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A. Comastri

Publications and source records attributed to A. Comastri.

At least 289 records · Page 16Linked to original sources
arXiv2002

The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.

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arXiv2002

The log(N)-log(S) and the Broadband Properties of the Sources in the HELLAS2XMM Survey

We present the first results from an XMM-Newton serendipitous medium-deep survey, which covers nearly three square degrees. We show the log(N)-log(S) distributions for the 0.5-2, 2-10 and 5-10 keV bands, which are found to be in good agreement with previous determinations and with the predictions of XRB models. In the soft band we detect a break at fluxes around 5x10^-15 cgs. In the harder bands, we fill in the gap at intermediate fluxes between deeper Chandra and XMM-Newton observations and shallower BeppoSAX and ASCA surveys. Moreover, we present an analysis of the broad-band properties of the sources.

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arXiv2001

The AGN content of hard X-ray surveys

Multiwavelength observations of the hard X-ray selected sources discovered by BeppoSAX, Chandra and XMM-Newton surveys have significantly improved our knowledge of the AGN population. The increasing number of X-ray obscured AGN so far discovered confirms the prediction of those AGN synthesis models for the X-ray background based on the Unified scheme. However, follow-up optical observations of hard X-ray selected sources indicate that their optical properties are quite varied and the simple relations between optical and X-ray absorption are by no means without exception. Moreover, there is evidence of a substantial number of luminous X-ray sources hosted by apparently normal galaxies. In this paper the results obtained from multiwavelength observations of hard X-ray selected sources discovered by BeppoSAX and XMM-Newton are presented and briefly discussed.

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arXiv2001

Disclosing the true nature of the Sy 2 galaxy NGC 3281: one more Compton-thick source

We present the BeppoSAX broad-band X-ray spectrum of the Seyfert 2 galaxy NGC 3281. The source high-energy spectrum is characterized by the nuclear transmitted component, with an absorbing column density of about 2 10^24 cm^-2, while the MECS spectrum is reflection-dominated, with a prominent (EW about 0.5-1.2 keV) iron K_alpha emission line. The source is detected at only the 5 sigma significance level in the LECS band, because of the strong obscuration which hampers at low energies the direct view of the active nucleus harbored in NGC 3281. BeppoSAX results are consistent with the scenario where NGC 3281 is inclined more than 60 degrees with respect to the line-of-sight. Combining the N_H value obtained from the present X-ray analysis with the A_V measurement, a N_H/A_V about 50 times the Galactic value is derived.

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arXiv2001

Chandra discovery of extended non-thermal emission in 3C 207 and the spectrum of the relativistic electrons

We report on the {\it Chandra} discovery of large scale non--thermal emission features in the double lobed SSRL quasar 3C 207 (z=0.684). These are: a diffuse emission well correlated with the western radio lobe, a bright one sided jet whose structure coincides with that of the eastern radio jet and an X-ray source at the tip of the jet coincident with the hot spot of the eastern lobe. The diffuse X-ray structure is best interpreted as inverse Compton (IC) scattering of the IR photons from the nuclear source and provides direct observational support to an earlier conjecture (Brunetti et al., 1997) that the spectrum of the relativistic electrons in the lobes of radio galaxies extends to much lower energies than those involved in the synchrotron radio emission. The X-ray luminous and spatially resolved knot along the jet is of particular interest: by combining VLA and {\it Chandra} data we show that a SSC model is ruled out, while the X-ray spectrum and flux can be accounted for by the IC scattering of the CMB photons (EIC) under the assumptions of a relatively strong boosting and of an energy distribution of the relativistic electrons as that expected from shock acceleration mechanisms. The X-ray properties of the hot spot are consistent with a SSC model. In all cases we find that the inferred magnetic field strength are lower, but close to the equipartition values. The constraints on the energy distribution of the relativistic electrons, imposed by the X-ray spectra of the observed features, are discussed. To this aim we derive in the Appendices precise semi--analytic formulae for the emissivities due to the SSC and EIC processes.

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arXiv2001

Chandra detection of the radio and optical double hot spot of 3C 351

In this letter we report a Chandra X-ray detection of the double northern hot spot of the radio quasar 3C 351. The hot spot has also been observed in the optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are interpreted as the results of the synchrotron and synchrotron-self-Compton (SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times smaller than the equipartition values. In the framework of shock acceleration theory, we show that the requirement for such a relatively small field strength is in agreement with the fitted synchrotron spectral models and with the sizes of the hot spots. Finally, we show that the combination of a lower magnetic field strength with the high frequencies of the synchrotron cut-off in the fitted synchrotron spectra provides strong evidence for electron acceleration in the hot spots.

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arXiv2001

BeppoSAX view of NGC 526A: a Seyfert 1.9 galaxy with a flat spectrum

In the present work we report the BeppoSAX observation of the Seyfert 1.9 galaxy NGC 526A in the band 0.1-150 keV. The high energy instrument onboard, PDS, has succeeded in measuring for the first time the spectrum of this source in the 13-150 keV range. The combined analysis of all Narrow Field Instruments provides a power law spectral index of ~ 1.6 and confirms the flat spectral nature of this source. Although NGC 526A varies strongly in the 2-10 keV over period of months/years, its spectral shape remains constant over these timescales. An Fe K-alpha line, characterized by a complex structure, has been detected in the 6-7 keV range. The line, which has an equivalent width of 120 eV, is not compatible with being produced in an absorbing torus with N_H ~ 10^22 cm^-2, but most likely originates by reflection in an accretion disk viewed at an intermediate inclination angle of ~ 42 deg. The reflection component is however small (R < 0.7) and so it is not sufficient to steepen the spectrum to photon index values more typical of AGNs. Instead, we find that the data are more consistent with a flat power law spectrum cut-off at around 100 keV plus a small reflection component which could explain the observed iron line. Thus NGC 526A is the only bona-fide Seyfert 2 galaxy which maintains a "flat spectrum" even when broad band data are considered: in this sense its properties, with respect to the general class of Seyfert 2's, are analogous to those of NGC 4151 with respect to the vast majority of Seyfert 1's.

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arXiv2001

Optical identification of sources from the HELLAS2XMM survey

We present preliminary results on the optical identifications of sources from the High Energy Large Area Survey performed with XMM-Newton (HELLAS2XMM). This survey covers about 3 square degrees of sky down to a 2-10 keV flux of 7 x 10^-15 erg cm-2 s-1 (Baldi & Molendi these proceedings, Baldi et al. 2001). The survey good sensitivity over a large area allows us to confirm and extend previous Chandra results about a population of X-ray luminous but optically dull galaxies emerging at 2-10 keV fluxes of about 10-14 erg cm-2 s-1. Although the statistics are still rather poor, it appears that at these X-ray fluxes the fraction of these galaxies is similar or even higher than that of narrow line AGN.

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arXiv2001

X-ray bright optically quiet galaxies: the case of P3

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at X-ray fluxes of the order of 10-14 erg cm-2 s-1. The nature of these objects is still unknown. We present the results of multiwavelength observations of what can be considered the best studied example: the Chandra source CXOUJ031238.9-765134 optically identified by Fiore et al. (2000) with an apparently normal galaxy at z=0.159 and called ``Fiore P3''.

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arXiv2001

The HELLAS2XMM survey: I. The X-ray data and the Log(N)-Log(S)

We present the first results from an XMM-Newton serendipitous medium-deep survey, which covers nearly three square degrees. We detect a total of 1022, 495 and 100 sources, down to minimum fluxes of about 5.9 x 10^-16, 2.8 x 10^-15 and 6.2 x 10^-15 erg cm^-2 s^-1, in the 0.5-2, 2-10 and 4.5-10 keV band, respectively. In the soft band this is one of the largest samples available to date and surely the largest in the 2-10 keV band at our limiting X-ray flux. The measured Log(N)-Log(S) are found to be in good agreement with previous determinations. In the 0.5-2 keV band we detect a break at fluxes around 5 x 10^-15 erg cm^-2 s^-1. In the harder bands, we fill in the gap at intermediate fluxes between deeper Chandra and XMM-Newton observations and shallower BeppoSAX and ASCA surveys.

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arXiv2001

The BeppoSAX High Energy Large Area Survey HELLAS, II: Number counts and X-ray spectral properties

The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85 deg^2 of sky in the 5-10 keV band down to a flux of 4-5 10^-14 erg cm-2 s-1. The source surface density of 16.9+-6.4 deg$^{-2}$ at the survey limit corresponds to a resolved fraction of the 5-10 keV X-ray background (XRB) of the order of 20-30 %. Hardness ratios analysis indicates that the spectra of a substantial fraction of the HELLAS sources (at least one third) are harder than a alpha_E = 0.6 power law. This hardness may be due to large absorbing columns. The hardness ratio analysis also indicates that many HELLAS sources may have a spectrum more complex than a single absorbed power law. A soft component, superimposed to a strongly cut-off power law, is likely to be present in several sources.

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arXiv2001

The BeppoSAX High Energy Large Area Survey HELLAS, III: testing synthesis models for the X-ray background

The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of square degrees of the sky in the 5--10 keV band down to a flux of about 5 10^-14 erg cm-2 s-1. The extrapolation of the HELLAS logN--logS towards fainter fluxes with an euclidean slope is consistent with the first XMM measurements, in the same energy band, which are a factor 20 more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the 5--10 keV counts over the range of fluxes covered by BeppoSAX and XMM a large fraction of highly absorbed (logN_H = 23--24 cm-2), luminous (L_X > 10^44 erg s-1) AGN is needed. A sizeable number of more heavily obscured, Compton thick, objects cannot be ruled out but it is not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators especially at high redshifts/luminosities where several broad line quasars show hardness ratios typical of absorbed power law models with logN_H=22--24 cm-2.

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arXiv2001

BeppoSAX Observations of 1 Jy BL Lacertae Objects. I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1 Jy sample plus one additional source. The collected data cover the energy range 0.1 - 10 keV (observer's frame), reaching ~ 50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the LBL type) display a relatively flat (alpha_x ~ 0.9) X-ray spectrum, which we interpret as inverse Compton emission. Four objects (2/3 of the LBL) show some evidence for a low-energy steepening which is likely due to the synchrotron tail merging into the inverse Compton component around ~ 1 - 3 keV. If this were generally the case with LBL, it would explain why the 0.1 - 2.4 keV ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones (by ~ 0.5 in alpha_x). The broad-band spectral energy distributions fully confirm this picture and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequencies, we confirm and clarify the dependence of the X-ray spectral index on synchrotron peak frequency originally found in ROSAT data.

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arXiv2001

The BeppoSAX view of the X-ray active nucleus of NGC4258

BeppoSAX observed NGC4258 on December 1998, when its 2-10 keV luminosity was \~1E41 erg/s. 100% variability is observed in the 3-10 keV band on timescales of a half a day while 20% variability is observed on timescales of one hour. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5+/-1.2)E22 cm-2; this component is detected up to 70 keV with S/N>=3 and with the steep power law energy index of 1.11+/-0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in ADAF models with strong winds, is ruled out by the data. The ratio between the nuclear radio and the X-ray luminosities is <=1E-5, similar to that of radio quiet AGN. X-ray variability and spectral shape, radio to X-ray and NIR to X-ray luminosity ratios suggest that the nucleus of NGC4258 could be a scaled-down version of a Seyfert nucleus. The soft (E<=2keV) X-ray emission is complex. There are at least two thermal-like components, with T1=0.6+/-0.1 keV and T2>=1.3 keV. The cooler (L(0.1-2.4keV)=1E40 erg/s) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI. The second component, which can be modeled equally well by an unobscured power law model, has L(0.1-2.4keV)~7E39 erg/s, consistent with that expected from discrete X-ray sources in the host galaxy. NGC4258 and other maser AGNs show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of maser emission from X-ray illumination. So a large column density gas may be a necessary property of masing AGNs.

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arXiv2001

The BeppoSAX High Energy Large Area Survey. IV. On the soft X-ray properties of the hard X-ray-selected HELLAS sources

We present a comprehensive study of the soft X-ray properties of the BeppoSAX High-Energy Large Area Survey (HELLAS) sources. A large fraction (about 2/3) of the hard X-ray selected sources is detected by ROSAT. The soft X-ray colors for many of these objects, along with the 0.5-2 keV flux upper limits for those undetected in the ROSAT band, do imply the presence of absorption. The comparison with the ROSAT Deep Survey sources indicates that a larger fraction of absorbed objects among the HELLAS sources is present, in agreement with their hard X-ray selection and the predictions of the X-ray background synthesis models. Another striking result is the presence of a soft (additional) X-ray component in a significant fraction of absorbed objects.

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arXiv2001

High Energy Large Area Surveys: from BeppoSAX to Chandra and XMM

Optical identification of hard X-ray selected BeppoSAX and Chandra sources indicates that a large fraction of the sources are ``intermediate'' AGN, i.e. type 1.8-1.9 AGN, broad-line quasars and even X-ray loud but optically dull (apparently normal) galaxies, all obscured in X-rays by columns of the order of logNH=22-23.5 cm-2. Because of this obscuring matter, these sources are more difficult to detect or select at other wavelengths, implying that a fraction of the accretion power in the Universe may have been missed so far.

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arXiv2001

BeppoSAX Observations of the Seyfert-2 Galaxies NGC 7172 and ESO 103-G35

We investigate the X-ray spectra of the type-2 Seyfert galaxies NGC 7172 and ESO 103-G35, using BeppoSAX observations, separated by approximately one year. We find that the X-ray spectra of both NGC 7172 and ESO 103-G35 can be well fitted using a power-law model with an Fe K emission line at 6.4 keV. We did not find any statistically significant evidence for the existence of a reflection component in the X-ray spectra of these two galaxies. The continuum flux has decreased by a factor of approximately two during this period, in both objects. However, the spectral index as well as the absorption column have remained constant. We find weak evidence for the decrease of the normalization of the Fe K emission line in a similar manner to the continuum in NGC7172. We also report evidence for a broad Fe K confirming previous observations. In contrast, in the case of ESO 103-G35 the line flux does not change while its width remains unresolved.

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