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Register a new userChandra and XMM Observations of Type II Quasars from the SDSS
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We are carrying out sensitive X-ray observations with Chandra and XMM of type II quasars selected from the Sloan Digital Sky Survey based on their optical emission line properties. We present observations of four objects at redshifts 0.4 < z < 0.8 and an analysis of the archival data for four additional objects in the same redshift range. Six of the eight were detected in X-rays; five of them have sufficient signal to derive spectral information. All of the detected sources have intrinsic luminosities L(2-10 keV) > 5 x 10^43 erg s^-1. The five with sufficient counts for spectral fitting show evidence for significant absorption (N_H >~ a few x 10^22 cm^-2). At least three of the objects likely have N_H > 10^23 cm^-2; some may be Compton-thick (N_H > 10^{24} cm^-2). In the five objects for which we could fit spectra, the slopes tend to be significantly flatter than is typically observed in AGN; it is possible that this is due either to reprocessing of the nuclear emission or to a line of sight that passes through patchy absorption.
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Chandra snapshot observations of the three most distant quasars then known, at redshifts 5.82, 5.99, and 6.28, gave signficant detections even in the short, 6 -- 8 ks, observations. The X-ray to optical luminosity ratios indicate that quasars will be detectable in X-rays if they exist at even larger redshifts. The present observations hint at two exciting discoveries. An extended X-ray source 23 arcsec from SDSS1306+0356 may be a jet emitting inverse Compton radiation from the Cosmic Microwave Background. SDSS 1030+0524 does not appear to be a point source, and may be a gravitationally lensed system, or contain a small scale X-ray jet.
We have conducted spectropolarimetry of 12 type II (obscured) quasar candidates selected from the spectroscopic database of the Sloan Digital Sky Survey based on their emission line properties. Polarization was detected in all objects, with nine being highly polarized (> 3%) and with polarization reaching as high as 17% in two objects. Broad lines were detected in the polarized spectra of five objects. These observations prove beyond a reasonable doubt that the objects in our sample are indeed type II quasars, in that they harbor luminous UV-excess AGNs in their centers and that the direct view to the AGN is highly obscured. For three of the objects in this paper, we have obtained HST images in three bands. The HST observations, combined with the spectropolarimetry data, imply that scattering off material outside the obscuration plane is the dominant polarization mechanism. In all three objects the sizes of scattering regions are a few kpc. For one object, the extent of the scattering region, coupled with the characteristics of the polarized spectrum, argue strongly that dust scattering rather than electron scattering dominates the polarized light. Our observations are well-described by the basic orientation-based unification model of toroidal obscuration and off-plane scattering, implying that the model can be extended to include at least some high-luminosity AGNs.
We have analyzed the {it XMM-Newton} and {it Chandra} data overlapping $sim$16.5 deg$^2$ of Sloan Digital Sky Survey Stripe 82, including $sim$4.6 deg$^2$ of proprietary {it XMM-Newton} data that we present here. In total, 3362 unique X-ray sources are detected at high significance. We derive the {it XMM-Newton} number counts and compare them with our previously reported {it Chandra} Log$N$-Log$S$ relations and other X-ray surveys. The Stripe 82 X-ray source lists have been matched to multi-wavelength catalogs using a maximum likelihood estimator algorithm. We discovered the highest redshift ($z=5.86$) quasar yet identified in an X-ray survey. We find 2.5 times more high luminosity (L$_x geq 10^{45}$ erg s$^{-1}$) AGN than the smaller area {it Chandra} and {it XMM-Newton} survey of COSMOS and 1.3 times as many identified by XBootes. Comparing the high luminosity AGN we have identified with those predicted by population synthesis models, our results suggest that this AGN population is a more important component of cosmic black hole growth than previously appreciated. Approximately a third of the X-ray sources not detected in the optical are identified in the infrared, making them candidates for the elusive population of obscured high luminosity AGN in the early universe.
Quasars at z>4 provide direct information on the first massive structures to form in the Universe. Recent ground-based optical surveys (e.g., the Sloan Digital Sky Survey) have discovered large numbers of high-redshift quasars, increasing the number of known quasars at z>4 to ~500. Most of these quasars are suitable for follow-up X-ray studies. Here we review X-ray studies of the highest redshift quasars, focusing on recent advances enabled largely by the capabilities of Chandra and XMM-Newton. Overall, analyses indicate that the X-ray emission and broad-band properties of high-redshift and local quasars are reasonably similar, once luminosity effects are taken into account. Thus, despite the strong changes in large-scale environment and quasar number density that have occurred from z~0-6, individual quasar X-ray emission regions appear to evolve relatively little.
We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project (ChaMP). Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2<z<5.4, representing some 36Msec of effective exposure. Spectroscopic redshifts are available for about 1/3 of the detected sample; elsewhere, redshifts are estimated photometrically. With 56 z>3 QSOs detected, we find no evidence for evolution out to z~5 for either the X-ray photon index Gamma or for the ratio of optical/UV to X-ray flux alpha_ox. About 10% of detected QSOs are obscured (Nh>1E22), but the fraction might reach ~1/3 if most non-detections are absorbed. We confirm a significant correlation between alpha_ox and optical luminosity, but it flattens or disappears for fainter AGN alone. Gamma hardens significantly both towards higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in non-thermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN.
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