Roberto Fusco-Femiano

HARD X-RAY RADIATION IN THE COMA CLUSTER SPECTRUM

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 σ) for nonthermal emission in excess of thermal emission above ~25 keV. The hard excess is very unlikely to be the result of X Comae, the Seyfert 1 galaxy that is present in the field of view of the PDS. A hard spectral tail that is due to inverse Compton scattering on cosmic microwave background 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 μG is derived, while the electron energy density of the emitting electrons is ~7×10−14 ergs cm-3.

The giant radio halo in Abell 2163

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 diuse 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 conrms that halos are associated with hot massive clusters, and conrms 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.

Confirmation of Nonthermal Hard X-Ray Excess in the Coma Cluster from Two Epoch Observations

We report the hard X-ray spectrum of the Coma Cluster obtained using the Phoswich Detection System data of two independent BeppoSAX observations performed with a time interval of about 3 yr. In both the spectra a nonthermal excess with respect to the thermal emission is present at a confidence level of ~3.4 σ. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ~4.8 σ at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised nonthermal X-ray flux that is slightly lower than that previously estimated (Fusco-Femiano et al.) and in agreement with that measured by two Rossi X-Ray Timing Explorer observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

Hard X-ray emission from the galaxy cluster A2256

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.

Chandra Observation of a 300 Kiloparsec Hydrodynamic Instability in the Intergalactic Medium of the Merging Cluster of Galaxies A3667

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. 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.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.

Probing the Astrophysics of Cluster Outskirts

In galaxy clusters the entropy distribution of the IntraClu ster Plasma modulates the latter’s equilibrium within the D ark 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 grow th. In such conditions, we predict the ICP temperature profiles to steep en into the cluster outskirts. The detailed expectations fr om our simple formalism agree with the X-ray data concerning five clusters whose temperature profiles have been recently measured out t o the virial radius. We predict steep temperature declines to prevail in clusters at low z, tempered only by rich environs including adjacent filamentary structures.

The BEPPOSAX View of the Hot Cluster Abell 2319

We present results from a BeppoSAX observation of the rich cluster Abell 2319. The broadband spectrum (2-50 keV) of the cluster can be adequately represented by an optically thin thermal emission model with a temperature of 9.6+/-0.3 keV and a metal abundance of 0.25+/-0.03 in solar units and with no evidence of a hard X-ray excess in the PDS spectrum. From the upper limit to the hard-tail component, we derive a lower limit of approximately 0.04 µG for the volume-averaged intracluster magnetic field. By performing spatially resolved spectroscopy in the medium energy band (2-10 keV), we find that the projected radial temperature and metal abundance profiles are constant out to a radius of 16&arcmin; (1.4 Mpc). A reduction of the temperature of one-third, when going from the cluster core out to 16&arcmin;, can be excluded in the present data at the 99% confidence level. From the analysis of the temperature and abundance maps, we find evidence of a temperature enhancement and of an abundance decrement in a region localized 6&arcmin;-8&arcmin; northeast of the core, where a merger event may be taking place. Finally, the temperature map indicates that the subcluster located northwest of the main cluster may be somewhat cooler than the rest of the cluster.

How Abundant Is Iron in the Core of the Perseus Cluster

The analysis of Perseus data collected with the Medium-Energy Concentrator Spectrometer (MECS) on board BeppoSAX shows that the ratio of the flux of the 8 keV line complex (dominated by Fe Kβ emission) to the 6.8 keV line complex (dominated by Fe Kα emission) is significantly larger than predicted by standard thermal emission codes. Moreover, the analysis of spatially resolved spectra shows that the above ratio decreases with increasing cluster radius. We find that, among the various explanations that we consider, the most likely requires the plasma to be optically thick for resonant scattering at the energy of the Fe Kα line. We argue that if this is the case, then measures of the iron abundance made using standard thermal emission codes that assume optically thin emission may significantly underestimate the true iron abundance. In the case of the core of Perseus, we estimate the true abundance to be ~0.9 solar in a circular region with a radius of ~60 kpc and centered on NGC 1275. Finally, we speculate that similar results may hold for the core of other rich clusters.

Nonthermal Hard X-Ray Excess in the Coma Cluster: Resolving the Discrepancy between the Results of Different PDS Data Analyses

The detection of a nonthermal excess in the Coma Cluster spectrum by two BeppoSAX observations analyzed with the XAS package (Fusco-Femiano et al.) has been disavowed by an analysis (Rossetti & Molendi) performed with a different software package (SAXDAS) for the extraction of the spectrum. To resolve this discrepancy we reanalyze the PDS data considering the same software used by Rossetti & Molendi. A correct selection of the data and the exclusion of contaminating sources in the background determination show that the SAXDAS analysis also reports a nonthermal excess with respect to the thermal emission at about the same confidence level of that obtained with the XAS package (~4.8 σ). In addition, we report the lack of the systematic errors investigated by Rossetti & Molendi and Nevalainen et al., taking into account the whole sample of the PDS observations off the Galactic plane, as already shown in our data analysis of Abell 2256 (Fusco-Femiano, Landi, & Orlandini). All this eliminates any ambiguity and confirms the presence of a hard tail in the spectrum of the Coma Cluster.

Nonthermal support for the outer intracluster medium

We submit that nonthermalized support for the outer intracluster medium in relaxed galaxy clusters is provided by turbulence, which is driven by inflows of intergalactic gas across the virial accretion shocks. We expect this component to increase briskly during the cluster development for z ≲ 1/2, owing to three factors. First, the accretion rates of gas and dark matter subside when they feed on the outer wings of the initial perturbations in the accelerating Universe. Second, the infall speeds decrease across the progressively shallower gravitational potential at the shock position. Third, the shocks eventually weaken and leave less thermal energy to feed the intracluster entropy, but relatively more bulk energy to drive turbulence into the outskirts. The overall outcome from these factors is physically modeled and analytically computed; thus we ascertain how these concur in setting the equilibrium of the outer intracluster medium, and predict how the observables in X-rays and μwaves are affected, so as to probe the development of outer turbulence over wide cluster samples. By the same token, we quantify the resulting negative bias to be expected in the total mass evaluated from X-ray measurements.