regions, and of decreasing outer entropy production as the accretion peters out. We propose these radial and time features to constitute a cluster Grand Design, that we use here to derive a number of predictions. For HE clusters we predict sustained outer temperature profiles. For LEs we expect the outer entropy ramp to bend over, hence the temperature decline to steepen at low z; this feature goes together with an increasing turbulent support, a condition that can be directly probed with the SZ effect. We finally discuss the looming out of two intermediate subsets: wiggled HE at low z that feature central temperature profiles retaining imprints of entropy discharged by AGNs or deep mergers; high-z LEs, where the cosmogony/cosmology had little time to enforce a sharp outer entropy bending.

In galaxy clusters the equilibria of the intracluster plasma (ICP) and of the gravitationally dominant dark matter (DM) are governed by the hydrostatic and the Jeans equation, respectively; in either case gravity is withstood by the corresponding, entropy-modulated pressure. Jeans, with the DM 'entropy' set to K ∝ r α and α ≈ 1.25 − 1.3 applying from groups to rich clusters, yields our radial α-profiles; these, compared to the empirical NFW distribution, are flatter at the center and steeper in the outskirts as required by recent gravitational lensing data. In the ICP, on the other hand, the entropy run k(r) is mainly shaped by shocks, as steadily set by supersonic accretion of gas at the cluster boundary, and intermittently driven from the center by merging events or by active galactic nuclei (AGNs); the resulting equilibrium is described by the exact yet simple formalism constituting our ICP Supermodel. With a few parameters, this accurately represents the runs of density n(r) and temperature T (r) as required by up-to-date X-ray data on surface brightness and spectroscopy for both cool core (CC) and non cool core (NCC) clusters; the former are marked by a middle temperature peak, whose location is predicted from rich clusters to groups. The Supermodel inversely links the inner runs of n(r) and T (r), and highlights their central scaling with entropy n c ∝ k −1 c and T c ∝ k 0.35 c , to yield radiative cooling times t c ≈ 0.3 (k c /15 keV cm 2 ) 1.2 Gyr. We discuss the stability of the central values so focused: against radiative erosion of k c in the cool dense conditions of CC clusters, that triggers recurrent AGN activities resetting it back; or against energy inputs from AGNs and mergers whose effects are saturated by the hot central conditions of NCC clusters. From the Supermodel we derive as limiting cases the classic polytropic β-models, and the 'mirror' model with T (r) ∝ σ 2 (r) suitable for NCC and CC clusters, respectively; these limiting cases highlight how the ICP temperature T (r) strives to mirror the DM velocity dispersion σ 2 (r) away from energy and entropy injections. Finally, we discuss how the Supermodel connects information derived from X-ray and gravitational lensing observations.

We present results from the combination of two Chandra pointings of the central region of the cluster of galaxies A3667. From the data analysis of the first pointing Vikhlinin et al. reported the discovery of a prominent cold front which is interpreted as the boundary of a cool gas cloud moving through the hotter ambient gas. Vikhlinin et al. discussed the role of the magnetic fields in maintaining the apparent dynamical stability of the cold front over a wide sector at the forward edge of the moving cloud and suppressing transport processes across the front. In this Letter, we identify two new features in the X-ray image of A3667: i) a 300 kpc arc-like filamentary X-ray excess extending from the cold gas cloud border into the hotter ambient gas; ii) a similar arc-like filamentary X-ray depression that develops inside the gas cloud. Both features are located beyond the sector identified by the cold front and are oriented in a direction perpendicular to the direction of motion. The temperature map suggests that the temperature of the filamentary excess is consistent with that inside the gas cloud while the temperature of the depression is consistent with that of the ambient gas. We suggest that the observed features represent the first evidence for the development of a large scale hydrodynamic instability in the cluster atmosphere resulting from a major merger. This result confirms previous claims for the presence of a moving cold gas cloud into the hotter ambient gas. Moreover it shows that, although the gas mixing is suppressed at the leading edge of the subcluster due to its magnetic structure, strong turbulent mixing occurs at larger angles to the direction of motion. We show that this mixing process may favor the deposition of a nonnegligible quantity of thermal energy right in the cluster center, affecting the development of the central cooling flow.