G. C. Perola

X-ray obscuration and obscured AGN in the local universe

In this paper we discuss the X-ray properties of 49 local (z<0.035) Seyfert 2 galaxies with HST/WFC2 high-resolution optical coverage. It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV) energy band. Our sample covers a range in the 2-10 keV observed flux from 3x10^{-11} to 6x10^{-15} erg cm

The HELLAS2XMM survey IV. Optical identifications and the evolution of the accretion luminosity in the Universe

^{-2}

Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies

s

The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy*

^{-1}

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

. The percentage of the objects which are likely obscured by Compton-thick matter (column density, N_H>1.6x10^{24} atoms/cm/cm) is ~50%, and reaches ~80% for log(F_{2-10})<12.3. Hence, K-alpha fluorescent iron lines with large Equivalent Width (EW>0.6 keV) are common in our sample (6 new detections at a confidence level>2 sigma). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a ~100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes following an idea originally proposed by Malkan et al. 1998, and Matt 2000). The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured - and therefore likely produced in extended gas structures - it may lead to a severe underestimation of the nuclear obscuration in z~1 absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous (~7x10^{40} erg/s) halo embedding the interacting galaxy pair Mkn266.

The X-Ray Afterglow of the Gamma-Ray Burst of 1997 May 8:Spectral Variability and Possible Evidence of an Iron Line

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discov- ered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the range 8 10 15 4 10 13 erg cm 2 s 1 and the total area surveyed is 0.9 square degrees. One of the most interesting results (which is found also in deeper sourveys) is that 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 R 10 (to be compared with 9 sources known from the deeper, pencil-beam surveys). Eight of them are narrow line QSO (seemingly the extension to very high luminosity of the type 2 Seyfert galax- ies), four are broad line QSO. The results from our survey are also used to make reliable predictions about the luminosity of the sources not yet spectroscopically identified, both in our sample and in deeper Chandra and XMM-Newton samples. We then use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, Chandra Deep Field North 1Msec, Chandra SSA13 and XMM-Newton Lockman Hole flux limited samples), 221 with measured redshifts, to evaluate the cosmo- logical evolution of the hard X-ray sources number and luminosity densities. Looking backward in time, the low luminosity sources (log L2 10 keV = 43 44 erg s 1 ) increase in number at a much slower rate than the very high luminosity sources (log L2 10 keV> 44: 5e rg s 1 ), reaching a maximum around z= 1 and then levelling o beyond z= 2. This translates into an accretion driven luminosity density which is dominated by sources with log L2 10 keV 44: 5e rg s 1 appear, with yet rather large uncertainties, to be comparable between z= 2 and 4.

Discovery of a Redshifted Iron K Line in the X-Ray Afterglow of GRB 000214

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{o. The most direct test of the theoretical predictions, namely that the equivalent width of the line, W, 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 of the power law is not conrmed. A weak evidence is found that the existence of a very shallow trend to increase on average with cannot be altogether excluded in both R and W, but needs to be tested with a larger sample. The energy Ef in the exponential cut{o spans a range from about 80 to more than 300 keV. A possible correlation is found, with Ef increasing on average with : if ignored, for instance by keeping Ef at a xed value in a sample study, it could be cause of articial steepening in a correlation between R and .

Broadband Spectrum of Cygnus X-1 in Two Spectral States with BeppoSAX

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 ergs 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 of better understanding 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: CXOU J031238.9� 765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z ¼ 0:159, usually known as FIORE P3. Analysis of the broadband energy distribution suggests the presence of a heavily obscured active nucleus. Subject headings: galaxies: active — galaxies: individual (P3) — galaxies: nuclei — X-rays: galaxies

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. IX. Ultraviolet observations of Fairall 9

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

X-ray reprocessing in Seyfert Galaxies: simultaneous XMM-Newton/BeppoSAX observations

We report the possible detection (99.3% of statistical significance) of redshifted iron line emission in the X-ray afterglow of gamma-ray burst GRB 970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. The line disappeared ~1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray afterglow of this gamma-ray burst. The spectrum gets harder during the flare, then becoming steep when the flux decreases. The variability, intensity, and width of the line indicate that the emitting region should have a mass 0.5 M? (assuming that the iron abundance is similar to its solar value), should have a size of ~3?1015 cm, is distributed anisotropically, and is moving with subrelativistic speed. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material, consistent with preexplosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase.