arXiv subjects

F. Fiore

Publications and source records attributed to F. Fiore.

At least 307 records · Page 17Linked to original sources

arXiv2002

The BeppoSAX High Energy Large Area Survey -V. The nature of the hard X-ray source populations and its evolution

We present optical spectroscopic identifications of hard X-ray (5-10 keV) selected sources belonging to the HELLAS sample obtained with BeppoSAX down to a 5-10 keV flux limit of f_{5-10 keV}~3x10^-14 erg cm^-2 s^-1. The sample consists of 118 sources. 25 sources have been identified trough correlations with catalogues of known sources. 49 have been searched for spectroscopic identification at the telescope. 13 fields resulted empty down to R=21. 37 sources have been identified with type 1 AGN and 9 with type 2 AGN. The remaining are: 5 narrow emission line galaxies, 6 Clusters, 2 BL Lac, 1 Radio Galaxy and 1 Star. Combining these objects with other hard X-ray selected AGNs from ASCA and HEAO1, we find that the local luminosity function of type 1 AGN in the 2-10 keV band is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution according to a pure luminosity evolution (PLE) model L_X(z)~(1+z)^k, with k=2.12 and k=2.22 in a (Omega_m,Omega_lambda)=(1.0,0.0) and in a (Omega_m,Omega_lambda)=(0.3,0.7) cosmology, respectively. The data show an excess of faint high redshift type 1 AGN which is well modeled by a luminosity dependent density evolution (LDDE), similarly to what observed in the soft X-rays. However, in both cosmologies, the statistic is not significant enough to distinguish between the PLE and LDDE models. The fitted models imply a contribution of AGN1 to the 2-10 keV X-ray background from 35% up to 60%.

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arXiv2002

Spectral properties of faint hard X-ray selected sources

We present preliminary results from a XMM-Newton spectroscopic survey of hard (2 -10 keV) X-ray sources detected serendipitously in seven EPIC FOVs. All observations were performed during the performance/verif ication phase. The resulting sample consists of 39 sources with fluxes ranging from 3 to 80 $\t imes$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 2-10 keV band. Most of these sources (23 out of 39, i.e. 60%) are fainter than 8 $\times $ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$. The sources have been selected in order to obtain detailed X-ray spectral inform ation, that are unprecedented at such low flux levels and for such a large sample. To date we also have optical counterparts for 23 out of 39 objects, with 0.0058 $\leq$ $z$ $\leq$ 1.187. About 70% of the sources have spectra well fitted with a simple power law model with Galactic absorption. For 9 objects (five of type 1, one of type 2 and three normal galaxies) absorption in excess to the Galactic one is required. Additional emission/absorption components (i.e. soft excess, warm absorber) are observed in 4 sources.

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arXiv2002

The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.

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arXiv2002

The log(N)-log(S) and the Broadband Properties of the Sources in the HELLAS2XMM Survey

We present the first results from an XMM-Newton serendipitous medium-deep survey, which covers nearly three square degrees. We show the log(N)-log(S) distributions for the 0.5-2, 2-10 and 5-10 keV bands, which are found to be in good agreement with previous determinations and with the predictions of XRB models. In the soft band we detect a break at fluxes around 5x10^-15 cgs. 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. Moreover, we present an analysis of the broad-band properties of the sources.

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arXiv2002

Chandra Discovery of a Tree in the X-ray Forest towards PKS 2155-304: the Local Filament?

We present the first X-ray detection of resonant absorption from warm/hot local gas either in our Galaxy, or in the Intergalactic space surrounding our Galaxy, along the line of sight toward the blazar PKS 2155-304. The Chandra HRCS-LETG spectrum of this z=0.116 source clearly shows unresolved OVII(Ka) and NeIX(Ka) resonant absorption lines at 21.603 A and 13.448 A (i.e. cz = [-340--200] km/s in the rest frame, from the OVII line). OVIII(Ka) and OVII(Kb) from the same system are also detected at a lower significance level, while upper limits are set on OVIII(Kb), NeX(Ka) and NeIX(Kb). The FUSE spectrum of this source shows complex OVI(2s-->2p) absorption at the same redshift as the X-ray system, made by at least two components: one relatively narrow and slightly redshifted, and one broader and blueshifted (cz = -135 km/s). We demonstrate that the physical states of the UV and X-ray absorbers are hard to reconcile with a single, purely collisionally ionized, equilibrium plasma. We propose, instead, that the X-ray and, at least the broader and blueshifted UV absorber are produced in a low density intergalactic plasma (partly photoionized by the diffuse extragalactic X-ray background), collapsing towards our Galaxy, consistent with the predictions of a Warm-Hot Intergalactic Medium (WHIM) from numerical simulations. We find that any reasonable solution requires overabundance of Ne compared to O by a factor of ~2, with respect to the solar value and propose several scenarios to account for this observation.

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arXiv2001

Nuclear and global X-ray properties of LINER galaxies: Chandra and BeppoSAX results for Sombrero and NGC 4736

We report on the 0.1--100 keV BeppoSAX observations of two nearby LINER galaxies, Sombrero and NGC 4736. Chandra ACIS-S observations supplement this broad-beam spectral study with a high resolution look into the nuclear region, and show a dominating central point source in Sombrero and a complex X-ray binary dominated/starburst region in NGC 4736. A compact non-thermal radio source, present in the nucleus of both galaxies, coincides with the central source in Sombrero, while in NGC 4736 its X-ray counterpart is a much fainter point source, not the brightest of the central region. On the basis of these and other results, we conclude that the LINER activity is linked to the presence of a low luminosity AGN in Sombrero and to a recent starburst in NGC 4736, and that Chandra's spectroscopic capabilities coupled to high resolution imaging are essential to establish the origin of the nuclear activity.

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arXiv2001

The evolution of AGNs in the Hard X-Rays and the Infrared

We present the estimate of the evolution of type 1 AGNs in the hard (2-10 keV) X-rays drawn from the HELLAS survey, and in the IR (15um) obtained from the ELAIS survey. We find that the local luminosity function (LF) of AGN1 in the 2-10 keV band is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution according to a pure luminosity evolution model Lx(z)~(1+z)^k, with k=2.12 and k=2.19 in a (Omega_m,Omega_lambda)=(1.0,0.0) and in a (Omega_m,Omega_lambda)=(0.3,0.7) cosmology respectively. In a (Omega_m,Omega_lambda)=(1.0,0.0) Universe the data show an excess of faint high redshift type 1 AGN which is well modeled by a luminosity dependent density evolution, similarly to what observed in the soft X-rays. In the IR band, with a (Omega_m,Omega_lambda)=(1.0,0.0) cosmology, the evolution found is similar to what observed in other wavebands, the LF is a double-power-law-function with a bright slope 2.9 and a faint slope 1.1, following a luminosity evolution model L(z)~(1+z)^3.0.

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arXiv2001

Optical identification of sources from the HELLAS2XMM survey

We present preliminary results on the optical identifications of sources from the High Energy Large Area Survey performed with XMM-Newton (HELLAS2XMM). This survey covers about 3 square degrees of sky down to a 2-10 keV flux of 7 x 10^-15 erg cm-2 s-1 (Baldi & Molendi these proceedings, Baldi et al. 2001). The survey good sensitivity over a large area allows us to confirm and extend previous Chandra results about a population of X-ray luminous but optically dull galaxies emerging at 2-10 keV fluxes of about 10-14 erg cm-2 s-1. Although the statistics are still rather poor, it appears that at these X-ray fluxes the fraction of these galaxies is similar or even higher than that of narrow line AGN.

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arXiv2001

X-ray bright optically quiet galaxies: the case of P3

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at X-ray fluxes of the order of 10-14 erg cm-2 s-1. The nature of these objects is still unknown. We present the results of multiwavelength observations of what can be considered the best studied example: the Chandra source CXOUJ031238.9-765134 optically identified by Fiore et al. (2000) with an apparently normal galaxy at z=0.159 and called ``Fiore P3''.

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arXiv2001

The complex iron line of NGC 5506

The bright Narrow Emission Line Galaxy, NGC 5506, has been observed simultaneously by XMM-Newton and BeppoSAX. The iron line is complex, with at least two components: one narrow and corresponding to neutral iron, the second one broad and corresponding to ionized iron. The latter line is equally well fitted by a truly broad line or by a blend of He- and H-like narrow lines. The bulk of the Compton reflection continuum originates in neutral matter, and is therefore associated with the narrow line: they are most likely emitted in distant matter. The origin of the ionized line(s) is less certain, but the solution in terms of a blend of narrow lines from photoionized matter seems to be preferable to emission from an ionized, relativistic accretion disc.

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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

Cosmology studies with GRB afterglows

GRB afterglows close to their peak intensity are among the brightest sources in the sky. We discuss how GRB optical-to-X-ray afterglows can be used as probes of the metal enrichment of the interstellar matter in the GRB host galaxies and of the heating history of the intergalactic matter in filaments and outskirts of clusters of galaxies along the line of sight. The key points of the proposed observation strategy are: 1) prompt GRB observations (from minutes to a few hours); 2) coordinated O-UV and X-ray observations; and 3) high resolution (1,500-6,000 in the 0.1-1 keV band and >10,000 in the OUV).

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arXiv2001

On the nature of X-ray absorption in Seyfert 2 galaxies

We have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: a) the Balmer decrement and the amount of X-ray absorption are anticorrelated on a wide range of column density: 10^{21} < N_H < 10^{24} atoms/cm/cm. The correlation does no longer apply to Compton-thick objects (N_H < 10^{24} atoms/cm/cm), although they span a comparable range in Balmer decrement; b) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich of dust on scales of the hundreds parsecs. On the other hand, Compton-thick Seyferts exhibit indifferently ``dust-poor'' and ``dust-rich'' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt, 2000), where Compton-thick Seyfert 2s are observed through compact ``torii'', whereas Compton-thin ones are obscured by dust on much larger scales

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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

Iron line emission in X-ray afterglows

Recent observations of X-ray afterglows reveal the presence of a redshifted Kalpha iron line in emission in four bursts. In GRB 991216, the line was detected by the low energy grating of Chandra, which showed the line to be broad, with a full width of ~15,000 km/s. These observations indicate the presence of a >1 solar mass of iron rich material in the close vicinity of the burst, most likely a supernova remnant. The fact that such strong lines are observed less than a day after the trigger strongly limits the size of the remnant, which must be very compact. If the remnant had the observed velocity since the supernova explosion, its age would be less than a month. In this case nickel and cobalt have not yet decayed into iron. We show how to solve this paradox.

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arXiv2001

The Chandra High Resolution Spectra of Mkn 421

Mkn 421 was observed by Chandra twice, on November 5, 1999 as part of the Chandra calibration program, with the ACIS-HETG configuration, and on May 29, 2000 following our Target Of Opportunity request aimed to catch the source in an ultra-high state, with both the ACIS-HETG and the HRC-LETG configurations. In this contribution we present and compare the two Chandra-MEG observations of Mkn 421, which lasted 26 and 19.6 ks respectively.

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arXiv2001

The BeppoSAX view of the X-ray active nucleus of NGC4258

BeppoSAX observed NGC4258 on December 1998, when its 2-10 keV luminosity was \~1E41 erg/s. 100% variability is observed in the 3-10 keV band on timescales of a half a day while 20% variability is observed on timescales of one hour. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5+/-1.2)E22 cm-2; this component is detected up to 70 keV with S/N>=3 and with the steep power law energy index of 1.11+/-0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in ADAF models with strong winds, is ruled out by the data. The ratio between the nuclear radio and the X-ray luminosities is <=1E-5, similar to that of radio quiet AGN. X-ray variability and spectral shape, radio to X-ray and NIR to X-ray luminosity ratios suggest that the nucleus of NGC4258 could be a scaled-down version of a Seyfert nucleus. The soft (E<=2keV) X-ray emission is complex. There are at least two thermal-like components, with T1=0.6+/-0.1 keV and T2>=1.3 keV. The cooler (L(0.1-2.4keV)=1E40 erg/s) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI. The second component, which can be modeled equally well by an unobscured power law model, has L(0.1-2.4keV)~7E39 erg/s, consistent with that expected from discrete X-ray sources in the host galaxy. NGC4258 and other maser AGNs show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of maser emission from X-ray illumination. So a large column density gas may be a necessary property of masing AGNs.

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