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C. Vignali

Publications and source records attributed to C. Vignali.

At least 415 records · Page 23Linked to original sources
arXiv2001

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

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.

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arXiv2001

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

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 160501.21-011220.0, 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, alpha_ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of approx 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.

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arXiv2001

The Chandra Deep Field North Survey. VII. X-ray Emission from Lyman Break Galaxies

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 keV 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 x 10^{41} erg 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.

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arXiv2001

The BeppoSAX High Energy Large Area Survey HELLAS, II: Number counts and X-ray spectral properties

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 source surface density of 16.9+-6.4 deg$^{-2}$ at the survey limit corresponds to a resolved fraction of 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 HELLAS sources (at least one third) are harder than a alpha_E = 0.6 power law. This hardness may be due to large absorbing columns. The hardness ratio analysis also indicates that many HELLAS sources may have a spectrum more complex than a single absorbed power law. A soft component, superimposed to a strongly cut-off power law, is likely to be present in several sources.

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arXiv2001

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

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.

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arXiv2001

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

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.

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arXiv2001

High Energy Large Area Surveys: from BeppoSAX to Chandra and XMM

Optical identification of hard X-ray selected BeppoSAX and Chandra sources indicates that a large fraction of the sources are ``intermediate'' AGN, i.e. type 1.8-1.9 AGN, broad-line quasars and even X-ray loud but optically dull (apparently normal) galaxies, all obscured in X-rays by columns of the order of logNH=22-23.5 cm-2. Because of this obscuring matter, these sources are more difficult to detect or select at other wavelengths, implying that a fraction of the accretion power in the Universe may have been missed so far.

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arXiv2000

The evolution of AGN in the Hard X-rays from HELLAS

We present optical spectroscopic identification of sources identified in the BeppoSAX MECS fields of the High Energy LLarge Area Survey (HELLAS). In total 62 sources from a sample of 115 brighter than F_{5-10keV}>5x10^{-14} cgs have been identified. We find a density of 13-21 sources/deg^2 at F_{5-10keV}>5x10^{-14} cgs, which contribute to 20-30 % of the hard X-ray background. Evidences are found for type 1 AGN being more absorbed with increasing redshift or luminosity. The low redshift (z<0.2) ratio of type 2 to type 1 AGN is 3+/-1.5, in agreement with the unified models for AGN. The luminosity function of type 1 AGN in the 2-10 keV band is preliminary fitted by a two-power law function evolving according to a pure luminosity evolution model: L~(1+z)^{2.2}.

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arXiv2000

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

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.

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arXiv2000

On the peculiar X-ray properties of the bright nearby radio-quiet quasar PDS456

BeppoSAX and ASCA observations of the nearby (z=0.184), high-luminosity, radio-quiet quasar PDS456 are presented. The X-ray spectrum is characterized by a prominent ionized edge at 8-9 keV (originally discovered by RXTE, Reeves et al. 2000) and by a soft excess below 1.5 keV. The lack of any significant iron K alpha emission line suggests for the edge an origin from line-of-sight material rather than from reflection from a highly ionized accretion disc. The hard X-ray continuum is indeed well modelled by transmission through a highly-ionized medium with a large column density (N_H warm = 4.5 x 10^24 cm^-2) plus an additional cold absorber with a lower column density (N_H cold = 2.7 x 10^22 cm^-2).

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arXiv2000

The complex X-ray absorbers of NGC 3516 observed by BeppoSAX

In this paper we present the analysis of two broad band (0.1-150 keV) BeppoSAX observations of the Seyfert 1 galaxy NGC 3516. The two observations were taken 4 months apart, on 1996 November 8 and 1997 March 12 respectively. We report a dramatic change in the degree of obscuration of the central source between the two observations, and propose, as possible explanations transient absorption by either a stationary-state cloud of cold gas crossing the line of sight, or a varying-state, initially neutral and dense amount of expanding gas with decreasing density and therefore decreasing opacity. We also report the detection of a second highly ionized absorber/emitter, which causes deep FeXVII-XXII K edges at about 7.8 keV to appear in both BeppoSAX spectra of NGC 3516, and possibly produces the soft X-ray continuum emission and the 1 keV blend of Fe L recombination lines detected during the epoch of heavy nuclear obscuration.

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arXiv2000

The BeppoSAX HELLAS survey: on the nature of faint hard X-ray selected sources

The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80 square degrees of sky down to a flux of F(5-10keV)~5E-14 cgs. Optical spectroscopic identification of about half of the sources in the sample (62) shows that many (~50%) are highly obscured AGN, in line with the predictions of AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et al. 1995). The X-ray data, complemented by optical, near-IR and radio follow-up, indicate that the majority of these AGN are ``intermediate'' objects, i.e. type 1.8-1.9 AGN,`red' quasars, and even a few broad line, blue continuum quasars, obscured in X-rays by columns of the order of logNH=22.5-23.5 cm-2, but showing a wide dispersion in optical extinction. The optical and near-IR photometry of the obscured objects are dominated by galaxy starlight, indicating that a sizeable fraction of the accretion power in the Universe may actually have been missed in optical color surveys. This also implies that multicolor photometry techniques may be efficiently used to assess the redshift of the hard X-ray selected sources.

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arXiv2000

Sub-mm and X-ray background: two unrelated phenomena?

Obscured AGNs are thought to contribute a large fraction of the hard X-ray background (2-10 keV), and have also been proposed as the powerhouse of a fraction of the SCUBA sources which make most of the background at 850um, thus providing a link between the two spectral windows. We have tackled this issue by comparing data at 2-10 keV and at 850um for a sample of 34 sources at fluxes (or limiting fluxes) which resolve most of the background in the two bands. We present here new SCUBA observations, and new correlations between separate data sets retrieved from the literature. Similar correlations presented by others are added for completeness. None of the 11 hard X-ray (2-10 keV) sources has a counterpart at 850um, with the exception of a Chandra source in the SSA13 field, which is a candidate type 2, heavily absorbed QSO at high redshift. The ratios F(850um)/F(5keV) (mostly upper limits) of the X-ray sources are significantly lower than the value observed for the cosmic background. In particular, we obtain that 2-10 keV sources brighter than 10^-15 erg s^-1 cm^-2, which make at least 75% of the background in this band, contribute for less than 7% to the submillimeter background. Out of the 24 SCUBA sources, 23 are undetected by Chandra. The ratios F(850um)/F(5keV) (mostly lower limits) of these SCUBA sources indicate that most of them must be powered either by starburst activity, or by an AGN which is obscured by a column Nh > 10^25 cm^-2, with a reflection efficiency in the hard X rays significantly lower than 1% in most cases. However, AGNs of this type could not contribute significantly to the 2-10 keV background.

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arXiv2000

Spectroscopic identification of ten faint hard X-ray sources discovered by Chandra

We report optical spectroscopic identifications of 10 hard (2-10 keV) X-ray selected sources discovered by Chandra. The X-ray flux of the sources ranges between 1.5 and 25 x 10-14 cgs, the lower value being 3 times fainter than in previous BeppoSAX and ASCA surveys. Their R band magnitudes are in the range 12.8-22. Six of the Chandra sources are broad line quasars with redshifts between 0.42 and 1.19, while the optical identification of the remaining four is quite varied: two are X-ray obscured, emission line AGN at z=0.272 and z=0.683, one is a starburst galaxy at z=0.016 and one, most unusually, is an apparently normal galaxy at z=0.158. These findings confirm and extend down to fainter X-ray fluxes the BeppoSAX results, in providing samples with a wide range of X-ray and optical properties. The ratio between the soft X-ray and the optical luminosity of the z=0.158 galaxy is a factor at least 30 higher than that of normal galaxies, and similar to those of AGN. The high X-ray luminosity and the lack of optical emission lines suggest an AGN in which either continuum beaming dominates, or emission lines are obscured or not efficiently produced.

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arXiv2000

Optically dim counterparts of hard X-ray selected AGNs

We present near-IR photometry and imaging observations of a small sample of sources identified in the BeppoSAX 5-10 keV survey (HELLAS) which resolves ~ 20-30% of the X-ray background at these energies. The near-IR data are combined with optical spectra and photometry. Only 40% of the sources in our sample have the blue, power law continuum typical of color-selected QSOs. The remaining 60% are dominated by a galactic component which, on the basis of the continuum colors and shape, have ages ranging from 10^9 to 10^10 years. The images show that the blue QSOs are pointlike at our angular resolution, while all the other sources are extended, consistent with their spectral appearance and low redshift. Since down to R=20 only about two thirds of the HELLAS sources have a counterpart, the preliminary HELLAS census comprises in roughly equal parts: i) blue QSOs (mostly at high redshifts); ii) optically dim, galaxy-dominated active nuclei (mostly at modest redshifts); and iii) empty fields (possibly highly absorbed QSOs at high redshifts).

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arXiv2000

Optical, near-infrared and hard X-ray observations of SAXJ1353.9+1820: a red quasar

We present the results of a follow-up ASCA observation and multicolour optical and near-infrared photometry carried out at the 3.5-m Italian National Telescope Galileo of SAXJ1353.9+1820. This object, serendipitously discovered by BeppoSAX in the 5-10 keV band, has been spectroscopically identified as a red quasar at z=0.217. The combined X-ray and optical-infrared data reveal the presence of a moderately luminous X-ray source (~ 10^{44} erg s^{-1}) obscured by a column density of the order of 10^{22} cm^{-2} in a otherwise optically passive early-type galaxy. The implications for the nature of red quasars and their possible contribution to the hard X-ray background are briefly outlined.

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