Searcharxiv⌕ Search

arXiv subjects

Andrea Comastri

Publications and source records attributed to Andrea Comastri.

At least 91 records · Page 5Linked to original sources
arXiv2006

The XMM-Newton wide-field survey in the COSMOS field: V. Angular Clustering of the X-ray Point Sources

We present the first results of the measurements of angular auto-correlation functions (ACFs) of X-ray point sources detected in the XMM-Newton observations of the 2 deg2 COSMOS field (XMM-COSMOS). A significant positive signals have been detected in the 0.5-2 (SFT) band, in the angle range of 0.5-24 arcminutes, while the positive signals were at the 2 and 3 sigma levels in the 2-4.5 (MED) and 4.5-10 (UHD) keV bands respectively. Correctly taking integral constraints into account is a major limitation in interpreting our results. With power-law fits to the ACFs without the integral constraint term, we find correlation lengths of theta_c=1.9+-0.3, 0.8+0.5-0.4 and 6+-2 arcseconds for the SFT, MED, and UHD bands respectively for gamma=1.8. The inferred comoving correlation lengths, also taking into account the bias by the source merging due to XMM-Newton PSF, are 9.8+-0.7, 5.8+1.4-1.7 and 12+-2 h-1 Mpc at the effective redshifts of 1.1, 0.9, and 0.6 for the SFT, MED, and UHD bands respectively. If we include the integral constraint term in the fitting process,assuming that the power-law extends to the scale length of the entire XMM-COSMOSfield, the correlation lengths become larger by 20%-90%. Comparing the inferred rms fluctuations of the spatial distribution of AGNs with those of the underlying mass, the bias parameters of the X-ray source clustering at these effective redshifts are in the range b(AGN)=1.5-4. (Edited to simple ascii.)

astro-ph↗
arXiv2006

The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era

We present a detailed and self-consistent modeling of the cosmic X-ray background (XRB) based on the most up-to-date X-ray luminosity functions (XLF) and evolution of Active Galactic Nuclei (AGN). The large body of observational results collected by soft (0.5-2 keV) and hard (2-10 keV) X-ray surveys are used to constrain at best the properties of the Compton-thin AGN population and its contribution to the XRB emission. The number ratio R between moderately obscured (Compton-thin) AGN and unobscured AGN is fixed by the comparison between the soft and hard XLFs, which suggests that R decreases from 4 at low luminosities to 1 at high luminosities. From the same comparison there is no clear evidence of an evolution of the obscured AGN fraction with redshift. The distribution of the absorbing column densities in obscured AGN is determined by matching the soft and hard source counts. A distribution rising towards larger column densities is able to reproduce the soft and hard AGN counts over about 6 dex in flux. The model also reproduces with excellent accuracy the fraction of obscured objects in AGN samples selected at different X-ray fluxes. The integrated emission of the Compton-thin AGN population is found to underestimate the XRB flux at about 30 keV and a population of heavily obscured -Compton-thick- AGN, as large as that of moderately obscured AGN, is required to fit the residual background emission. Remarkably, the fractions of Compton-thick AGN observed in the Chandra Deep Field South and in the first INTEGRAL and Swift catalogs of AGN selected above 10 keV are in excellent agreement with the model predictions. [abridged]

astro-ph↗
arXiv2006

New insights on chemical evolution of galaxies: XMM-Newton observations of M82

We report on the ongoing data analysis of a very deep (~100 ks) XMM-Newton observation of the starburst galaxy M82. We show some details of data analysis and a few results from spatially-resolved spectroscopy with the EPIC cameras. Since M82 is a bright object with a complex spectrum, the data reduction of such a deep observation has posed many challenges both about the involved astrophysical processes and the available data analysis techniques. Vertical (with respect to the galaxy plane) abundance gradients are discovered. The hints for an under-abundance of Oxygen stemming from our previous study are confirmed. The hot X-ray emitting gas is shown to have a multi-temperature distribution.

astro-ph↗
arXiv2006

Relativistic Iron lines at high redshifts

The shape and the intensity of the 6.4 keV iron line bring unique information on the geometrical and physical properties of the supermassive black hole and the surrounding accreting gas at the very center of Active Galactic Nuclei. While there are convincing evidences of a relativistically broadened iron line in a few nearby bright objects, their properties at larger distances are basically unknown. We have searched for the presence of iron line by fully exploiting Chandra observations in the deep fields. The line is clearly detected in the average spectra of about 250 sources stacked in several redshift bins over the range z=0.5-4.0. We discuss their average properties with particular enphasys on the presence and intensity of a broad component.

astro-ph↗
arXiv2005

Gaseous abundances in M82

We present the preliminary analysis of a deep (100ks) XMM-Newton observation of M82. The spatial distribution of the abundances of chemical elements (Fe, O, Ne, Mg, Si, S) is investigated through narrow-band imaging analisys and spatially-resolved spectroscopy. We find that the abundances of alpha-elements follow a bipolar distribution, these elements being more abundant in the gaseous outflow than in the galaxy centre. This behaviour is found to be more marked for lighter elements (O, Ne) than for heavier elements.

astro-ph↗
arXiv2005

The X-ray luminosity function and number counts of spiral galaxies

(abridged) A detailed comparison is performed of the LFs compiled at infrared, radio and optical wavelengths and converted into XLFs using available relations with the XLF directly estimated in the 0.5--2 keV energy band from X-ray surveys (Norman et al). We find that the XLF from the local sample of IRAS galaxies (Takeuchi et al) provides a good representation of all available data samples; pure luminosity evolution of the form (1+z)^η, with η< ~3, is favoured over pure density. The local X-ray luminosity density is also well defined. We discuss different estimates of the galaxies LogN-LogS, selected from the Chandra Deep Fields with different selection criteria: these have similar slopes, but normalisations scattered within a factor ~2, of the same order of the Poissonian error on the counts. We compare the observed LogN-LogS with the counts predicted by integrating our reference z=0 XLF. By using number counts alone, it is not possible to discriminate between density and luminosity evolution; however, the evolution of galaxies must be stopped in both cases at z~1-2. The contribution of galaxies to the X-ray background is found to be in the range 6%--12%. Making use of cosmic star formation models, we find that the X-ray LogN-LogS might be not compatible with very large star formation rates at z ~ 3 as suggested by sub-mm observations in Blain et al. 1999. As to the content of current and, possibly, future X-ray surveys, we determine the fraction of galaxies around the current flux limit: (30+-12 %). At fainter fluxes the fraction of galaxies will probably rise, and overcome the counts from AGN at fluxes < ~10^{-17} erg/s/cm^2.

astro-ph↗
arXiv2005

The Iron Line Background

We investigate the presence of iron line emission among faint X-ray sources identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep Field North. Individual source spectra are stacked in seven redshift bins over the range z=0.5-4. We find that iron line emission is an ubiquitous property of X-ray sources up to z~3. The measured line strengths are in good agreement with those expected by simple pre-Chandra estimates based on X-ray background synthesis models. The average rest frame equivalent width of the iron line does not show significant changes with redshift.

astro-ph↗
arXiv2005

X-ray spectral analysis of optically faint sources in the Chandra Deep Fields

We present the results of a detailed spectral analysis of optically faint hard X-ray sources in the Chandra deep fields selected on the basis of their high X-ray to optical flux ratio (X/O). The stacked spectra of high X/O sources in both Chandra deep fields, fitted with a single power-law model, are much harder than the spectrum of the X-ray background (XRB). The average slope is also insensitive to the 2-8 keV flux, being approximately constant around Gamma~1 over more than two decades, strongly indicating that high X/O sources represent the most obscured component of the XRB. For about half of the sample, a redshift estimate (in most of the cases a photometric redshift) is available from the literature. Individual fits of a few of the brightest objects and of stacked spectra in different redshift bins imply column densities in the range 10^{22-23.5} cm^{-2}. A trend of increasing absorption towards higher redshifts is suggested.

astro-ph↗
arXiv2004

A broad iron K alpha line at z=1.146

We report the discovery of a strong iron K alpha line in the hard X-ray selected source CXOJ 123716.7+621733 in the Chandra Deep Field North survey at z=1.146. The analysis is made possible by the very deep exposure ~2 Ms and low background of the ACIS detector. The line profile seems to be inconsistent with a narrow feature. The best fit solution is achieved with a broad line. Most of the flux in the broad component originates at energies below 6.4 keV with a shape similar to that expected from emission in the innermost regions of the accretion disk.

astro-ph↗
arXiv2004

X-ray number counts of star forming galaxies

The behaviour of the X-ray number counts of normal galaxies at faint (-18<Log F<-15 cgs in the 0.5-2.0 keV band) fluxes is investigated. The joint use of information from radio, far infrared and X-ray surveys allows the determination of the LogN-LogS of galaxies within a factor-of-3 over the whole flux range considered.

astro-ph↗
arXiv2004

The density and masses of obscured Black Holes

Recent Chandra and XMM-Newton surveys have uncovered a large fraction of the obscured AGN responsible of the hard X-ray background. One of the most intriguing results of extensive programs of follow-up observations concerns the optical and near-infrared properties of the hard X-ray sources counterparts. More specifically, for a significant fraction of hard X-ray obscured sources the AGN responsible of the high X-ray luminosity remains elusive over a wide range of wavelengths from soft X-rays to near-infrared. This very observational result opens the possibility to investigate the host of bright obscured quasars in some detail. Here we briefly report on some preliminar results obtained for a small sample of elusive AGN in the HELLAS2XMM survey.

astro-ph↗
arXiv2004

Compton Thick AGN: the dark side of the X-ray background

The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several pieces of observational and theoretical evidence indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. The absorbing matter is often very thick, with column densities exceeding N_H > 1.5 10^24 cm-2, the value corresponding to unity optical depth for Compton scattering. These sources are called ``Compton thick'' and appear to be very numerous, at least in the nearby universe. Although Compton thick Active Galactic Nuclei (AGN) are thought to provide an important contribution to the overall cosmic energy budget, their space density and cosmological evolution are poorly known. The properties of Compton thick AGN are reviewed here, with particular emphasis on their contributions to the extragalactic background light in the hard X-ray and infrared bands.

astro-ph↗
arXiv2004

The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South

The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-Ray Luminosity Function for Galaxies in these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~ (1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0). This provides the most direct measure yet of the X-ray derived cosmic star-formation history of the Universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of AGN, finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the H-alpha LF indicating that additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, Radio, H-alpha) gives an interesting constraint on the IMF, and X-rays may be measuring directly the binary star formation history of the Universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future x-ray missions.

astro-ph↗
arXiv2003

High Redshift Supermassive Black Holes: X-ray observations

The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several observational and theoretical evidences indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. X-ray surveys are the most efficient way to trace accretion onto supermassive black holes, since obscured, accreting sources are more difficult to select at all other wavelengths. The current status of hard X-ray surveys after the recent observations carried out with Chandra and XMM-Newton satellites is reviewed along with the results of extensive follow-up multiwavelength observations. In particular recent results concerning the physical and evolutive properties of the supermassive black holes hosted by X-ray selected Active Galactic Nuclei at high redshifts will be discussed.

astro-ph↗
arXiv2002

The 2-10 keV luminosity as a Star Formation Rate indicator

Radio and far infrared luminosities of star forming galaxies follow a tight linear relation. Making use of BeppoSAX and ASCA observations of a well-defined sample of star forming galaxies, we argue that a tight linear relation holds between the 2-10 keV X-ray luminosity and both the radio and far infrared ones. It is suggested that the hard X-ray emission is directly related to the Star Formation Rate. Preliminary results obtained from deep Chandra and radio observations of the Hubble Deep Field North show that a similar relation might hold also at high (0.2<z<1.2) redshift.

astro-ph↗
arXiv2000

The BeppoSAX view of NLS1s

The main results of broad-band (0.1-10 keV) BeppoSAX observations of a selected sample of NLS1s are presented and discussed. It is shown that all the available data are consistent with a scenario in which NLS1s are running at a high accretion rate.

astro-ph↗
arXiv2000

What's wrong with AGN models for the X-ray background ?

The origin of the hard X-ray background (XRB) as a superposition of unabsorbed and absorbed Active Galactic Nuclei is now widely accepted as the standard model. The identification of faint X-ray sources in ROSAT, ASCA, and BeppoSAX medium-deep surveys and their average spectral properties are in broad agreement with the model predictions. However, AGN models, at least in their simplified version, seem to be at odds with some of the most recent findings calling for substantial revisions. I will review the recent XRB "best fit" models and discuss how the foreseen XMM and Chandra surveys will be able to constrain the allowed parameter space.

astro-ph↗