arXiv subjects

S. Molendi

Publications and source records attributed to S. Molendi.

At least 127 records · Page 7Linked to original sources

arXiv1999

Mapping the circumnuclear matter of NGC1068 in X-rays

An-energy dependent variation of the X-ray emission from the archetypical "Compton-thick'' Seyfert 2 galaxy NGC1068 has been observed between two BeppoSAX observations, performed one year apart. This variation (at the 2.6 sigma level) is highest in the 3--10 keV band and declines towards lower and higher energies. The most straightforward explanation is a variation of the primary nuclear continuum, which is scattered along our line of sight by an electron plasma. If this is indeed the case, this evidence allows us to obtain the first direct estimate of the location of the scattering medium, at ~1 pc from the nucleus.

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arXiv1999

The BeppoSAX view of the hot cluster Abell 2319

We present results from a BeppoSAX observation of the rich cluster Abell 2319. The broad band 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 ~0.04 \muG 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 1/3, 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 NE of the core, where a merger event may be taking place. Finally, the temperature map indicates that the subcluster located NW of the main cluster may be somewhat cooler than the rest of the cluster.

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arXiv1999

The BeppoSAX 1-8 keV cosmic background spectrum

The spectrum of the 1.0-8.0 keV cosmic X-ray background (CXB) at galactic latitudes > 25 degrees has been measured using the BeppoSAX LECS and MECS instruments. The spectrum is consistent with a power-law of photon index 1.40 +/- 0.04 and normalization 11.7 +/- 0.5 photon cm^{-2} s^{-1} keV^{-1} sr^{-1} at 1 keV. Our results are in good agreement with previous ROSAT PSPC (Georgantopuolos et al. 1996), ASCA GIS (Myaji et al. 1998) and rocket (McCammon & Sanders 1990) measurements. On the contrary, previous measurements with the HEAO1 A2 (Marshall et al. 1980) and the ASCA SIS (Gendreau et al. 1995) instruments, are characterized by normalizations which are respectively 35% and 25% smaller than ours.

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arXiv1999

Diffuse Thermal Emission from Very Hot Gas in Starburst Galaxies

BeppoSAX observations of the nearby archetypical starburst galaxies NGC253 and M82 are presented. Spectral analysis shows that the 2-10 keV spectra of both galaxies, extracted from the central 4' regions, are best fitted by a thermal emission model with kT~6-9 keV and metal abundances ~ 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. These results clearly rule out a LLAGN as the {\it main} origin of the X-ray emission in NGC 253. For M82, the presence of an Fe-K line at ~ 6.7 keV, and the convex profile of its 2-10 keV continuum, indicate a significant thermal component. Contributions from point sources (e.g. X-ray binaries, supernova remnants, and/or a LLAGN) and Compton emission are also likely. Altogether, BeppoSAX results provide compelling evidence for the existence of a hot interstellar plasma in both galaxies, possibly in the form of superwind outflows from the disks of these galaxies. Order-of-magnitude estimates and some implications, such as the expelled mass and the energetics of the outflowing gas of this superwind scenario, are discussed. These new results also suggest some similarity between the X-ray emission from these galaxies and that from the Galactic Ridge.

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arXiv1999

BeppoSAX unveils the nuclear component in NGC 6240

The direct nuclear X-ray emission of the ultraluminous IRAS galaxy NGC 6240 has been observed for the first time by BeppoSAX. It is seen through an absorber with N_H=2*10^{24} cm^{-2}. The 2-10 keV X-ray luminosity >10^{44} erg s^{-1} definitely proves that a powerful AGN is hosted in this galaxy, despite the LINER optical appearance. Assuming the typical L_X/L_bol ratio for quasars, our result implies that it is the AGN and not the starburst that dominates the energy output of NGC 6240.

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arXiv1999

The contribution of faint AGN to the hard X-ray background

Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as AGN, from sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage this corresponds to resolving about 30 % of the hard Cosmic X-ray Background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level respectively), thus supporting the scenario in which a significant fraction of the XRB is made by obscured AGN.

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arXiv1999

A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: the Catalog and the LogN-LogS

We describe the selection of an X-ray flux-limited sample of bright clusters of galaxies in the southern hemisphere, based on the first analysis of the ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an identification of candidate clusters in RASS1, and their X-ray fluxes are remeasured using the Steepness Ratio Technique. This method is better suited than the RASS1 standard algorithm for measuring flux from extended sources. The final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so that due to the distribution of NH, its effective flux limit varies between about 3-4 x 10**-12 ergs cm**-2 s**-1 over the selected area. This covers the Decl<2.5 deg part of the south Galactic cap region (b<-20 deg) - with the exclusion of patches of low RASS1 exposure time and of the Magellanic Clouds area - for a total of 8235 deg**2. 130 candidate sources fulfill our selection criteria for bonafide clusters of galaxies in this area. Of these, 101 are Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of these clusters, 126 (97%) clusters have a redshift and for these we compute an X-ray luminosity. 20% of the cluster redshifts come from new observations, as part of the ESO Key Program REFLEX Cluster Survey that is under completion. Considering the intrinsic biases and incompletenesses introduced by the flux selection and source identification processes, we estimate the overall completeness to be better than 90%. The observed number count distribution, LogN-LogS, is well fitted by a power law with slope alpha = 1.34 +/- 0.15 and normalization A = 11.87 +/- 1.04 sr**-1 (10**-11 ergs cm**-2 s**-1)**alpha, in good agreement with other measurements.

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arXiv1999

The BeppoSAX view of the hard X-ray background

First results on a medium-deep X-ray survey in the "new" 5-10 keV band carried out with the MECS detectors onboard BeppoSAX are presented. The High Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where the energy density of the X-ray background is more than twice than that in the soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant population at 5-10 keV fluxes of the order of 10e-13 cgs. We discuss the implications of these findings for the AGN synthesis models for the XRB.

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arXiv1999

Diffuse Thermal Emission from Very Hot Gas in Starburst Galaxies: Spatial Results

New BeppoSAX observations of the nearby prototypical starburst galaxies NGC 253 and M82 are presented. A companion paper (Cappi et al. 1998;astro-ph/9809325) shows that the hard (2-10 keV) spectrum of both galaxies, extracted from the source central regions, is best described by a thermal emission model with kT ~ 6-9 keV and abundances ~ 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. This quite clearly rules out a LLAGN as the main responsible for their hard X-ray emission. Significant contribution from point-sources (i.e. X-ray binaries (XRBs) and Supernovae Remnants (SNRs)) cannot be excluded; neither can we at present reliably estimate the level of Compton emission. However, we argue that such contributions shouldn't affect our main conclusion, i.e., that the BeppoSAX results show, altogether, compelling evidence for the existence of a very hot, metal-poor interstellar plasma in both galaxies.

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arXiv1999

The X-ray Luminosity Function of Bright Clusters in the Local Universe

We present the X-ray luminosity function (XLF) for clusters of galaxies derived from the RASS1 Bright Sample. The sample, selected from the ROSAT All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains 130 clusters with flux limits in the range ~ 3-4 x 10^-12 ergs/cm^2/s in the 0.5-2.0 keV band. A maximum-likelihood fit with a Schechter function of the XLF over the entire range of luminosities (0.045 - 28. x 10^44 ergs/s), gives alpha = 1.52 +/- 0.11, L_* = 3.80 +0.70 -0.55 x 10^44 ergs/s, and A = 5.07 +/- 0.45 x 10^-7 Mpc^-3 (10^44 ergs/s)^(α-1). We investigate possible evolutionary effects within the sample, out to our redshift limit (z ~ 0.3), finding no evidence for evolution. Our results are in good agreement with other local estimates of the XLF, implying that this statistic for the local universe is now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6), shows that no evolution is present for L_X < 10^{44} ergs/s. However, we detect differences at the 3 sigma level, between our local XLF and the distant one estimated by Henry et al. for the EMSS sample. This difference is still present when considering the EMSS sample revised by Nichol et al.

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arXiv1999

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 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.

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arXiv1998

One more surprise from the Circinus Galaxy: BeppoSAX discovery of a transmission component in hard X-rays

The Circinus Galaxy has been observed, for the first time above 10 keV, by BeppoSAX. An excess emission above the extrapolation of the best fit 0.1--10 keV spectrum is apparent in the PDS data. The most likely explanation is that we are observing the nucleus through a column density of about 4x10**24 cm-2, i.e. large enough to completely block the radiation below 10 keV but small enough to permit partial transmission above this energy.

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arXiv1998

BeppoSAX confirms extreme relativistic effects in the X-ray spectrum of MCG-6-30-15

We report the first simultaneous measure of the X-ray broadband (0.1--200 keV) continuum and of the iron K-alpha fluorescent line profile in the Seyfert 1 galaxy MCG-6-30-15. Our data confirms the ASCA detection of a skewed and redshifted line profile (Tanaka et al. 1995). The most straightforward explanation is that the line photons are emitted in the innermost regions of a X-ray illuminated relativistic disk. The line Equivalent Width (~200 eV) is perfectly consistent with the expected value for solar abundances, given the observed amount of Compton reflection. We report also the discovery of a cut-off in the nuclear primary emission at the energy of ~160 keV.

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arXiv1998

The BeppoSAX High Energy Large Area Survey (HELLAS)

We have surveyed about 50 deg-2 of sky in the largely unexplored 5-10 keV band using the BeppoSAX MECS instrument, finding 180 sources. After correction for the non uniform sky coverage we find about 18 sources deg-2 with F(5-10keV) >= 5E-14 cgs, and resolve 30-40 % of the hard 5-10 keV Cosmic X-ray Background (XRB). Optical identification of a first small sample of sources show that most (11 out of 14) are AGN. Six of these show evidence of absorption/extinction in X-ray/optical, thus providing support to the scenario in which the hard XRB is largely made by obscured AGN (Setti & Woltjer 1989, Comastri et al. 1995).

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arXiv1998

1H0419-577: A "two-state" soft X-ray Seyfert galaxy

In this paper we report on the first simultaneous optical and X-ray (Beppo-SAX) observations of the radio-quiet AGN 1H0419-577. The optical spectrum clearly leads us to classify this source as a Seyfert 1. The X-ray spectrum is, however, somewhat at odds with this classification: a simple flat (Gamma ~ 1.55) and featureless power--law is a good description of the whole 1.8-40 keV spectrum, even if the upper limit to a broad iron line is not very tight. An analysis of a still unpublished ROSAT observation of the same target reveals that the soft X-ray spectrum has undergone a transition from a steep (Gamma ~ 2.5) to a flat (Gamma ~ 1.55) state, at least in the 0.7-2 keV band. If this difference is due to a remarkably variable soft excess, it is unlikely that a single component is responsible for the optical/UV/soft X-ray spectral distribution. The hypothesis that the difference is due to a change in the primary X-ray continuum and its implications for current Comptonization models are discussed.

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arXiv1998

Beppo-Sax Observation of the Coma cluster

We present first results of the BeppoSAX observation of the Coma Cluster. Thanks to the unprecedented sensitivity of the PDS instrument, the source has been detected up to ~80 keV. There is clear evidence for emission in excess to the thermal one above ~25 keV, very likely of non-thermal origin. We have therefore, for the first time, detected the long sought Inverse Compton emission on CMB photons predicted in clusters, like Coma, with radio halos. Combining X and radio observations, a value of 0.16 micro Gauss for the volume-averaged intracluster magnetic field is derived.

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arXiv1998

Gamma-Ray Burst 980329 and its X-Ray Afterglow

GRB 980329 is the brightest gamma-ray burst detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. With respect to its fluence (2.6 X 10**-5 erg/s/cm**2 in 50 to 300 keV) it would be in the top 4% of gamma-ray bursts in the 4B catalog (Meegan et al. 1998). The time-averaged burst spectrum from 2 to 20 and 70 to 650 keV can be well described by the empirical model of Band et al. (1993). The resulting photon index above the break energy is exceptionally hard at -1.32 +/- 0.03. An X-ray afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 h after the event within the error box as determined with the Wide Field Cameras. Its peak flux is (1.4 +/- 0.2) X 10**-12 erg/s/cm**2 (2 to 10 keV). The afterglow decayed according to a power law function with an index of -1.35 +/- 0.03. GRB 980329 is characterized by being bright and hard, and lacking strong spectral evolution.

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arXiv1998

Beppo-SAX Observations of Galaxy Clusters

The high spatial resolution of the MECS experiment on board Beppo-SAX has encouraged a few scientists, including the author, to perform observations of galaxy clusters. Results from the analysis of the first few observed objects are encouraging. After having reviewed the Beppo-SAX observing program for galaxy clusters and referenced contributions to these proceedings by other authors on the same topic, I present results from the analysis of the Perseus cluster.

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