arXiv subjects

Silvano Molendi

Publications and source records attributed to Silvano Molendi.

64 records · Page 4Linked to original sources

arXiv2001

XMM-EPIC observation of MCG-6-30-15: Direct evidence for the extraction of energy from aspinning black hole?

We present XMM-Newton European Photon Imaging Camera (EPIC) observations of the bright Seyfert 1 galaxy MCG-6-30-15, focusing on the broad Fe K$α$ line at ~6keV and the associated reflection continuum, which is believed to originate from the inner accretion disk. We find these reflection features to be extremely broad and red-shifted, indicating its origin from the very most central regions of the accretion disk. It seems likely that we have caught this source in the ``deep minimum'' state first observed by Iwasawa et al. (1996). The implied central concentration of X-ray illumination is difficult to understand in any pure accretion disk model. We suggest that we are witnessing the extraction and dissipation of rotational energy from a spinning black hole by magnetic fields connecting the black hole or plunging region to the disk.

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arXiv2001

The chemical enrichment of the ICM with BeppoSAX

We review observations on the chemical enrichment of the intracluster medium (ICM) performed using BeppoSAX MECS data. The picture emerging is that non-cooling flow clusters have flat metallicity profiles, whereas a strong enhancement in the abundance is found in the central regions of the cooling flow clusters. All the non-cooling flow clusters present evidence of recent merger activity suggesting that the merger events redistributes efficiently the metal content of the ICM. The observed abundance excess in the central regions of cooling flow clusters is probably due to metals ejected from the cD galaxy located in the cluster core. Cooling flow cluster have also enhanced Nickel abundances in their cores with respect to the non cooling flow clusters.

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arXiv2001

On the metal abundance in the core of M87

We revisit the XMM-Newton observation of M87 to make a new and more detailed measurement of the metal abundance profile. After having verified that spectral fits with single temperature models show a dramatic abundance drop within 1 arcmin of the cluster core, we show that more appropriate models, which include a multi-temperature component to account for the strong temperature gradient observed in M87, and a power-law component to account for the emission of the nucleus and knot A, give a substantially flat abundance profile. The drastic abundance decrement found by fitting a single temperature component to the data is an artifact known as "Fe-bias" (Buote 2000a,b) following from the application of an oversimplified spectral model to the data.

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arXiv2001

Is the gas in cooling-flows multi-phase?

Employing XMM-Newton EPIC data we perform a detailed comparison between different spectral models to test whether the gas in cooling-flows is multi-phase or not. Our findings all point in the same direction, namely that gas in cooling-flows does not show the wide distribution of temperatures expected from standard multi-phase models. This result has profound implications for cooling-flow models. Firstly, the large absorption column depths inferred by previous analysis of cooling-flow spectra are most likely an artifact following from the application of an incorrect spectral model to the data. Secondly, the mass deposition and mass flow are likely to be much smaller than previously thought. Last, but perhaps not least, the term "cooling-flow" cluster is probably no longer appropriate, as it describes a phenomenon of smaller entity and impact than previously thought. We propose to substitute it with that of "cool-core" cluster. The latter definition is less ambitious than the first, as it reflects only an observational fact rather than an inferred physical property, the flow, but has the undeniable advantage of being firmer.

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arXiv2000

Metallicity Gradients in X-ray Clusters of Galaxies

We present the projected metallicity profiles for a sample of 17 rich galaxy clusters observed by BeppoSAX. We find that the 8 non-cooling flow clusters have flat metallicity profiles. On the contrary, a strong enhancement in the abundance is found in the central regions of the cooling flow clusters. All the non-cooling flow clusters present evidence of recent merger activity suggesting that the merger events redistributes efficiently the metal content of the intracluster medium. For the cooling flow clusters with better statistics and available optical data (A85, A496, A2029 and Perseus) we have tested whether the observed abundance excess is due to metals ejected from the galaxies located in the cluster core. We find that at a resolution > 100 kpc the observed projected abundance excess profiles are consistent with originating from a deprojected metal excess distribution tracing the optical light distribution. In the one case (i.e. Perseus) with higher resolution (~ 50 kpc), we find that the observed metal abundance excess distribution is broader than the predicted one. Such a difference can be reconciled if we assume that the metals have drifted away from their parent ejecting galaxies by a few tens of kpc, or, alternatively, if we assume that the cluster light profile has become significantly more centrally peaked because of the formation process of the central dominant cluster galaxy since the last major merger occurred.

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arXiv2000

Interaction in Abell 2256: the BeppoSAX view

We present results from a spatially resolved spectral analysis of the merging cluster Abell 2256. The long integration time (135 ks) and the good spatial resolution of the MECS onboard BeppoSAX allow us to derive a new and substantially improved measurement of the temperature structure. We find that, within a central region of the cluster, where the effects of the merger are visible in the ROSAT surface brightness image, the azimuthally averaged projected temperature is remarkably flat; outside this region the projected temperature rapidly declines. The BeppoSAX data also shows clear evidence of an azimuthal temperature gradient in the 4'-8' (0.4-0.8 Mpc) radial bin, oriented in the same direction as the merger itself. Our metal abundance profile shows, for the first time, firm evidence of an abundance gradient in a rich merging cluster. Intriguingly the abundance map shows, in the SE sector, i.e. the one furthest away from the merger, a sharp factor of two drop in abundance at a radius comparable to the core radius of the cluster. A possible interpretation is that, prior to the merger event, a cooling flow had already developed in the core of the infalling subgroup, as suggested by Fabian and Daines (1991). The interaction between the substructures would have disrupted the cooling flow thereby re-heating and re-mixing the gas. As the merger in A2256 is still in a relatively early stage, the gas located on the side opposite to the merger event would still retain the low abundances associated with the intra-cluster medium (ICM) prior to the cooling flow disruption.

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arXiv1999

Spatially resolved spectroscopy of the cooling flow cluster PKS 0745-191 with BeppoSAX

We present results from a BeppoSAX observation of the cooling flow cluster PKS 0745-191 (z=0.1028). By performing spatially resolved spectroscopy, we find that the projected temperature profile is consistent with being constant. We can rule out, at more than the 99% confidence level, a temperature decrement of a factor 2 when going from the cluster core out to 1.2 Mpc. On the contrary, the projected metal abundance is found to drop from 0.4 (solar units) within the cluster core to 0.2 (solar units) at radii larger than 300 kpc, this decrement is significant at more than the 99.9% confidence level.

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arXiv1999

A BeppoSAX observation of the merging cluster Abell 3266

We present results from a BeppoSAX observation of the rich cluster Abell 3266. The broad band spectrum (2-50 keV) of the cluster, when fitted with an optically thin thermal emission model, yields a temperature of 8.1 +/- 0.2 keV and a metal abundance of 0.17 +/- 0.02 in solar units, and with no evidence of a hard X-ray excess in the PDS spectrum. By performing a spatially resolved spectral analysis we find that the projected temperature drops with increasing radius, going from ~ 10 keV at the cluster core to ~ 5 keV at about 1.5 Mpc. Our BeppoSAX temperature profile is in good agreement with the ASCA temperature profile of Markevitch et al. (1998). From our two-dimensional temperature map we find that the gradient is observed in all azimuthal directions. The temperature gradient may have been caused by a recent merger event also responsible for a velocity dispersion gradient measured in the optical band. The projected metal abundance profile and two-dimensional map are both consistent with being constant.

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arXiv1999

A BeppoSAX observation of the cooling flow cluster Abell 2029

We present results from a BeppoSAX observation of the cooling flow cluster Abell 2029. The broad band spectrum (2-35 keV) of the cluster, when fitted with a model including a cooling flow component and a one temperature thermal component, yields a temperature of 8.3+/-0.2 keV and a metal abundance of 0.46+/-0.03 in solar units, for the latter. No evidence of a hard X-ray excess is found in the PDS spectrum. By performing a spatially resolved spectral analysis we find that the projected temperature and abundance drop with increasing radius, going from about 8 keV and 0.5 (solar units) at the cluster core to about 5 keV and 0.2 (solar units) at about 1.2 Mpc.

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arXiv1995

ROSAT observations of radio-selected BL Lac objects

ROSAT observations of a homogeneous sample of radio--selected BL Lac objects are presented. Results of a detailed spectral analysis in the soft 0.1-2.0 keV energy range are discussed and compared with similar previously published results. The X--ray spectral shape is discussed in relation to the overall energy distribution with particular emphasys on the high energy gamma--ray emission recently detected by CGRO--EGRET for about half of the objects in the sample. Dividing the objects in our sample on the basis of the radio to X--ray flux ratio (alpha_rx) we show that objects with alpha_rx > 0.75 have flatter X--ray spectra and are likely to be stronger gamma--ray emitters than objects with alpha_rx < 0.75. Moreover we note that the value of the peak energy of the synchrotron component, in a E * F(E) plot, correlates with alpha_rx and alpha_x.

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