We present the final results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low $z$ quasars. The main results are: 1. There is no evidence for significant soft excess emission or excess foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating that there is no steep soft component below 0.2 keV. 3. The occurrence of warm absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN. 4. The strongest correlation found is between the spectral slope, alpha_x, and the Hb FWHM. This remarkably strong correlation may result from a dependence of alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where the direct X-ray source is obscured, and only scattered X-rays are observed. 6. Thin accretion disk models cannot reproduce the observed optical to soft X-ray spectral shape. An as yet unknown physical mechanism maintains a strong correlation between the optical and soft X-ray emission. 7. The well known difference in alpha_x between radio-loud and radio-quiet quasars may be due only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray columns implies that He in the diffuse H II component of the Galactic ISM is ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol. 447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8 figures available at http://physics.technion.ac.il/~laor/rosat/paper.p

A large population of heavily obscured, Compton-thick active galactic nuclei (AGNs) is predicted by AGN synthesis models for the cosmic X-ray background and by the "relic" supermassive black hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGNs. Alternative selection criteria, combining mid-infrared with near-infrared, and optical photometry, have instead been successful in pinpointing a large population of Compton-thick AGNs. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the Cosmic Evolution Survey (COSMOS) field to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density, and therefore large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24 μm fluxes higher than 550 μJy. For the MIPS sources not directly detected in the Chandra images, we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population, we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations, including both obscured AGN and star-forming galaxies. The volume density of Compton-thick QSOs (log L(2-10 keV) = 44-45 erg s^–1, or logλL _λ(5.8 μm) = 44.79-46.18 erg s^–1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10^–6 Mpc^–3 in the redshift bin 1.2-2.2. This density is ~44% of all X-ray-selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of the most up-to-date AGN synthesis models for the cosmic X-ray background (Gilli et al. 2007). The density of lower luminosity Compton-thick AGNs (log L(2-10 keV) = 43.5-44) at z = 0.7-1.2 is (3.7 ± 1.1) × 10^–5 Mpc^–3, corresponding to ~67% of X-ray-selected AGNs. The comparison between the fraction of infrared-selected, Compton-thick AGNs to the X-ray selected, unobscured, and moderately obscured AGNs at high and low luminosity suggests that Compton-thick AGNs follow a luminosity dependence similar to that discovered for Compton-thin AGNs, becoming relatively rarer at high luminosities. We estimate that the fraction of AGNs (unobscured, moderately obscured, and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24 μm fluxes. We discuss how our findings can constrain AGN feedback models.

We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg² of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^-15 erg cm^-2 s^-1 (~5 × 10^-15 erg cm^-2 s^-1). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray-to-optical associations we have used the "likelihood ratio technique." Here we present the identification method and its application to the CFHT I-band and photometric catalogs. We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I_AB > 25) or to the presence of multiple optical sources, with similar likelihoods of being the correct identification, within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS, and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis of this statistically meaningful sample of X-ray sources reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical-to-near-infrared colors: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray-to-optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical-to-near-infrared colors, and span a larger range of X-ray-to-optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the "extended" BL AGNs lie at redshift <1.5, while the redshift distribution of the full BL AGN population peaks at z ~ 1.5. The most likely explanation is that in these objects the nuclear emission is not dominant with respect to the host galaxy emission in the observed ACS band. Our analysis also suggests that the type 2/type 1 ratio decreases toward high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.

Using Chandra observations in the 2.15 deg² COSMOS-legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3–7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT ∼ 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Γ = 1.45 ± 0.02. The 1 keV normalization of the extragalactic component is 10.91 ± 0.16 keV cm⁻² s⁻¹ sr⁻¹ keV⁻¹. Removing all X-ray-detected sources, the remaining unresolved CXB is best fit by a power law with normalization 4.18 ± 0.26 keV cm⁻² s⁻¹ sr⁻¹ keV⁻¹ and photon spectral index Γ = 1.57 ± 0.10. Removing faint galaxies down to leaves a hard spectrum with and a 1 keV normalization of ∼1.37 keV cm⁻² s⁻¹ sr⁻¹ keV⁻¹. This means that ∼91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute ∼8%–9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another ∼1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. According to these limits, we investigate a scenario where early black holes totally account for non-source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the accreted mass density, such a population of black holes must grow in Compton-thick envelopes with 1.6 × 10²⁵ cm⁻² and form in extremely low-metallicity environments .

We present two Rossi Xray Timing Explorer (RXTE) PCA observations of the type~2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE All-Sky Monitor. NGC 4388 was found at a somewhat high continuum level (f(2-10 keV)=8x10(-11)erg cm(-2) s(-1)) and with a column density (N_H~3x10(22)cm(-2)), a factor ~10 lower than normal. The second PCA observation, four hours later, gave N_H<2x10(21)cm(-2) indicating, at the 3.1 sigma level, variability so rapid puts the absorber on a few 100 Schwartzschild radii scale, similar to the Broad Emission Line Region, or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of Unified Schemes for type 1 and type 2 AGNs.Comment: 5 pages, 3 figures. ApJLetters, in press. Author list correcte

We have carried out a survey with the Chandra X-ray Observatory of a sample of 10 optically bright broad absorption line (BAL) QSOs. Eight out of ten sources are detected. The 6 brightest X-ray sources have only high ionization BALs (hiBALs), while the 4 faintest all show low ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Gamma=1.8+/-0.35, consistent with the mean slope for radio-quiet quasars from ASCA, but BALQSOs require a (restframe) absorbing column of 6.5e22, with a partial covering fraction of 80%. Removing the absorption component from our best-fit spectral model yields a range of alphaOX from 1.55 to 2.28. All 6 hiBAL QSOs have de-absorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs - both the underlying power-law slope and alphaOX provide the first conclusive evidence that BALQSOs have appeared to be X-ray weak because of intrinsic absorption, and that their underlying emission is consistent with non-BAL QSOs. By contrast, removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the 4 loBAL QSOs as unusually X-ray faint for their optical luminosities.Comment: 12 pages, Latex, emulateapj style, including 3 tables and 3 figures. Accepted May 15, 2001 for publication in ApJ Main Journal. See also http://hea-www.harvard.edu/~pgreen/Papers.html Replaced May 21 since 2 lines were missing from Table