arXiv subjects

L. Bassani

Publications and source records attributed to L. Bassani.

At least 181 records · Page 10Linked to original sources

arXiv2002

First Results from a XMM-Newton Survey of a Distance-Limited (D<22 Mpc) Sample of Seyfert Galaxies: I- the Agns

We report here preliminary results from a survey of nearby Seyfert galaxies using the EPIC CCDs on board XMM-Newton. The total sample consists of 28 Seyfert galaxies, and constitute a well-defined, complete (B<12.5 mag), and volume-limited (D<22 Mpc) sample of Seyfert galaxies in the northern (Delta > 0 deg.) hemisphere. The survey has been initiated in June, 2001, and we report here the results for the 6 objects analyzed so far, namely: NGC3185, NGC3486, NGC3941, NGC4138, NGC4565, and NGC5033. The main goal of this survey is to obtain a better and unbiased understanding of the ``typical'' Seyfert X-ray spectrum (e.g. the distribution of their absorption column density) in the local Universe. This is crucial to verify the predictions and, thus, to validate unified models, and is a fundamental parameter of synthesis models for the X-ray background. A companion poster (paper II: L. Foschini et al., these proceedings) illustrates how this survey will also allow a comprehensive spectral study of the brightest (highest-luminosity) off-nuclear sources in the galaxies.

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arXiv2002

First Results from a XMM-Newton Survey of a Distance-Limited (D < 22 Mpc) Sample of Seyfert Galaxies. II - The Galactic Serendipitous Sources

We present preliminary results from a study with XMM-Newton of the galactic serendipitous sources in a sample of 28 nearby Seyfert galaxies (with D < 22 Mpc). We concentrate here on a subset of four galaxies, namely: NGC3185, NGC3486, NGC3941, and NGC4565. In these galaxies we found six ultraluminous X-ray source (ULX). The large collecting area of XMM-Newton makes the statistics sufficient to perform spectral fitting with simple models. We shortly discuss the results obtained so far.

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arXiv2002

Spectral properties of faint hard X-ray selected sources

We present preliminary results from a XMM-Newton spectroscopic survey of hard (2 -10 keV) X-ray sources detected serendipitously in seven EPIC FOVs. All observations were performed during the performance/verif ication phase. The resulting sample consists of 39 sources with fluxes ranging from 3 to 80 $\t imes$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 2-10 keV band. Most of these sources (23 out of 39, i.e. 60%) are fainter than 8 $\times $ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$. The sources have been selected in order to obtain detailed X-ray spectral inform ation, that are unprecedented at such low flux levels and for such a large sample. To date we also have optical counterparts for 23 out of 39 objects, with 0.0058 $\leq$ $z$ $\leq$ 1.187. About 70% of the sources have spectra well fitted with a simple power law model with Galactic absorption. For 9 objects (five of type 1, one of type 2 and three normal galaxies) absorption in excess to the Galactic one is required. Additional emission/absorption components (i.e. soft excess, warm absorber) are observed in 4 sources.

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arXiv2001

Mkn 463 field observed by BeppoSAX

In this work we present the observation of the Mkn 463 field performed with the MECS instrument on-board BeppoSAX in the 1.8-10.5 keV band. The Mkn 463 field is an example of an extragalactic field crowded with absorbed X-ray sources: apart from the Seyfert 2 galaxy Mkn 463 and the well known QSO PG 1352+183 (the only object showing no absorption), two other objects are detected with a column density in excess to the galactic value. The first 1SAX J1353.9+1820 is a red QSO from the BeppoSAX High Energy Large Area Survey (HELLAS). The second 1SAX J1355.4+1815 is optically unidentified, but its X-ray spectral characteristics indicate that it too is an AGN hidden behind a large column density.

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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

Unveiling the AGN powering the "Composite" Seyfert/Star-forming galaxy NGC 7679: BeppoSAX and ASCA results

We discuss BeppoSAX observations and archive ASCA data of NGC 7679, a nearby, nearly face-on SB0 galaxy in which starburst and AGN activities coexist. The X-ray observations reveal a bright (L_{0.1-50 keV} \sim 2.9 \times 10^{43} erg s^{-1}) and variable source having a minimum observed doubling/halving time scale of \sim 10 - 20 ksec. A simple power law with photon index of Γ\sim 1.75 and small absorption (N_H < 4\times 10^{20} cm^{-2}) can reproduce the NGC 7679 spectrum from 0.1 up to 50 keV. These X-ray properties are unambiguous signs of Seyfert 1 activity in the nucleus of NGC 7679. The starburst activity, revealed by the IR emission, optical spectroscopy and Hαimaging, and dominating in the optical and IR bands, is clearly overwhelmed by the AGN in the X-ray band. Although, at first glance, this is similar to what is observed in other starburst-AGN galaxies (e.g. NGC 6240, NGC 4945), most strikingly here and at odds with the above examples, the X-ray spectrum of NGC 7679 does not appear to be highly absorbed. The main peculiarity of objects like NGC 7679 is not the strength of their starburst but the apparent optical weakness of the Seyfert 1 nucleus when compared with its X-ray luminosity. To date NGC 7679 is one of the few Seyfert 1/ Starburst composites for which the broad-band X-ray properties have been investigated in detail. The results presented here imply that optical and infrared spectroscopy could be highly inefficient in revealing the presence of an AGN in these kinds of objects, which instead is clearly revealed from X-ray spectroscopic and variability investigations.

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arXiv2001

On the Origin of the FeK$α$ Line in the Seyfert 2 Galaxy NGC 7172

The Seyfert 2 Galaxy NGC 7172 was observed twice by BeppoSAX narrow field instruments approximately one year apart. As found in previous observations, the source is variable on short time scales (hours) by a factor of $\sim$30%, confirming the presence of a type 1 nucleus at the center. A strong flux variability (by a factor of 2) on a longer time scale was observed between the two observations. Indeed, in November 1997, the lowest flux ever recorded was detected by BeppoSAX. The broad band spectra obtained with the BeppoSAX narrow field instruments show marginal evidence of a reflection component. This structure could explain the very flat spectra previously observed in the 2-10 keV band. An FeK$α$ line was also detected in both observations. The line intensity appears to remain almost constant between the two observations even if the associated large errors cannot exclude some variability. In order to understand the problem of the origin of the line, data from previous observations, performed with other satellites, were also used. The scenario favored is one where the line is produced at a distance of approximatively 8 light years from the continuum source region, rather than having its origin in the accretion disk. In the framework of the standard unified models for AGN, this suggests that its origin is located in the molecular torus.

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arXiv2000

BeppoSAX discovery of a new Seyfert 2 galaxy: 1SAXJ2234.8-2541

In the present work we report the BeppoSAX serendipitous discovery of the type 2 AGN 1SAXJ2234.8-2541 in the MECS field of view when pointing at the Seyfert 2 galaxy NGC7314. The source is optical identified with the bright (m_B=14.40) galaxy ESO533-G50 at redshift z=0.026. The source is clearly detected at energies above 4 keV but barely visible below this energy implying heavy obscuration intrinsic to the source. Spectral analysis indicates a column density of the order of 2-3 x 10^(23) cm^-2$ and a power law photon index compatible with values often seen in active galaxies. These X-ray characteristics suggest a Seyfert 2 classification for 1SAXJ2234.8-2541. Subsequent spectroscopy of the optical conterpart, ESO533-G50, performed with the ESO 1.52m telescope at La Silla Observatory, confirms the type 2 nature of this source and therefore its identification with the X-ray source. We also report the marginal detection of an iron line centered at 7.29 +/- 0.47 keV and having an equivalent width of 464 (+636 -398) eV; although marginal, this result is indicative of the presence of warm material in the source. If instead the line is associated to cold material, we estimate an upper limit to its equivalent width of 290 eV. The overall characteristics of 1SAXJ2234.8-2541 strongly suggest that the source is Compton thin.

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arXiv2000

The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX

Results from BeppoSAX observations of Compton--thick Seyfert 2 galaxies are summarized and reviewed, and their general properties derived and discussed. In five out of the seven observed sources, the nucleus is directly visible at high X-ray energies, where the photons penetrate absorbers with column densities in the range 1.1-4.3x10^{24} cm-2 (in the other two sources, NGC 1068 and NGC 7674, the nucleus is instead totally obscured at all energies, implying even larger column densities). In most sources there is unambiguous evidence of a reflection component from optically thick, cold matter, while in two (or maybe four) cases there is also evidence of reflection from ionized matter. For the sources with a measured X-ray luminosity, a comparison with the infrared luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the IR emission appears to be dominated by starburst activity, in the other three sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by reprocessing of the UV and X-ray photons emitted by an AGN.

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arXiv2000

The hard X-ray properties of the Seyfert nucleus in NGC 1365

We present BeppoSAX observations of the Seyfert 1.8 galaxy NGC1365 in the 0.1--100 keV range. The source was 6 times brighter than during an ASCA observation 3 years earlier. The 4--10 keV flux is highly variable during the BeppoSAX observation, while the soft (0.1-4 keV) emission is constant within the errors. Both a cold and a warm reflector and a cold absorber are required to explain the observed spectrum. The comparison between ASCA and BeppoSAX spectra strongly suggests that the circumnuclear material has a more complex structure than a simple homogeneous torus, with quite different absorbing gas columns along different lines of sight. A broad iron K_alpha line is also present in the spectrum, with the peak energy significantly redshifted. This can be explained by means of a relativistic disk line model. Alternatively, a warm absorption Fe line system with N_H ~ 10^23 cm^(-2) could account for the observed line profile.

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arXiv2000

A broadband X-ray view of NGC4945

We present the results of a BeppoSAX observation of the nearby spiral galaxy NGC4945 in the 0.1-200 keV energy band. The nuclear X-ray emission emerges above ~7 keV, through an absorber with column density N_H ~ a few 10^24/cm/cm. Its remarkable variability (doubling/halving timescale ~10^4 s) is not associated with any appreciable spectral changes, ruling out variations of the interposed absorber or changes in the primary continuum shape. The intense iron K-ALPHA fluorescent emission line is likely to be produced by the same nuclear absorbing matter. Our estimate of the intrinsic nuclear power suggests that the active nucleus dominates the bolometric energy output of NGC4945, if its nucleus has a typical quasar L_X/L(bol) ratio. This supports the idea that X-rays are the best energy band to search for absorbed AGN. The 0.1-5 keV emission is extended along the plane of the galaxy, and most likely due to a population of unresolved binaries, as believed to happen in several early-type galaxies. Alternatively, hot gas associated with a starburst-driven superwind outflow could substantially contribute to the extended emission above 1 keV.

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arXiv1999

BeppoSAX uncovers a type-2 QSO in the hyperluminous infrared galaxy IRAS 09104+4109

We studied with BeppoSAX the infrared luminous galaxy IRAS 09104+4109 over a very wide X-ray band from 0.1 to 80 keV. Our observations indicate the dominance of a thermal component at energies below 8 keV, which we attribute to the free-free emission from the intracluster (IC) plasma surrounding the source. Above 10 keV we find evidence for the existence of flux in excess with respect to the free-free IC plasma emission. This, together with the marginal detection of a neutral iron line at $\sim 6.4$ keV, gives a strong indication for the presence of an AGN deeply buried within the source. This component is best modelled by a strongly absorbed (N_H > 5 10^{24} cm^{-2}) power-law plus unabsorbed reflection spectrum (0.15<R<0.3), which is also responsible for the cold iron line. The unabsorbed broad-band (2-100 keV) X-ray emission of this AGN is 2.5 10$^{46} erg/s, well within the range of quasar luminosities. Our results indicate that IRAS 09104+4109 is indeed the prototype of a rare class of sources, the luminous type-2 QSOs. The association of this source with a huge cooling-flow of $\sim 1000 M_\odot$ in the cluster, as indicated by the X-ray data, might suggest that such condition of extremely fast mass accumulation could favour the survival of a thick obscuring envelope, which would otherwise be quickly destroyed by the very luminous central source.

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arXiv1999

The low luminosity AGN in the LINER galaxy M81: BeppoSAX discovery of highly ionized gas

The LINER nucleus of the nearby spiral galaxy M81 was pointed by BeppoSAX, which caught it at the highest (2-10) keV flux level observed so far. The LECS, MECS and PDS data, extending over (0.1-100) keV, are used to investigate the physical similarities and differences between LINERs and AGNs. The continuum is well fitted by a power law of photon index \sim 1.84, modified by little absorption due to cold material. Superimposed on the continuum BeppoSAX detects a 6.7 keV emission line (confirming an ASCA result) and an absorption edge at 8.6 keV. Both spectral features are consistent with being produced by iron at the same high ionization level, and probably also with the same column density. So, we suggest that they originate from transmission through highly ionized thin material. Concerning the origin of the continuum emission, we do not observe signs of reflection from the optically thick material of an accretion disk, as usually found in Seyfert 1's (a 6.4 keV emission line and a broad bump peaking at 10-20 keV). The low bolometric luminosity of the nucleus of M81 is consistent with being produced by advection dominated accretion; in this case the X-ray emission should be dominated by Comptonization, rather than by bremsstrahlung, in order to reproduce the steep spectrum observed over the (0.1-100) keV band.

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arXiv1999

Hard X-ray detection of the high redshift quasar 4C 71.07

BATSE/OSSE observations of the high redshift quasar 4C 71.07 indicate that this is the brightest and furthest AGN so far detected above 20 keV. BATSE Earth occultation data have been used to search for emission from 4C 71.07 from nearly 3 years of observation. The mean source flux over the whole period in the BATSE energy range 20-100 keV is (13.2 +/- 1.06) x 10^(-11) erg cm^(-2) s^(-1) corresponding to a luminosity of 2 x 10^(48) erg s^(-1). The BATSE light curve over the 3 years of observations shows several flare-like events, one of which (in January 1996) is associated with an optical flare (R=16.1) but with a delay of 55 days. The OSSE/BATSE spectral analysis indicates that the source is characterized by a flat power spectrum (Gamma about 1.1 - 1.3) when in a low state; this spectral form is consistent within errors with the ASCA and ROSAT spectra. This means that the power law observed from 0.1 to 10 keV extends up to at least 1 MeV but steepens soon after to meet EGRET high energy data. BATSE data taken around the January 1996 flare suggests that the spectrum could be steeper when the source is in a bright state. The nuF-nu representation of the source is typical of a low frequency peaked/gamma-ray dominated blazar, with the synchrotron peak in the mm-FIR band and the Compton peak in the MeV band. The BATSE and OSSE spectral data seem to favour a model in which the high energy flux is due to the sum of the synchrotron self-Compton and the external Compton contributions; this is also supported by the variability behaviour of the source.

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arXiv1999

BeppoSAX Observation of NGC 7582: Constraints on the X-ray absorber

This paper presents a BeppoSAX observation of NGC 7582 made during 1998 November and an optical spectrum taken in 1998 October. The new X-ray data reveal a previously unknown hard X-ray component in NGC 7582, peaking close to 20 keV. Rapid variability is observed with correlated changes in the 5-10 and 13-60 keV bands indicating that a single continuum component, produced by the active nucleus, provides the dominant flux across both bands. Changes in the nuclear X-ray flux appear unrelated to the gradual decline in optical flux noted since the high-state in 1998 July. The X-ray continuum is attenuated by a thick absorber of N_H ~ 1.6 x 10E24 cm^-2 covering ~60% of the nucleus, plus a screen with N_H ~ 1.4 x 10E23 cm^-2 covering the entire nucleus. Comparison of the BeppoSAX and ASCA spectra shows an increase in the full screen by N_H ~7 x 10E22 cm^-2 since 1994, confirming the absorption variability found by Xue et al. The increase in soft X-ray flux between 1994 and 1998 is consistent with the appearance of holes in the full screen producing some clear lines-of-sight to the broad-line-region.

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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

Soft X-ray spectroscopy of Compton-thick Seyfert 2 galaxies with BeppoSAX

We present a X-ray spectroscopic study of the bright Compton-thick Seyfert 2s NGC1068 and the Circinus Galaxy, performed with BeppoSAX. Matt et al. (1997 and 1998) interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC1068 spectrum is rich in emission lines, mainly due to K-alpha transitions of He-like elements from oxygen to iron, plus a K-alpha fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, of the warm scatterer less than a few 10^21 atoms/cm/cm. In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with ionization parameter U<5 and density about a few times 10^22 atoms/cm/cm. In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC1068 an optically thin plasma emission with kT~500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed.

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