arXiv subjects

C. Vignali

Publications and source records attributed to C. Vignali.

At least 361 records · Page 20Linked to original sources

arXiv2005

The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei

We perform a detailed investigation of moderate-to-high quality X-ray spectra of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to z~6.28). This study includes five new XMM observations and five archived X-ray observations (four by XMM and one by Chandra). We find that the X-ray power-law photon indices of our sample, composed of eight radio-quiet sources and two that are moderately radio loud, are not significantly different from those of lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are not significantly absorbed. A joint fit performed on our eight radio-quiet sources, with a total of ~7000 photons, constrains the mean photon index of z>4 radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their lower-redshift counterparts. All this suggests that the X-ray production mechanism and the central environment in radio-quiet AGNs have not significantly evolved over cosmic time. The mean equivalent width of a putative neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place constraints on the mean Compton reflection component (R<~1.2). None of the AGNs varied on short (~1 hr) timescales, but on longer timescales (months-to-years) strong variability is observed in four of the sources. In particular, the X-ray flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4 over a rest-frame period of 73 d. This is the most extreme X-ray variation observed in a luminous z>4 radio-quiet AGN.

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arXiv2005

On the Exotic Hard X-ray Source Populations in the Hellas2XMM survey

Recent hard X-ray surveys have proven to be effective in discovering large numbers of X-ray sources that, despite the likely association with active nuclei, appear to be characterized by "peculiar" properties. Among these "exotic" source populations, we will focus on the nature of two classes of hard X-ray sources: those characterized by high X-ray-to-optical flux ratios -- a fraction of these are associated with the rather elusive Type 2 quasars -- and the X-ray bright optically normal galaxies, also known as XBONGs.

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arXiv2005

X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra

We report on Chandra observations of a sample of 11 optically luminous (Mb<-28.5) quasars at z=3.96-4.55 selected from the Palomar Digital Sky Survey and the Automatic Plate Measuring Facility Survey. These are among the most luminous z>4 quasars known and hence represent ideal witnesses of the end of the "dark age ''. Nine quasars are detected by Chandra, with ~2-57 counts in the observed 0.5-8 keV band. These detections increase the number of X-ray detected AGN at z>4 to ~90; overall, Chandra has detected ~85% of the high-redshift quasars observed with snapshot (few kilosecond) observations. PSS 1506+5220, one of the two X-ray undetected quasars, displays a number of notable features in its rest-frame ultraviolet spectrum, the most prominent being broad, deep SiIV and CIV absorption lines. The average optical-to-X-ray spectral index for the present sample ( =-1.88+/-0.05) is steeper than that typically found for z>4 quasars but consistent with the expected value from the known dependence of this spectral index on quasar luminosity. We present joint X-ray spectral fitting for a sample of 48 radio-quiet quasars in the redshift range 3.99-6.28 for which Chandra observations are available. The X-ray spectrum (~870 counts) is well parameterized by a power law with Gamma=1.93+0.10/-0.09 in the rest-frame ~2-40 keV band, and a tight upper limit of N_H~5x10^21 cm^-2 is obtained on any average intrinsic X-ray absorption. There is no indication of any significant evolution in the X-ray properties of quasars between redshifts zero and six, suggesting that the physical processes of accretion onto massive black holes have not changed over the bulk of cosmic time.

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arXiv2004

X-rays from the First Massive Black Holes

We briefly review some recent results from Chandra and XMM-Newton studies of the highest redshift (z > 4) active galactic nuclei (AGNs). Specific topics covered include radio-quiet quasars, radio-loud quasars, moderate-luminosity AGNs in X-ray surveys, and future prospects. No significant changes in AGN X-ray emission properties have yet been found at high redshift, indicating that the small-scale X-ray emission regions of AGNs are insensitive to the dramatic changes on larger scales that occur from z = 0-6. X-ray observations are also constraining the environments of high-redshift AGNs, relevant emission processes, and high-redshift AGN demography.

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arXiv2004

A Chandra observation of the z=2.285 galaxy FSC10214+4724: Evidence for a Compton-thick quasar?

We present a ~20 ks Chandra ACIS-S observation of the strongly lensed z=2.285 ultra-luminous infrared galaxy FSC10214+4724. Although this observation achieves the equivalent sensitivity of an up-to ~4 Ms Chandra exposure (when corrected for gravitational lensing), the rest-frame 1.6-26.3 keV emission from FSC10214+4724 is weak (L_X~2E42 erg/s for a lensing boost of ~100); a significant fraction of this X-ray emission appears to be due to vigorous star-formation activity. If FSC10214+4724 hosts a quasar, as previously suggested, then it must be obscured by Compton-thick material. We compare FSC10214+4724 to high-redshift SCUBA galaxies and discuss the X-ray identification of Compton-thick AGNs at high redshift.

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arXiv2004

Obscured accreting black holes at high redshift

A significant fraction of the accreting black holes powering high redshift AGN are obscured by large columns of dust and gas. For this reason, luminous type 2 quasars can be efficiently discovered combining hard X-ray and near-infrared observations. We will briefly discuss the most recent results.

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arXiv2004

Type II quasar candidates in the Sloan Digital Sky Survey: evidence for X-ray obscuration

Recent X-ray surveys with Chandra and XMM-Newton have found several genuine Type II quasars, the long sought-after population of high-luminosity (high-redshift) Seyfert 2 galaxies, and a sizable number of Type II quasar candidates. However, at present it is hard to know whether the on-going X-ray surveys are providing a reliable and nearly complete census of the Type II quasar population. In order to address this open issue and shed light on the broad-band properties of Type II quasars, we used the sample of 291 high-ionization narrow emission-line AGN at z=0.3-0.83 selected from the Sloan Digital Sky Survey by Zakamska et al. (2003). Using archival X-ray information, we were able to place constraints on the X-ray emission of 17 of these objects (three X-ray detections and 14 upper limits). Using the [O III]5007 line luminosities to predict the intrinsic X-ray power of the AGN, we found that at least 47% of the objects in the observed sample provides evidence for the presence of X-ray absorption (with N_H>10^22 cm^-2), including the four highest luminosity sources with predicted unobscured luminosities of ~10^45 erg/s. For the only source with moderate-quality XMM-Newton spectral data, all of the pieces of the puzzle (2-10 keV luminosity of ~4x10^44 erg/s, N_H~1-3x10^22 cm^-2, and unobscured magnitude M_B=-26 assure that this is a genuine Type II quasar.

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arXiv2004

XMM-Newton observations of Extremely Red Objects and the link with luminous, X-ray obscured Quasars

We present the results of a deep (about 80 ks) XMM-Newton survey of the largest sample of near-infrared selected Extremely Red Objects (R-K>5) available to date to K_S< 19.2. At the relatively bright X-ray fluxes (F_{2-10 keV}> 4x10^{-15} cgs) and near-infrared magnitude probed by the present observations, the fraction of AGN (i.e., X-ray detected) among the ERO population is small (~3.5%); conversely, the fraction of EROs among hard X-ray selected sources is much higher (~20%). The X-ray properties of EROs detected in our XMM-Newton observation indicate absorption in excess of 10^{22} cm^{-2} in a large fraction of them. We have also considered additional samples of X-ray detected EROs available in the literature. X-ray spectral analysis of the highest S/N sources unambiguously indicates that large columns of cold gas (even >10^{23} cm^{-2}) are the rule rather than the exception. The X-ray, optical, and near-infrared properties of those X-ray selected EROs with a spectroscopic or photometric redshift nicely match those expected for quasars 2, the high-luminosity, high-redshift obscured AGNs predicted in baseline XRB synthesis models. A close correlation is detected between X- and K-band fluxes. For the AGN EROs this is consistent, under reasonable assumptions, with the relation established locally between the host galaxies and their central black holes. This suggest that the majority of EROs are powered by massive black holes accreting, on average, at about 0.03-0.1 of the Eddington limit.

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arXiv2004

Evidence for X-ray Obscuration in Type II Quasar Candidates from the Sloan Digital Sky Survey

Recently, Zakamska et al. (2003) selected 291 high-ionization narrow emission-line AGN in the redshift range 0.3-0.83 from the Sloan Digital Sky Survey spectroscopic data. The sample includes both Seyfert II galaxies and their higher luminosity "cousins", Type II quasar candidates. Here we present the results on the X-ray properties of 17 of these objects for which archival X-ray data (ROSAT and XMM-Newton) are available. Three sources have been significantly (>6 sigma) detected, one being the most radio-loud source of the sample; its X-ray emission, possibly enhanced by jet emission, is consistent with the absence of absorption. Another source has a ~6 sigma detection in the ROSAT All-Sky Survey, possibly complex radio structure, and no evidence for strong X-ray absorption. For the third X-ray detection, the XMM-Newton spectrum indicates a column density of NH=1.26+0.75/-0.51x10^22 cm^-2; this result, coupled with the 2-10 keV luminosity of ~4x10^44 cgs, makes this source a genuine Type II quasar. Using the O[III]5007 line luminosities, we estimated the intrinsic X-ray power of the AGN and found that >47% of the observed sample shows indications of X-ray absorption, with column densities >10^22 cm^-2. This provides further evidence that a considerable fraction are obscured quasars. Support to our conclusions also comes from the recent analysis of RASS data performed by Zakamska et al. (2004), who found five additional lower significance (~2.1-3.6 sigma) X-ray matches.

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arXiv2004

The z>4 Quasar Population Observed by Chandra and XMM-Newton

The current status of our Chandra and XMM-Newton project on high-redshift (z>4) quasars is briefly reviewed. We report the main results obtained in the last few years for the detected quasars, along with a few (~10%) intriguing cases where no detection has been obtained with Chandra snapshot observations.

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arXiv2004

Chandra Observations of Radio-Loud Quasars at z > 4: X-rays from the Radio Beacons of the Early Universe

We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radio-quiet quasar (RQQ) at z = 4.1-4.4. These observations cover a representative sample of RLQs with moderate radio-loudness (R ~ 40-400), filling the X-ray observational gap between optically selected RQQs and the five known blazars at z > 4 (R ~ 800-27000). We study the relationship between X-ray luminosity and radio-loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic N_H, with a best-fit neutral intrinsic column density of N_H = 2.4^{+2.0}_{-1.8} x 10^{22} cm^{-2}, consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background (CMB) at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low-to-moderate redshifts. Overall, the z ~ 4-5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well.

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arXiv2004

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

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.

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arXiv2004

X-ray Survey Results on Active Galaxy Physics and Evolution

This "pedagogical" review describes the key Chandra and XMM-Newton extragalactic surveys to date and details some of their implications for AGN physics and evolution. We additionally highlight two topics of current widespread interest: (1) X-ray constraints on the AGN content of luminous submillimeter galaxies, and (2) the demography and physics of high-redshift (z > 4) AGN as revealed by X-ray observations. Finally, we discuss prospects for future X-ray surveys with Chandra, XMM-Newton, and upcoming missions.

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arXiv2004

The HELLAS2XMM 1dF Survey: a window on exotic hard X-ray selected sources

Recent Chandra and XMM-Newton surveys have confirmed that the cosmic X-ray background is mostly due to accretion onto super-massive black holes, integrated over cosmic time. Here we review the results obtained from the photometric and spectroscopic follow-up observations of the 122 X-ray sources detected by the HELLAS2XMM 1dF Survey down to a 2-10 keV flux of ~10^{-14} erg cm^-2 s^-1. In particular, we focus on the multiwavelength properties of a few intriguing classes of X-ray sources: high X-ray-to-optical flux ratio sources, Type 2 quasars, and XBONGs.

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arXiv2004

The HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios

We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks<19) and red colors (R-K>5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift'', ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

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arXiv2003

X-rays from the Dawn of the Modern Universe. Chandra and XMM-Newton Observations of z>4 Quasars

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.

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arXiv2003

X-raying Active Galaxies Found and Missed by the Sloan Digital Sky Survey

Current X-ray observatories, archival X-ray data, and the Sloan Digital Sky Survey (SDSS) represent a powerful combination for addressing key questions about active galactic nuclei (AGN). We describe a few selected issues at the forefront of X-ray AGN research and the relevance of the SDSS to them. Bulk X-ray/SDSS AGN investigations, X-ray weak AGN, red AGN, hard X-ray selected AGN, high-redshift AGN demography, and future prospects are all briefly discussed.

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arXiv2003

The HELLAS2XMM 1dF survey: on the nature of high X-ray to optical flux ratio sources

We present results from the photometric and spectroscopic identification of 122 X-ray sources 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 sourveys) 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.

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