arXiv subjects

X. Barcons

Publications and source records attributed to X. Barcons.

At least 73 records · Page 4Linked to original sources

arXiv2003

On the XMM-Newton spectra of soft X-ray selected QSOs

We study the XMM-Newton spectra of a sample of 32 soft X-ray selected QSOs. Our goal is to check, using the spectra of moderate redshift (z~1.5), faint (f (0.2-8 keV) > 5x10^-15 cgs) broad-line QSOs, previous claims for either significant intrinsic absorption or spectral hardening at high energies. We derive hardness ratios for all sources and furthermore we perform spectral fits for the 11 brighter sources. The majority of sources have steep spectra (photon index >1.9). We find a few QSOs with large amounts of intrinsic absorption, as high as N_H~10^23 cm-2. We find no strong evidence for spectral hardening above 2 keV. The coadded QSO spectrum is well described by a single power-law with photon index of ~1.9, demonstrating that, on average, any effects of absorption are not important. This suggests that the discrepancy between the X-ray background and the (soft X-ray selected) QSO spectrum holds well at the faint fluxes probed here.

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arXiv2003

On the origin of the X-ray emission from a narrow-line radioquasar at z>1

We present new XMM-Newton X-ray observations of the z=1.246 narrow-line radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray spectrum previously measured by ROSAT and ASCA is shown to be the result of a steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident with the nucleus of the radio source that we have resolved in new sensitive VLA observations at 3.6 and 6 cm, implying that scattering in the radio lobes is not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint patch of X-ray emission is apparent 14'' to the NE of the main X-ray source. The former is positionally coincident with an apparently extended optical object with R~21.9, but there is no associated radio emission, thus ruling out the possibility that this represents a hotspot in a jet emanating from the primary X-ray source. No reflection features are detected in the X-ray spectrum of the narrow-line radioquasar, although an Fe line with equivalent width of up to 600 eV cannot be ruled out.

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arXiv2003

Cosmological constraints from the cluster contribution to the power spectrum of the soft X-ray background. New evidence for a low sigma_8

We use the X-ray power spectrum of the ROSAT all-sky survey in the R6 band (approximately 0.9-1.3 keV) to set an upper limit on the galaxy cluster power spectrum. The cluster power spectrum is modelled with a minimum number of robust assumptions regarding the structure of the clusters. The power spectrum of ROSAT sets an upper limit on the Omega_m-sigma_8 plane which excludes all the models with sigma_8 above sigma_8 = 0.5/(Omega_m^0.38) in a flat LCDM universe. We discuss the possible sources of systematic errors in our conclusions, mainly dominated by the assumed L_x-T relation. Alternatively, this relation could be constrained by using the X-ray power spectrum, if the cosmological model is known. Our conclusions suggest that only models with a low value of sigma_8 (sigma_8 < 0.8 for Omega_m = 0.3) may be compatible with our upper limit. We also find that models predicting lower luminosities in galaxy clusters are favoured. Reconciling our cosmological constraints with these arising by other methods might require either a high entropy floor or wide-spread presence of cooling flows in the low-redshift clusters.

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arXiv2002

The XMM-Newton Survey Science Centre Medium Sensitivity Survey

We present preliminary results on the XMM-Newton Survey Science Centre medium sensitivity survey (XMS), with 0.5-4.5 keV flux limit 2 10^{-14} erg/cm2/s. At present, 19 fields have been examined with a total of 239 X-ray sources. Identifications for about 2/3 of these reveal that type 1 AGN dominate, but type 2 AGN, absorption-line galaxies and clusters as well as stars are also present. We also discuss on a few X-ray selected Broad-Absorption-Line QSOs found in this survey.

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arXiv2002

H1320+551: A Seyfert 1.8/1.9 galaxy with an unabsorbed X-ray spectrum

We present new optical spectroscopic and XMM-Newton X-ray observations of the Active Galactic Nucleus H1320+551. The optical data (consistent with but of better quality than a previously published spectrum) show this source to be a Seyfert 1.8/1.9 galaxy at z=0.0653. The narrow line region is significantly reddened, with a Balmer decrement Ha/Hb~6 and the broad line region, with a barely detectable Hb broad component, shows a much pronnounced Balmer decrement (Ha/Hb>27). In spite of this, the EPIC-pn X-ray spectrum exhibits a power-law continuum with a soft excess that is well fitted by a black body, with no photoelectric absorption above the Galactic value. A Fe K emission line is also seen at a rest-frame energy ~6.5 keV with an equivalent width of ~400 eV, far too weak for the source being Compton-thick. Reconciling the optical and X-ray data requires the narrow line region being internally reddened but with small covering factor over the nuclear emission and the Balmer decrement of the broad line region being an intrinsic property rather than caused by reddening/absorption. The H1320+551 Seyfert 1.8/1.9 galaxy is not consistent with being an obscured Seyfert 1 nucleus, i.e., it does not match the basic AGN unified scheme hypothesis.

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arXiv2002

The XMM-Newton SSC Survey of the Galactic Plane

The XMM-Newton Survey Science Center is currently conducting an optical identification programme of serendipitous EPIC sources at low galactic latitudes. The aim of this study is to quantify the various populations contributing to the overall X-ray emission of the Galaxy and elaborate identification rules that can be later applied to the bulk of the low galactic latitude EPIC detections. We report here on preliminary results from an optical campaign performed in two very low b XMM-Newton fields and discuss the contributions of the various X-ray populations. This paper is presented on behalf of the Survey Science Center and of the AXIS collaboration.

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arXiv2002

The XMM-Newton Bright Serendipitous Source Sample (XMM-Newton BSS)

The XMM-Newton Survey Science Center is constructing a large (~1000 sources) and complete sample of bright serendipitous XMM-Newton sources at high galactic latitude, so as to allow both the discovery of sources of high individual interest as well as statistical population studies. The sample, known as ``The XMM-Newton Bright Serendipitous Source Sample" (XMM-Newton BSS), has a flux limit of ~10^(-13) cgs in the 0.5--4.5 keV energy band and will be fundamental in complementing other medium and deep survey programs (having fluxes 10 to 100 times fainter) in order to: a) characterize the X-ray sky and its constituents, b) understand the evolution of the population selected and c) define statistical identification procedures to select rare and interesting populations of X-ray sources (e.g. high z clusters of galaxies, BL Lacs, type 2 QSOs, etc..). We discuss here the scientific rationale and the details of the project and we present some preliminary results.

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arXiv2001

The XMM-Newton Serendipitous Survey. II. First results from the AXIS high galactic latitude medium sensitivity survey

We present the first results on the identifications of a medium sensitivity survey (X-ray flux limit 2E-14 erg/cm2/s in the 0.5-4.5 keV band) at high galactic latitude (|b|>20 deg) carried out with the XMM-Newton X-ray observatory within the AXIS observing programme. This study is being conducted as part of the XMM-Newton Survey Science Centre activities towards the identification of the sources in the X-ray serendipitous sky survey. The sample contains 29 X-ray sources in a solid angle of 0.26deg2 (source density 113 +/- 21 sources/deg2), out of which 27 (93%) have been identified. The majority of the sources are broad-line AGN (19), followed by narrow emission line X-ray emitting galaxies (6, all of which turn out to be AGN), 1 nearby non-emission line galaxy (NGC 4291) and 1 active coronal star. Among the identified sources we find 2 broad-absorption line QSOs (z~1.8 and z~1.9), which constitute ~10% of the AGN population at this flux level, similar to optically selected samples. Identifications of a further 10 X-ray sources fainter than our survey limit are also presented.

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arXiv2001

The European Large Area ISO Survey V: a BeppoSAX hard X-ray survey of the S1 region

We present BeppoSAX observations of the Southern S1 region in the European Large Area ISO Survey (ELAIS). These observations cover an area of ~1.7 sq. deg. and reach an on-axis (~0.7 sq. deg) 2-10 keV (HX) sensitivity of ~10E-13 cgs. This is the first HX analysis of an ISOCAM survey. We detect 9 sources with a signal to noise ratio SNR_HX>3, 4 additional sources with a 1.3 to 10 keV (T) SNR_T>3 and 2 additional sources which appear to be associated with QSOs with SNR_T>2.9. The number densities of the SNR_HX>3 sources are consistent with the ASCA and BeppoSAX logN-logS functions. Six BeppoSAX sources have reliable ISOCAM 15 micron counterparts within ~60 arcsec. All these ISOCAM sources have optical counterparts of R<20 mags. Five of these sources have been previously optically classified giving 4 QSOs and 1 BALQSO at z=2.2. The remaining unclassified source has X-ray and photometric properties consistent with that of a nearby Seyfert galaxy. One further HX source has a 2.6sigma ISOCAM counterpart associated with a galaxy at z=0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z=3.2 and assess the HX/MIR survey depth. The BALQSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to QSOs but different to those found for low redshift BALQSOs. This difference can be explained in terms of absorption, and suggests that high redshift BALQSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high redshift QSO population to be determined (ABRIDGED).

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arXiv2000

XMM-Newton observation of the Lockman Hole; I. The X-ray Data

We report on the first deep X-ray survey with the XMM-Newton observatory during the performance verification phase. The field of the Lockman Hole, one of the best studied sky areas over a very wide range of wavelengths, has been observed. A total of ~100 ksec good exposure time has been accumulated. Combining the images of the European Photon Imaging Camera (EPIC) detectors we reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1}, respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of 10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S) relation in the three bands is compared with previous results. In particular in the 5-10 keV band these observations present the deepest X-ray survey ever, about a factor 20 more sensitive than the previous BeppoSAX observations. Using X-ray spectral diagnostics and the set of previously known, spectroscopically identified ROSAT sources in the field, the new sources can be classified. XMM-Newton detects a significant number (~40%) of X-ray sources with hard, probably intrinsically absorbed X-ray spectra, confirming a prediction of the population synthesis models for the X-ray background.

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arXiv2000

On the intensity of the cosmic X-ray background

Measurements of the intensity of the cosmic X-ray background (XRB) carried out over small solid angles are subject to spatial variations due to the discrete nature of the XRB. This cosmic variance can account for the dispersion of XRB intensity values found within the ASCA, BeppoSAX and ROSAT missions separately. However there are differences among the values obtained in the different missions which are not due to spatial fluctuations but, more likely, to systematic cross-calibration errors. Prompted by recent work which shows that ROSAT PSPC has calibration differences with all the other missions, we compute a bayesian estimate for the XRB intensity at 1 keV of 10.0^{+0.6}_{-0.9} keV/cm2/s/keV/sr (90 per cent confidence errors) using the ASCA and BeppoSAX data points. However, this value is still significantly larger than the HEAO-1 intensity measured over many thousands of square degrees (8 keV/cm2/s/keV/sr).

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arXiv2000

Clustering in the X-ray Universe

In this paper we discuss various possibilities of using X-ray observations to gain information about the large-scale structure of the Universe. After reviewing briefly the current status of these investigations we explore different ways of making progress in this field, using deep surveys, large area surveys and X-ray background observations.

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arXiv2000

X-ray sources as tracers of the large-scale structure in the Universe

We review the current status of studies of large-scale structure in the X-ray Universe. After motivating the use X-rays for cosmological purposes, we discuss the various approaches used on different angular scales including X-ray background multipoles, cross-correlations of the X-ray background with galaxy catalogues, clustering of X-ray selected sources and small-scale fluctuations and anisotropies in the X-ray background. We discuss the implications of the above studies for the bias parameter of X-ray sources, which is likely to be moderate for X-ray selected AGN and the X-ray background (~1-2). We finally outline how all-sky X-ray maps at hard X-rays and medium surveys with large sky coverage could provide important tests for the cosmological models.

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arXiv1999

XEUS - The X-ray Evolving Universe Spectroscopy Mission

XEUS is under study by ESA as part of the Horizon 2000+ program to utilize the International Space Station (ISS) for astronomical applications. XEUS will be a long-term X-ray observatory with an initial mirror area of 6m2 at 1 keV that will be grown to 30m2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5'' HEW. XEUS will consist of separate detector and mirror spacecraft aligned by active control to provide a focal length of 50m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting sensitivity will then be ~4 10-18 erg/cm2/s - around 250 times better than XMM, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.

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arXiv1999

The gaseous extent of galaxies and the origin of Lyman alpha absorption systems. IV: Lyman alpha absorbers arising in a galaxy group

We present new GHRS observations of Lyman alpha absorption lines associated with a group of galaxies towards the QSO 1545+2101. We have identified eight distinct Lyman alpha absorption features in the spectrum of QSO 1545+2101 at a mean redshift of z=0.2648 with a velocity dispersion of 163 km/s. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of z=0.2645 and velocity dispersion 239 km/s. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that the Lyman alpha absorption lines are associated with individual galaxies in the group, although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment.

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arXiv1998

Discovery of an X-ray selected radio-loud obscured AGN at z=1.246

We have discovered an obscured active galaxy at redshift z = 1.246 identified with the ROSAT X-ray source RX J1011.2+5545. We report on multiwavelength observations of this source and discuss its X-ray, optical and radio properties. This is the first X-ray selected, obscured active galaxy at high redshift to be shown to be radio-loud, with a radio counterpart exhibiting a classical double-lobe morphology.

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arXiv1998

The Gaseous Extent of Galaxies and The Origin of QSO Absorption Line Systems

We present results of an ongoing program to study the gaseous extent of galaxies and the origin of QSO absorption line systems. For \lya absorption systems, we find that absorption equivalent width depends strongly on galaxy impact parameter and galaxy $B$-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy $B$-band luminosity as $r\propto L_B^{0.40\pm0.09}$. Applying the results to galaxies in the Hubble Deep Field to calculate the predicted number density of \lya absorption lines as a function of redshift and comparing it with observations, we find that at least 50% and perhaps as much as 100% of \lya absorption systems with $W\apg0.32$ Åcan be explained by the extended gaseous envelops of normal galaxies. The anti-correlation analysis has also been performed on \civ absorption line systems, and the results show that the ionized gas cross section scales with galaxy $B$-band luminosity as $r\propto L_B^{0.76\pm0.26}$.

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arXiv1998

Clustering of X-ray selected Active Galactic Nuclei

A total of 235 Active Galactic Nuclei (AGN) from two different soft X-ray surveys (the ROSAT Deep Survey -DRS- and the ROSAT International X-ray Optical Survey -RIXOS-) with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2 sigma significant detection of clustering of such objects is found on scales <40-80/h Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5<~ r_0 <~ 3.3/h Mpc is implied for comoving evolution, while 1.9 <~ r_0 <~ 4.8 for stable clustering and 2.2 <~ r_0 <~ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent UV and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be <~1.7 on scales <~ 10/h Mpc, a significantly smaller value than is inferred from local large-scale dynamical studies.

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