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Context. There have been recent claims that a significant fraction of type 2 AGN accrete close to or even above the Eddington limit. In type 2 AGN, the bolometric luminosity (L(b)) is generally inferred from the [OIII] emission line luminosity (L(OIII)). The key issue in estimating the bolometric luminosity in these AGN, is therefore to know the bolometric correction to be applied to L(OIII). A complication arises from the observed L(OIII) being affected by extinction, most likely from dust within the narrow line region. The extinction-corrected [OIII] luminosity (L(OIII)(c)) is a better estimator of the nuclear luminosity than L(OIII). However, only the bolometric correction to be applied to the uncorrected L(OIII) has been evaluated so far. Aims. This paper is devoted to estimating the bolometric correction C(OIII) = L(b)/L(OIII)(c) for deriving the Eddington ratios for the type 2 AGN in a sample of SDSS objects. Methods. We collected 61 sources from the literature with reliable estimates of both L(OIII)(c) and X-ray luminosities (L(X)). To estimate C(OIII), we combined the observed correlation between L(OIII)(c) and L(X) with the X-ray bolometric correction. Results. In contrast to previous studies, we found a linear correlation between L(OIII)(c) and L(X). We estimated C(OIII) using an earlier luminosity-dependent X-ray bolometric correction, and we found a mean value of C(OIII) in the luminosity ranges log L(OIII) = 38-40, 40-42, and 42-44 of 87, 142, and 454, respectively. We used it to calculate the Eddington ratio distribution of type 2 SDSS AGN at 0.3 < z < 0.4 and found that these sources are not accreting near their Eddington limit, contrary to previous claims.

We report on the XMM-Newton observation of the bright Seyfert 1 galaxy, MCG+8-11-11. Data from the EPIC/p-n camera, the Reflection Gratings Spectrometers ( RGS), and the Optical Monitor ( OM) were analyzed. The p-n spectrum was fitted well by a power law, a spectrally unresolved Fe K alpha line, a Compton reflection component ( whose high value, when compared to the iron line equivalent width, suggests iron underabundance), and absorption by warm material. Absorption lines are apparent in the RGS spectra, but their identification is uncertain and would require large matter velocities. The UV fluxes measured by the OM are well above the extrapolation of the X-ray spectrum, indicating the presence of a UV bump.

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1 dF sample). One of the most interesting results (which is found also in deeper surveys) is that similar to 20% of the sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes 10 times lower than in the HELLAS2XMM sample, many of the extreme X/O sources in our sample have R less than or similar to 25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with extreme X/O values. While four of these sources are broad line QSO, eight of them are narrow line QSO, seemingly the extension to very high luminosity of the type 2 Seyfert galaxies.

We selected a sample of eight bright unobscured (at least at the iron line energy) Seyfert galaxies observed simultaneously by XMM-Newton and BeppoSAX, taking advantage of the complementary characteristics of the two missions. The main results of our analysis can be summarized as follows: narrow neutral iron lines are confirmed to be an ubiquitous component in Seyfert spectra; none of the analyzed sources shows unambiguously a broad relativistic iron line; all the sources of our sample (with a single exception) show the presence of a Compton reflection component; emission lines from ionized iron are observed in some sources; peculiar weak features around 5-6 keV (possibly arising from rotating spots on the accretion disk) are detected in two sources. The scenario emerging from these results strongly requires some corrections for the classical model of reprocessing from the accretion disk. As for materials farther away from the Black Hole, our results represent a positive test for the Unification Model, suggesting the presence of the torus in (almost) all sources, even if unobscured.

The analysis of a simultaneous XMM-Newton/BeppoSAX plus three previous BeppoSAX observations revealed that NGC 7213 is a rather peculiar Seyfert 1. No significant Compton reflection component was observed, while an iron line complex, best explained in terms of three narrow lines, is clearly apparent in the data. Due to the absence of the reflection component, the neutral iron line is likely not produced in a Compton-thick material, but current data do not allow to choose between a Compton-thin torus and the BLR. The two ionized iron lines from Fe X X V and Fe X X V I may be produced in a photoionized gas with a column density of a few 10(23) cm(-2), in analogy with the case of NGC 5506.

We present a detailed discussion of the spectral and spatial components of NGC 5506, based on XMM-Newton, Chandra and BeppoSAX observations. The overall picture consists of a nucleus absorbed by cold gas with column density of approximate to10(22) cm(-2) and surrounded by a Compton-thick torus, whose existence is inferred by a cold reflection component and an iron Kalpha line. On a much larger scale, a photoionized gas extended on approximate to350 pc reprocesses the nuclear radiation, producing a soft excess and ionized iron lines. Noteworthy, we find no evidence for the presence of the accretion disc.

We present and discuss Chandra and XMM-Newton observations of the Seyfert 2 galaxy and Compton-thick absorbed source, Tololo 0109-383. The hard X-ray emission (i.e. above similar to2 keV), is dominated by a spatially unresolved reflection component, as already discovered by previous ASCA and BeppoSAX observations. The soft X-ray emission is partly (similar to15%) extended over about 1 kpc. Below 2 keV, the spectrum is very steep and two emission lines, probably due to recombination to He-like ions of oxygen and neon, are clearly present. Combining X-rays and optical information taken from the literature, we propose an overall scenario for the nuclear regions of this source.

We present optical spectroscopic identifications of hard X-ray (5-10 keV) selected sources belonging to the High-Energy Large-Area Survey sample obtained with BeppoSAX down to a 5-10 keV flux limit of f(5-10 keV) similar to 3 x 10(14) ergs cm(-2) s(-1). The sample consists of 118 sources. There are 25 sources that have been identified through correlations with catalogs of known sources. A spectroscopic identification for 49 more has been searched for with the telescope. The results of 13 fields were empty down to R = 21. There were 37 sources identified as type 1 active galactic nuclei (AGNs) and nine as type 2 AGNs. The remaining sources are five narrow emission-line galaxies, six clusters, two BL Lac objects, one radio galaxy, and one star. Combining these objects with other hard X-ray selected AGNs from ASCA and HEAO1, we find that the local luminosity function of type 1 AGNs (AGN 1s) in the 2-10 keV band is fairly well represented by a double power law function. There is evidence for significant cosmological evolution according to a pure luminosity evolution (PLE) model [L-X(z) proportional to (1+ z)(k)], with k = 2.12 and 2.22 (sigma(k) similar or equal to 0.14) in a (Omega(m), Omega(lambda)) = (1.0; 0.0) and a (Omega(m), Omega(lambda)) = (0.3; 0.7) cosmology, respectively. The data show an excess of faint high-redshift type AGN 1s, which is well modeled by a luminosity-dependent density evolution (LDDE), similar to what is observed in soft X-rays. However, in both cosmologies, the statistics are not significant enough to distinguish between the PLE and LDDE models. The fitted models imply a contribution of AGN 1s to the 2 10 keV X-ray background from 35% up to 60%.

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 0.1-10keV spectrum is apparent in the PDS data. The most likely explanation is that we are observing the nucleus through a column density of similar to 4 x 10(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.

The direct nuclear X-ray emission of the ultraluminous IRAS galaxy NGC 6240 has been observed by BeppoSAX. It is seen through an absorber with N-H similar to 2 x 10(24) cm(-2) The 2-10 keV X-ray luminosity of > 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.

On 1998 May 9-11,BeppoSAX observed the low-luminosity Seyfert 1 galaxy NGC 4051 in an ultra-dim X-ray state. The 2-10 keV flux (1.26 x 10-(12) erg cm(-2) s(-1)) was about 20 times fainter than its historical average value, and remained steady along the whole observation (similar to 2.3 d). The observed flat spectrum (Gamma similar or equal to 0.8) and intense iron line (EW similar or equal to 600 eV) are best explained by assuming that the active nucleus has switched off, leaving only a residual reflection component visible.

Extended (R similar to 4.5 kpc) X-ray emission in the Seyfert 2 galaxy NGC 4388 has been discovered with the ROSAT HRI. No evidence for a nuclear, unresolved component has been found; the upper limit for such a component is about 20% of the total flux. The interpretation of the extended emission in terms of nuclear radiation scattered by a plasma ionized by the nuclear photons can be excluded on the basis of a simple photoionization calculation. The two most likely alternatives, i.e. thermal emission from a collisionally heated plasma and the sum from many unresolved discrete sources, are briefly discussed.

The capability of the Stellar X-ray Polarimeter, a focal plane instrument of the SODART telescope, to detect linear polarization in the X-ray emission of some bright AGNs is examined.

A method is proposed for the estimate of the black hole mass in an active galactic nucleus which is based on the response of the intensity, centroid energy and width of the iron Kalpha fluorescence line to variations of the ionizing continuum. The method is illustrated for the case of a geometrically thin and optically thick, X-ray illuminated disc, in Keplerian rotation around a Schwarzschild black hole, using a detailed model of the line emissivity. This method is complementary to the one that makes use of variations in detailed line profiles, and could be applied also to measurements obtained at moderate energy resolution.

The intensity and profile of the iron K-alpha fluorescence line from a flat, optically thick accretion disk rotating around a Schwarzschild black hole and illuminated by a central X-ray source are computed using a fully relativistic treatment of the photon intensity and shifts. The X-ray source is modelled as an isotropic point source located on the symmetry axis at a height h in units of the gravitational radius. These calculations represent a refinement and an extension of those presented in Paper I for h = 20, carried out using a weak field approximation and therefore of validity limited to inclination angles less-than-or-similar-to 70-degrees. Here it is shown that at high inclination angles purely relativistic effects lead to the growth of features between the two Doppler horns and that, as a consequence, the line equivalent width maintains a sizeable value, while the centroid energy and the line width go through a broad maximum at about approximately 80-degrees. The statistical implications for the expected distribution of the line parameters in a sample of randomly oriented disks in Seyfert galaxies are briefly discussed.

Ginga observations of several bright Seyfert 1 galaxies have recently revealed the presence in their X-ray spectra of both the iron K fluorescence line and a high energy bump, which together are currently interpreted as due to the reprocessing of the primary radiation by cold matter located near the putative black hole. A number of quantitative model predictions on the reprocessed radiation, when the matter is assumed to be distributed in an accretion disk, are presented in this paper; they were obtained, on the basis of Monte Carlo simulations, for different geometries of the disk surface and of the primary X-ray source. The case of a flat disk and a point source placed above the disk on its symmetry axis is treated in greater detail, taking into account the general relativistic and kinematic effects, which have dramatic consequences on the shape of the K line and edge. These effects are less relevant for the equivalent width (W-alpha) of the line and for the strength of the high energy bump, and were therefore ignored for simplicity in computing these two quantities for other source geometries (an optically thin corona, which turns out to be well approximated by the point source, and an optically thick sphere), and for other configurations of the disk surface (conical and concave). Concerning W-alpha, its maximum value is about 150 eV for a flat, 180 eV for a conical and 230 eV for a concave disk seen face-on. Concerning the strength of the bump, it turns out to be an excellent indicator of strong deviations from the planar configuration, and it is shown that, being practically independent of the iron abundance, it can be used to constrain the abundance when used in conjunction with W-alpha. Several observational implications are discussed, in particular it is noted that the relativistic and kinematic effects on the shape of the edge might lead to a misinterpretation of low resolution measurements in terms of absorption by highly ionized iron.

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Lyα + N v emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Lyα + N v, for which we find that the centroid of the continuum-emission-line cross-correlation function is τcent ≈ 14-20 days. We measure a lag τcent ≲ 4 days for He II λ1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Lyα + N v and He II λ1640 at a high level of significance. We are unable to determine a reliable lag for C IV λ1550, but we note that the profiles of the variable parts of Lyα and C IV λ1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.

We present BeppoSAX observations of the southern S1 region in the European Large-Area Infrared Space Observatory (ISO) Survey (ELAIS). These observations cover an area of similar to1.7 deg(2) and reach an on-axis (similar to0.7 deg(2)) 2-10 keV (hard X-ray, or HX) sensitivity of similar to 10(-13) ergs s(-1) cm(-2). This is the first HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio SNRHX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T) SNRT > 3, and two additional sources that seem to be associated with QSOs having SNRT > 2.9. The number densities of the SNRHX > 3 sources are consistent with the ASCA and BeppoSAX log N-log S functions. Six BeppoSAX sources have reliable ISOCAM 15 mum counterparts within similar to 60". All these ISOCAM sources have optical counterparts of R < 20 mag. Five of these sources have been previously optically classified, four as QSOs and one as a broad absorption line (BAL) QSO at z = 2.2. The remaining unclassified source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 ISOCAM counterpart associated with a galaxy at z = 0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z = 3.2 and assess the HX/MIR survey depth. The BAL QSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to those of QSOs but different from those found for low-redshift BAL QSOs. This difference can be explained in terms of absorption, and it suggests that high-redshift BAL QSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high-redshift QSO population to be determined. The QSOs cover a wide redshift range (0.4

We present the broad band analysis of two BeppoSAX observations of the Sy 1 Mkn509. In 2000 the source was in a typical flux state, F{2-10keV}=2.7x10^{-11}erg cm^{-2} s^{-1}, while in 1998 it was found in a high flux state, F{2-10keV}=5.7x10^{-11} erg cm^{-2} s^{-1}. A comparison between the two states shows a energy-dependent flux variation. A soft excess, a narrow iron line and a Compton reflection hump above 10 keV, are clearly apparent in the residuals after fitting the spectra with a simple power law. We tested two alternative models. In the first the iron line and the high energy bump are well reproduced by reprocessing in a cold and Compton thick material. The intensity of the iron line (also consistent with a Chandra measurement) as well as the normalization of the reflection hump are consistent with a constant in the two epochs: this, combined with the fact that the line is narrow as observed by Chandra, suggests a common origin from distant and optically thick matter. This model further requires a component to model the soft excess: the empirical choice of two black bodies accounts well for the excess in both observations, their combined strength was a factor about three higher in the high than in the low flux state defined above. However the relative contribution of the soft excess is higher in the low flux state. In the second model we attempted to reproduce all spectral features, except for the narrow cold line, with reflection from an ionized disc. This model is successful only in the high flux state,but it fails in the low flux state, when the soft excess is only partially accounted for. In either models, the slope of the power law is larger in the high than in the low flux state, in agreement with a behaviour known to be shared by several objects of the same type., Comment: 9 pages, 6 figures. Accepted for publication in Astronomy & Astrophysics

We report the spectral analysis of a long XMM-Newton observation of the well-studied, moderate luminosity Broad Line Seyfert 1 galaxy NGC 5548. The source was at an historically average brightness and we find the hard (3-10 keV) spectrum can be well fitted by a power law of photon index gamma ~ 1.75, together with reflection. The only feature in the hard X-ray spectrum is a narrow emission line near 6.4 keV, with an equivalent width of ~ 60 eV. The energy and strength of this line is consistent with fluorescence from `neutral' iron distant from the central continuum source. We find no evidence for a broad Fe K line, with an upper limit well below previous reports, suggesting the inner accretion disc is now absent or highly ionised. The addition of simultaneous BeppoSAX data allows the analysis to be extended to 200 keV, yielding important constraints on the total reflection. Extrapolation of the hard X-ray power law down to 0.3 keV shows a clear `soft excess' below ~ 0.7 keV. After due allowance for the effects of a complex warm absorber, measured with the XMM-Newton RGS, we find the soft excess is better described as a smooth upward curvature in the continuum flux below ~ 2 keV. The soft excess can be modelled either by Comptonised thermal emission or by enhanced reflection from the surface of a highly ionised disc., Comment: 9 pages, 11 figures, accepted by MNRAS; minor changes to text and figures

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., to appear in MNRAS

We have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: (i) the Balmer decrement and the amount of X-ray absorption are anticorrelated over a wide range of column density, 10(21) less than or similar to N-H less than or similar to 10(24) cm(-2) - the correlation no longer applies to Compton-thick objects (N-H greater than or similar to 10(24) cm(-2)), although they span a comparable range in Balmer decrement; (ii) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich in dust on scales of hundreds of parsecs. On the other hand, Compton-thick Seyferts indifferently exhibit 'dust-poor' and 'dust-rich' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt), where Compton-thick Seyfert 2s are observed through compact 'tori', whereas Compton-thin ones are obscured by dust on much larger scales.

The evidence of a decrease with increasing luminosity of the fraction f_{abs} of absorbed and Compton-thin among X-ray (2-10 keV) selected AGN is observationally rather well supported, while that of an increase of f_{abs} with redshift is rather controversial. In Lamastra, Perola & Matt (2006) the gravitational effect of the SMBH on the molecular interstellar gas, in the central region of the host galaxy, was shown to predict an anti-correlation between f_{abs} and M_{BH}. The most recent findings on the distribution of the Eddington ratio \lambda=L_b/L_E as a function of M_{BH} and z are used to convert that relationship into one between f_{abs} and both bolometric (L_b) and X-ray (L_X) luminosities at various values of z. The findings on \lambda(M_{BH},z) are properly treated in order to ensure completeness in the prediction of f_{abs} above a certain luminosity, at values of z=0.1, 0.35, 0.7 and >1. To verify the consequence of these findings alone, we adopted in a first istance a distribution of gas surface density \Sigma, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z. Assuming in the \lambda(M_{BH},z) distribution the Eddington limit, \lambda=1, as a ``natural'' cut-off, the predictions are consistent with the existence of an anti-correlation between f_{abs} and L_X, but fail to reproduce an increase of f_{abs} with z. Because the early type galaxies on average are much poorer in molecular gas than late type ones, a quantitative agreement with the local value of f_{abs} requires the existence of a correlation between \Sigma and the central activity. An increase of typical values of \Sigma with z, correlated with the activity, might explain an increase of f_{abs} with z. However, at the highest luminosities f_{abs} could hardly exceed about 0.3., Comment: 12 pages, 12 figures, accepted for publication in A&A

The fraction of AGN with photoelectric absorption in the X-rays ranging from NH of 10^{22} up to about 10^{24} cm^{-2} (Compton-thin) appears observationally to be anticorrelated to their luminosity Lx. This recently found evidence is used to investigate the location of the absorbing gas. The molecular torus invoked in the unified picture of AGN, while it can be regarded as confirmed on several grounds to explain the Compton-thick objects, do not conform to this new constraint, at least in its physical models as developed so far. In the frame of observationally based evidence that in Compton-thin sources the absorbing gas might be located far away from the X-ray source, it is shown that the gravitational effects of the black hole (BH) on the molecular gas in a disk, within 25-450 pc (depending on the BH mass, from 10^6 to 10^9 M_solar, leads naturally to the observed anticorrelation, under the assumption of a statistical correlation between the BH mass and Lx. Its normalization is also reproduced provided that the surface density, Sigma, of this gas is larger than about 150-200 M_solar pc^{-2}, and assuming that the bolometric luminosity is one tenth of the Eddington limit. Interestingly, the required values are consistent with the value of the 300 pc molecular disk in our own galaxy, namely 500 M_solar pc^{-2}. In a sample of nearby galaxies from the BIMA SONG survey, it is found that half of the objects have central Sigma larger than 150 M_solar pc${-2}. Given the simplicity of the proposed model, this finding is very encouraging, waiting for future higher resolution surveys in CO on more distant galaxies., Astronomy and Astrophysics, in press

(Abridged) We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources (6e-15 1000, while their Lx, L(R) and Nh are similar. This suggests that while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured., Comment: Accepted for publication in A&A, 18 pages, 19 figures

We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red colors (6.3, Comment: 7 pages, 5 figures, accepted for publication in A&A

We have determined the cosmological evolution of the density of AGN and of their Nh distribution as a function of the un-absorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGN. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low luminosity (Lx~10^43 erg s^-1) AGN peak at z~0.7, while high luminosity AGN (Lx>10^45 erg s^-1) peak at z~2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (Nh>10^22 cm^-2) AGN decreases with the intrinsic X-ray luminosity, and increases with the redshift. Our best fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGN as a function of the flux in the 10^-1510^44 erg s^-1) AGN have a density of 267 deg^-2 at fluxes S(2-10)>10^-15 erg s^-1 cm^-2. Using these results, we estimate a density of supermassive black holes in the local Universe of 3.2 (h^2_70) x 10^5 M_sol Mpc^-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies., Comment: 16 pages, 20 figures. Scheduled on ApJ, Vol. 635, 2005 December 20

The Seyfert 1 galaxy NGC4151 is characterized by complex X-ray absorption, well described by a dual absorber, composed of a uniform mildly ionized gas and a cold system that partially covers the central source. However, in one of the 5 BeppoSAX observations, the spectrum shows two peculiar features. An absorption feature is detected around 8.5-9 keV with a statistical significance of 99.96%. This feature can be fitted either with an absorption edge at E=8.62^{+0.34}_{-0.52} keV with optical depth \tau=0.06\pm0.03 or with an absorption line with 9.5^{+1.3}_{-0.6} keV, width \sigma=0.95^{+1.2}_{-0.7} keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII (E_{rest}=8.4-8.5 keV). In the second case the feature could be identified with a blend of FeXXV and FeXXVI lines, with an outflow velocity v \approx (0.09-0.26)c. This spectrum is also characterized by a substantial reduction of the absorption column density and the covering fraction of the dual absorber. In particular the column density of the mildly ionized and cold absorbers is \approx 3-5 times lower than observed in the other states, and the covering fraction is reduced by \approx 40 per cent. We propose a possible explanation linking the two properties in terms of a multi-phase ionized absorber., Comment: 5 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics Letters

The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is described. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH>22) Active Galactic Nuclei (AGN) in the 2--10 keV flux range 0.8-20E-14 erg/cm2/s. This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is significant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above logNH>21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit logNH>22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ~1E-14} erg/cm2/s of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by logNH>22 absorption., A&A in press

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the range 8E-15-4E-13 cgs and the total area surveyed is 0.9 deg2. About 20% of the hard X-ray selected sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more than one order of magnitude lower than the HELLAS2XMM survey sources, many of the extreme X/O sources in our sample have R10: 8 are narrow line QSO (i.e. QSO2), four are broad line QSO. We use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, CDFN 1Msec, SSA13 and Lockman Hole flux limited samples), 221 with measured z, to evaluate the cosmological evolution of the hard X-ray source's number and luminosity densities. Looking backward in time, the low luminosity sources (logL(2-10keV) = 43-44 erg/s) increase in number at a rate different than the high luminosity sources (logL(2-10keV)>44.5 erg/s), reaching a maximum around z=1 and then levelling off beyond z=2. This translates into an accretion driven luminosity density which is dominated by sources with logL(2-10keV) < 44.5 erg/s up to at least z=1, while the contribution of the same sources and of those with logL(2-10keV)>44.5 erg/s appear to be comparable between z=2 and 4., v2, minor changes, A&A in press

We present extensive optical, radio and infrared follow-up observations of a sample of 35 hard X-ray (2-10 keV) selected sources discovered serendipitously in the PV XMM-Newton observation of the radio-loud quasar PKS 0312-77 field, for which also an archival Chandra observation is available. The observations have been carried out as part of the HELLAS2XMM survey, a program aimed to understand the nature of the sources responsible for the bulk of the hard X-ray Background (XRB). The identification of the optical counterparts greatly benefits from the positional accuracy obtained from Chandra and radio observations. As a consequence, the spectroscopic completeness of the present sample (80%) is limited only by the faintness of the optical counterparts. The multiwavelength coverage of our survey allows us to unveil a large spread in the overall properties of hard X-ray selected sources. At low redshift (z1 are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X-ray absorption (N(H)>10^22 cm^-2), further supporting previous evidence of a decoupling between optical and X-ray properties at high luminosities and redshifts. Finally, a non negligible fraction (15%) of the hard X-ray sources are not detected down to the limiting magnitude of the optical images. The corresponding high X-ray to optical flux ratio, X-ray and optical-infrared colors strongly suggest that they are high redshift, obscured AGN., 16 pages, 10 figures, accepted for publication in Astronomy & Astrophysics (aa.cls included)

We present results of a complete radio follow-up obtained with the VLA and ATCA radio telescopes down to a 6 cm flux limit of about 0.3 mJy of all the 147 X-ray sources detected in the BeppoSAX HELLAS survey. We found 53 X-ray/radio likely associations, corresponding to about one third of the X-ray sample. Using the two point spectral index alpha_ro=0.35 we divided all the HELLAS X-ray sources in radio quiet and radio loud. We have 26 sources classified as radio-loud objects, corresponding to about 18% of the HELLAS sample. In agreement with previous results, the identified radio-loud sources are associated mainly with Type 1 AGNs with L(5-10 keV) > 10^44 erg/s, while all the identified Type 2 AGNs and Emission Line Galaxies are radio quiet objects with L(5-10 keV) < 10^44 erg/s. The analysis of the radio spectral index suggests that Type 1 AGNs have a mean radio spectral index flatter than Type 2 AGNs and Emission Line Galaxies., 12 pages, 5 figures, MNRAS, accepted

A sample of nine bright Seyfert 1 and NELG type galaxies, observed with BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the issue of the spectral contributions of Compton reflection and iron line fluorescence from circumnuclear gas. The empirical description adopted for the direct continuum is the commonly used power law with an exponential cut-off. The most direct test of the theoretical predictions, namely that the equivalent width of the line, Walpha, and the strength R of the reflection relative to the direct continuum are closely related to each other, gives a substantially positive result, that is their mean ratio is very close to expectation, and only a modest spread in the iron abundance seems implied. The existence of a steep correlation between R and the slope Gamma of the power law is not confirmed. A weak evidence is found that the existence of a very shallow trend to increase on average with Gamma cannot be altogether excluded in both R and Walpha, but needs to be tested with a larger sample. The energy Ef in the exponential cut-off spans a range from about 80 to more than 300 keV. A possible correlation is found, with Ef increasing on average with Gamma: if ignored, for instance by keeping Ef at a fixed value in a sample study, it could be cause of artificial steepening in a correlation between R and Gamma., accepted for publication in A&A

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., Comment: 7 pages, 5 figures included, LaTeX emulateapj5.sty, accepted for publication by The Astrophysical Journal

We report on a dramatic transition between a Compton-thick, reflection-dominated state and a Compton-thin state in the Seyfert 2 galaxy UGC4203, discovered by comparing a recent (May 2001) XMM-Newton observation with ASCA observations performed about six years earlier. This transition can be explained either as a change in the column density of the absorber, maybe due to moving clouds in a clumpy torus, or as the revival of a transient active nucleus, which was in a phase of very low activity when observed by ASCA. If the latter explanation is correct, spectral transitions of this kind provide observational support to the idea that Compton-thick and Compton-thin regions coexist in the same source, the former likely to be identified with the "torus", the latter with dust lanes on much larger scales., Comment: 6 Latex pages, 5 figures, To appear in Astronomy & Astrophysics

We used high quality BeppoSAX data of 6 Seyfert galaxies to test realistic thermal Comptonization models. Our main effort was to adopt a Comptonization model taking into account the anisotropy of the soft photon field. The best fit parameter values of the temperature and optical depth of the corona and of the reflection normalization obtained fitting this class of models to the data are substantially different from those derived fitting the same data with the power law + cut--off model commonly used. The two models also provide different trends and correlation between the physical parameters, which has major consequences for the physical interpretation of the data, Comment: 6 pages, 4 figures (uses newpasp.sty included). To appear in Proc. of "X-ray astronomy 2000",(Palermo Sep. 2000), Eds. R. Giacconi, L. Stella, S. Serio, ASP Conf. Series, in press

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., to appear in MNRAS

We test accurate models of Comptonization spectra over the high quality data of the BeppoSAX long look at NGC 5548, allowing for different geometries of the scattering region, different temperatures of the input soft photon field and different viewing angles. We find that the BeppoSAX data are well represented by a plane parallel or hemispherical corona viewed at an inclination angle of 30$^{\circ}$. For both geometries the best fit temperature of the soft photons is close to 15$^{+3}_{-9}$ eV. The corresponding best fit values of the hot plasma temperature and optical depth are $kT_{\rm e}\simeq$ 250--260 keV and $\tau\simeq$ 0.16--0.37 for the slab and hemisphere respectively. These values are substantially different from those derived fitting the data with a power-law + cut off approximation to the Comptonization component ($kT_{\rm e}\lta$ 60 keV, $\tau\simeq$ 2.4). This is due to the fact that accurate Comptonization spectra in anisotropic geometries show "intrinsic" curvature which reduces the necessity of a high energy cut-off. The Comptonization parameter derived for the slab model {is} larger than predicted for a two phase plane parallel corona in energy balance, suggesting that a more ``photon-starved'' geometry is necessary. The spectral softening detected during a flare which occurred in the central part of the observation corresponds to a decrease of the Comptonization parameter, probably associated with an increase of the soft photon luminosity, the {hard} photon luminosity remaining constant., Comment: 36 pages, 9 figures, accepted by ApJ

The Narrow Line Seyfert 1 galaxy Mrk 766 has been observed by BeppoSAX on May 1997. The source was fairly variable, both in flux and spectrum, during the observation. The variability is the largest around 2 keV, possibly due to variations in the warm absorber properties. An oxygen line is clearly detected, at least in the first part of the observation. An edge around 7.5 keV, coupled with the lack of any detectable iron line, suggests either reprocessing of the primary X-rays by a mildly ionized disc or absorption by a further, thicker and more ionized material., Comment: accepted for publication in Astronomy & Astrophysics

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., Comment: 26 pages, 7 figures, 1 table. LaTeX with encapsulated postscript. Scheduled for publication in the Astrophysical Journal vol 531, March 2000

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., Comment: 4 pages, Latex, 5 Postscript figures, accepted for publication in Astronomy & Astrophysics

We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light curves for the continuum fluxes exhibited two well-defined local minima or 'dips,' the first lasting is less than or approximately 20 days and the second is less than or approximately 4 days, with additional episodes of relatively rapid flickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became 'harder' when brighter. The variations in the continuum occurred simultaneously at all wavelengths (delta(t) is less than 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N V (which is relatively weak and badly blended with Ly(alpha), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or 'lag.' As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of (formally) approximately 0 days for He II + O III), and approximately 4 days for Ly(alpha) and C IV. The data further suggest lags of approximately 4 days for Si IV + O IV) and 8-30 days for Si III + C III). Mg II lagged the 1460 A continuum by approximately 9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe II lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of is less than or approximately 2 days, and that the character of the continuum variability may change with time.

Results from observations of the Seyfert galaxy NGC 4151 obtained with Exosat (0.1-25 keV) and IUE (1200-3000 A) during November 1983, and between December 1984 and January 1985, are presented. The stability of the flux during large-scale variations in the UV and X-ray regions suggests that the soft X-ray excess noted by Holt et al. (1980) represents a new spectral component whose temperature is estimated to be 1.8 million K, and luminosity to be 1.7 x 10 to the 42nd erg/s, if it is thermal emission from a hot gas. The observed variable UV and X-ray continua are well correlated down to a time scale of days, and the standard model of a power law with a uniform cold absorber gives a bad fit at low energies in the 1.2-25 keV range. The spectral index of the power law is found to correlate closely with the flux in the 2-10 keV range, with a shallower slope for lower intensity, and the iron fluorescence line at 6.4 keV is seen to respond very slowly to changes in the continuum.

We report the first pointed X-ray observation, made by ASCA, of the nearby Seyfert 2 galaxy in Circinus. The spectrum consists of a very prominent (EW similar to 2 keV) fluorescent 6.4-keV iron line, a very flat continuum (photon spectral index less than 1) and other lines, including one at similar to 7 keV that is probably due, at least in part, to iron K beta fluorescent emission. The spectrum is consistent with Compton scattering and fluorescent emission from cold matter illuminated by an obscured active nucleus.