A. Baldi

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

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.

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 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

The HELLAS2XMM survey. VI. X-ray absorption in the 1df AGN sample through a spectral analysis

The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is de- scribed. 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 0.8−20 × 10 −14 erg cm −2 s −1 . This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is signifi- cant 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 log NH > 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 log NH > 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 ∼10 −14 erg cm −2 s −1 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 log NH > 22 absorption.

The HELLAS2XMM survey - III. Multiwavelength observations of hard X-ray selected sources in the PKS 0312-77 field

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 (z 1 are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X-ray absorption ( NH> 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.

The HELLAS2XMM survey - VIII. Optical identifications of the extended sample

Aims. Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN. Methods. We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. Their 2–10 keV fluxes cover the range 6 × 10 −15 −4 × 10 −13 erg cm −2 s −1 and the total area surveyed is ∼0. 5d eg 2 at the bright flux limit. We spectroscopically identified 59 new sources, bringing the spectroscopic completeness of the full HELLAS2XMM sample to almost 70% over a total area of ∼1. 4d eg 2 at the bright flux limit. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to R ∼ 25. We measured the flux and luminosity of the [OIII]λ5007 emission line for 59 of these sources. Results. Assuming that most high X-ray-to-optical flux ratio sources are obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their log N− logS . We find obscured QSOs surface density of 45 ± 15 and 100–350 deg −2 down to flux limits of 10 −14 and 10 −15 erg cm −2 s −1 , respectively. At these flux limits, the fraction of X-ray-selected obscured QSOs turns out to be similar to that of unobscured QSOs. Since X-ray selection misses most Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs. We find that hard X-ray selected AGNs with a detected [OIII] emission span a wide range of L2−10 keV/L[OIII] with a logarithmic median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of optically selected AGNs (median 1.69 and interquatile range 0.30), suggesting that optically selected samples are at least partly incomplete and/or that [OIII] emission is not a perfect isotropic indicator of the nuclear power. The seven X-ray bright, optically normal galaxy (XBONG) candidates in the sample have L2−10 keV/L[OIII] > 1000, while their X-ray and optical luminosities and obscuring column density are similar to those of narrow-line AGNs in the same redshift interval (0.075–0.32). 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.

The HELLAS2XMM 1dF Survey: On the Nature of High X-Ray/Optical Flux Sources

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). One of the most interesting results (which is found also in deeper surveys) is that ∼ 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≲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.