Showing papers by "Cristian Vignali published in 2001"

The HELLAS2XMM survey: I. The X-ray data and the Log(N)-Log(S)

Abstract: We present the first results from an XMM-Newton serendipitous medium-deep survey, which covers nearly three square degrees. We detect a total of 1022, 495 and 100 sources, down to minimum fluxes of about 5.9 x 10^-16, 2.8 x 10^-15 and 6.2 x 10^-15 erg cm^-2 s^-1, in the 0.5-2, 2-10 and 4.5-10 keV band, respectively. In the soft band this is one of the largest samples available to date and surely the largest in the 2-10 keV band at our limiting X-ray flux. The measured Log(N)-Log(S) are found to be in good agreement with previous determinations. In the 0.5-2 keV band we detect a break at fluxes around 5 x 10^-15 erg cm^-2 s^-1. In the harder bands, we fill in the gap at intermediate fluxes between deeper Chandra and XMM-Newton observations and shallower BeppoSAX and ASCA surveys.

The Chandra deep field-north survey. VII. X-ray emission from Lyman break galaxies

Abstract: We present results from stacking analyses, using the 1 Ms Chandra Deep Field-North data, that constrain the X-ray emission of Lyman break galaxies at z ≈ 2-4. Stacking the counts from 24 individually undetected Lyman break galaxies located within the Hubble Deep Field-North, we have obtained average detections of these objects in the resulting 0.5-8.0 and 0.5-2.0 keV images; these images have effective exposure times of 22.4 Ms (260 days). Monte Carlo testing empirically shows the detections to be highly significant. The average rest-frame 2-8 keV luminosity of a Lyman break galaxy is derived to be ≈3.2 × 1041 ergs s-1, comparable to that of the most X-ray luminous starbursts in the local universe. The observed ratio of X-ray to B-band luminosity is somewhat, but not greatly, higher than that seen from local starbursts. The X-ray emission probably arises from a combination of high-mass X-ray binaries, "super-Eddington" X-ray sources, and low-luminosity active galactic nuclei.

The BeppoSAX High Energy Large Area Survey (HELLAS) – III. Testing synthesis models for the X‐ray background

Abstract: The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of deg2 of the sky in the band down to a flux of about . The source surface density of at the survey limit corresponds to a resolved fraction of the X-ray background (XRB) of the order of per cent. The extrapolation of the HELLAS towards fainter fluxes with a Euclidean slope is consistent with the first XMM–Newton measurements, in the same energy band, which are a factor of 20 times more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the counts over the range of fluxes covered by BeppoSAX and XMM–Newton a large fraction of highly absorbed , luminous active galactic nuclei is needed. A sizeable number of more heavily obscured, Compton-thick, objects cannot be ruled out but they are not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators, especially at high redshifts/luminosities where several broad-line quasars show hardness ratios typical of absorbed power-law models with .

Exploratory Chandra Observations of the Highest-Redshift Quasars: X-Rays from the Dawn of the Modern Universe

Abstract: We report exploratory Chandra observations of 14 high-redshift (z = 4.06?5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z > 4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z = 4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the broad absorption-line quasars SDSSp J112956.10-014212.4 and SDSSp J160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435+3057. A comparison of the quasars' spectral energy distributions (by means of the optical?to?X-ray spectral index ?ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of ?2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z > 4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and XEUS, are also provided.

The BeppoSAX High Energy Large Area Survey HELLAS, III: testing synthesis models for the X-ray background

Abstract: The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of square degrees of the sky in the 5--10 keV band down to a flux of about 5 10^-14 erg cm-2 s-1. The extrapolation of the HELLAS logN--logS towards fainter fluxes with an euclidean slope is consistent with the first XMM measurements, in the same energy band, which are a factor 20 more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the 5--10 keV counts over the range of fluxes covered by BeppoSAX and XMM a large fraction of highly absorbed (logN_H = 23--24 cm-2), luminous (L_X > 10^44 erg s-1) AGN is needed. A sizeable number of more heavily obscured, Compton thick, objects cannot be ruled out but it is not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators especially at high redshifts/luminosities where several broad line quasars show hardness ratios typical of absorbed power law models with logN_H=22--24 cm-2.

The BeppoSAX High Energy Large Area Survey (HELLAS) – II. Number counts and X‐ray spectral properties

Abstract: The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85 deg 2 of sky in the 5–10 keV band down to a flux of 4−5×10 −14 erg cm −2 s −1 . The sourcesurface density of 16.9±6.4 deg −2 at the survey limit corresponds to a resolved fractionof the 5–10 keV X–ray background (XRB) of the order of 20–30 %.Hardness ratios analysis indicates that the spectra of a substantial fraction of the HEL-LAS sources (at least one third) are harder than a α E = 0.6 power law. This hardnessmay be due to large absorbing columns. The hardness ratio analysis also indicates thatmany HELLAS sources may have a spectrum more complex than a single absorbedpower law. A soft component, superimposed to a strongly cut-off power law, is likely tobe present in several sources.Key words: X–ray: selection – background – galaxies – AGN 1 INTRODUCTIONHard X-ray observations are very efficient in tracing emis-sion due to accretion mechanisms, like in Active GalacticNuclei (AGN). Hard X-ray selection is not affected by strongbiases present at other wavelengths. For example, a columnof a few times 10

BeppoSAX observations of Narrow-Line Seyfert 1 galaxies. II. Ionized iron features in Arakelian 564

Abstract: The BeppoSAX observations of the bright Narrow-Line Seyfert 1 galaxy Ark 564 are presented along with a high quality optical spectrum taken at the 1.5m telescope at La Silla. The 0.1-10 keV X-ray spectrum is characterized by a strong soft component which is best described by blackbody-like emission with a temperature of about 160 eV. At higher energies a steep (Gamma = 2.4) power-law tail is present. There is evidence of an ionized reflector in the form of an iron line and edge. We do not find significant evidence of soft X-ray features if the spectrum is modelled with a two component continuum. The optical and X-ray spectral properties support the hypothesis of a high accretion rate onto a low mass black hole.

The BeppoSAX High Energy Large Area Survey - IV. On the soft X-ray properties of the hard X-ray-selected HELLAS sources

Abstract: We present a comprehensive study of the soft X-ray properties of the BeppoSAX High-Energy Large Area Survey (HELLAS) sources. A large fraction (about 2/3) of the hard X-ray selected sources is detected by ROSAT . The soft X-ray colors for many of these objects, along with the 0.5-2 keV flux upper limits for those undetected in the ROSAT band, do imply the presence of absorption. The comparison with the ROSAT Deep Survey sources indicates that a larger fraction of absorbed objects among the HELLAS sources is present, in agreement with their hard X-ray selection and the predictions of the X-ray background synthesis models. Another striking result is the presence of a soft (additional) X-ray component in a significant fraction of absorbed objects.

The Chandra Deep Field North Survey. X. X-ray Emission from Very Red Objects

Abstract: The multi-wavelength properties of Very Red Objects (VROs; I-K>=4) are largely unknown as many of these sources are optically faint (I>=24) and undetected at most wavelengths. Here we provide constraints on the X-ray properties of VROs using the 1 Ms Chandra exposure of an 8.4'x8.4' region within the Hawaii Flanking-Field area containing the HDF-N. We find that VROs detected in the hard band (2-8 keV) have flat X-ray spectral slopes (Gamma ~ 0.9) and X-ray properties consistent with those expected from luminous obscured AGN. The fraction of such sources in the K 1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star-formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (>=99% confidence) in the soft band and the full band (0.5-8 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from non-active VROs with low X-ray-to-optical flux ratios [log(FX/FI) ~ -2]; this is consistent with that expected if the majority of these VROs are ~ M* elliptical galaxies. A number of VROs are also detected with mid-IR (15um) and radio emission, and we provide constraints on the nature of this emission.

The BeppoSAX High Energy Large Area Survey. IV. On the soft X-ray properties of the hard X-ray-selected HELLAS sources

Abstract: We present a comprehensive study of the soft X-ray properties of the BeppoSAX High-Energy Large Area Survey (HELLAS) sources. A large fraction (about 2/3) of the hard X-ray selected sources is detected by ROSAT. The soft X-ray colors for many of these objects, along with the 0.5-2 keV flux upper limits for those undetected in the ROSAT band, do imply the presence of absorption. The comparison with the ROSAT Deep Survey sources indicates that a larger fraction of absorbed objects among the HELLAS sources is present, in agreement with their hard X-ray selection and the predictions of the X-ray background synthesis models. Another striking result is the presence of a soft (additional) X-ray component in a significant fraction of absorbed objects.