arXiv subjects

F. Fiore

Publications and source records attributed to F. Fiore.

At least 361 records · Page 20Linked to original sources

arXiv1999

The complex 0.1-200 keV spectrum of the Seyfert 1 Galaxy NGC4593

We report on the first observation of the Seyfert 1 galaxy NGC4593 in the 0.1-200 keV band, performed with the BeppoSAX observatory. Its spectral components are for the first time simultaneously measured: a power-law with photon spectral index ~1.9; the Compton-reflection of the primary power-law; a moderately broad (>60 eV) K-alpha fluorescent line from neutral iron; and an absorption edge, whose threshold energy is consistent with K-shell photoionization from OVII. The amount of reflection and the iron line properties are consistent with both being produced in a plane-parallel, X-ray illuminated relativistic accretion disc surrounding the nuclear black hole, seen at an inclination of 30 degrees. Any cutoff of the intrinsic continuum is constrained to lay above 150 keV. The claim for a strongly variable soft excess is dismissed by our data and by a reanalysis of archival ASCA and ROSAT data.

astro-ph↗

arXiv1999

The contribution of faint AGN to the hard X-ray background

Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as AGN, from sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage this corresponds to resolving about 30 % of the hard Cosmic X-ray Background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level respectively), thus supporting the scenario in which a significant fraction of the XRB is made by obscured AGN.

astro-ph↗

arXiv1999

The BeppoSAX X-ray view of reflection-dominated Seyfert Galaxies

We present new results from BeppoSAX observations of reflection-dominated Seyfert galaxies, and namely: 1) the Compton-thick Seyfert 2s NGC1068 and Circinus Galaxy; 2) the Seyfert 1 NGC4051, whose nucleus was observed on May 1998 to have switched off, leaving only a residual reflection component as an echo of its past activity. Our main focus in this paper is on the soft X-ray continuum properties and on the X-ray line spectroscopy.

astro-ph↗

arXiv1999

The BeppoSAX view of the hard X-ray background

First results on a medium-deep X-ray survey in the "new" 5-10 keV band carried out with the MECS detectors onboard BeppoSAX are presented. The High Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where the energy density of the X-ray background is more than twice than that in the soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant population at 5-10 keV fluxes of the order of 10e-13 cgs. We discuss the implications of these findings for the AGN synthesis models for the XRB.

astro-ph↗

arXiv1998

X-Ray Surveys

A review of recent developments in the field of X-ray surveys, especially in the hard (2-10 and 5-10 keV) bands, is given. A new detailed comparison between the measurements in the hard band and extrapolations from ROSAT counts, that takes into proper account the observed distribution of spectral slopes, is presented. Direct comparisons between deep ROSAT and BeppoSAX images show that most hard X-ray sources are also detected at soft X-ray energies. This may indicate that heavily cutoff sources, that should not be detectable in the ROSAT band but are expected in large numbers from unified AGN schemes, are in fact detected because of the emerging of either non-nuclear components, or of reflected, or partially transmitted nuclear X-rays. These soft components may complicate the estimation of the soft X-ray luminosity function and cosmological evolution of AGN.

astro-ph↗

arXiv1998

One more surprise from the Circinus Galaxy: BeppoSAX discovery of a transmission component in hard X-rays

The Circinus Galaxy has been observed, for the first time above 10 keV, by BeppoSAX. An excess emission above the extrapolation of the best fit 0.1--10 keV spectrum is apparent in the PDS data. The most likely explanation is that we are observing the nucleus through a column density of about 4x10**24 cm-2, i.e. large enough to completely block the radiation below 10 keV but small enough to permit partial transmission above this energy.

astro-ph↗

arXiv1998

BeppoSAX confirms extreme relativistic effects in the X-ray spectrum of MCG-6-30-15

We report the first simultaneous measure of the X-ray broadband (0.1--200 keV) continuum and of the iron K-alpha fluorescent line profile in the Seyfert 1 galaxy MCG-6-30-15. Our data confirms the ASCA detection of a skewed and redshifted line profile (Tanaka et al. 1995). The most straightforward explanation is that the line photons are emitted in the innermost regions of a X-ray illuminated relativistic disk. The line Equivalent Width (~200 eV) is perfectly consistent with the expected value for solar abundances, given the observed amount of Compton reflection. We report also the discovery of a cut-off in the nuclear primary emission at the energy of ~160 keV.

astro-ph↗

arXiv1998

The BeppoSAX High Energy Large Area Survey (HELLAS)

We have surveyed about 50 deg-2 of sky in the largely unexplored 5-10 keV band using the BeppoSAX MECS instrument, finding 180 sources. After correction for the non uniform sky coverage we find about 18 sources deg-2 with F(5-10keV) >= 5E-14 cgs, and resolve 30-40 % of the hard 5-10 keV Cosmic X-ray Background (XRB). Optical identification of a first small sample of sources show that most (11 out of 14) are AGN. Six of these show evidence of absorption/extinction in X-ray/optical, thus providing support to the scenario in which the hard XRB is largely made by obscured AGN (Setti & Woltjer 1989, Comastri et al. 1995).

astro-ph↗

arXiv1998

The High Energy Resolution View of Ionized Absorbers in AGN

A number of emission and absorption features are expected to be visible in high energy resolution X-ray spectra of type 1 AGN with ionized gas along the line of sight (so called "warm absorbers"). Emission strongly depends on the geometrical configuration of the gas, while absorption along the line of sight does not. Absorption features include photoelectric absorption K and L edges along with many strong K$α$, K$β$ and L resonance absorption lines from the most abundant elements. We present detailed simulations of our ``photoelectric + resonant absorption'' model with the high energy resolution gratings and calorimeters of AXAF, XMM and Constellation-X, and discuss the relevant physics which can be addressed with the new generation of X-ray spectrometers.

astro-ph↗

arXiv1998

The Equilibrium Photoionized Absorber in 3C351

We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an `ionized absorber' in its X-ray spectrum. The factor 1.7 change in flux in the $\sim$2~years between the observations allows a test of models for this ionized absorber. The absorption feature at ~0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity - and hence the ionizing flux at the absorber - is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agrement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n_e > 2 x 10^4 cm^{-3}; and the distance of the ionized gas from the central source R < 19 pc. This produces a strong test for a photoionized absorber in 3C~351: a factor 2 flux change in ~1 week in this source must show non-equilibrium effects in the ionized absorber.

astro-ph↗

arXiv1998

Ionized Absorbers in AGN: the Role of Collisional Ionization and Time-Evolving Photoionization

In this paper we explore collisional ionization and time-evolving photoionization in the, X-ray discovered, ionized absorbers in Seyfert galaxies. These absorbers show temporal changes inconsistent with simple equilibrium models. We develop a simple code to follow the temporal evolution of non-equilibrium photoionized gas. As a result several effects appear that are easily observable; and which, in fact, may explain otherwise paradoxical behavior. Specifically we find that: 1) In many important astrophysical conditions pure collisional and photoionization equilibria can be distinguished with moderate spectral resolution observations, due to a strong absorption structure between 1 and 3 keV. 2) In time-evolving non-equilibrium photoionization models the response of the ionization state of the gas to sudden changes of the ionizing continuum is smoothed and delayed at low gas densities, even when the luminosity increases. 3) If the changes of the ionizing luminosity are not instantaneous, and the electron density is low enough (the limit depends on the average ionization state of the gas), the ionization state of the gas can continue to increase while the source luminosity decreases, so a maximum in the ionization state of a given element may occur during a minimum of the ionizing intensity (the opposite of the prediction of equilibrium models). 4) Different ions of different elements reach their equilibrium configuration on different time-scales. These properties are similar to those seen in several ionized absorbers in AGN, properties which had hitherto been puzzling. We applied these models to a high S/N ROSAT PSPC observation of the Seyfert 1 galaxy NGC 4051.

astro-ph↗

arXiv1998

Signatures of Resonant Absoprtion in X-ray Spectra of Type 1 AGN

We present photoionization models accounting for both photoelectric and resonant absorption. We demonstrate that resonance absorption lines are detectable even in moderate resolution X-ray spectra of type 1 AGN. The spectra transmitted by gas illuminated by the ionizing continua typical of flat X-ray spectrum, broad optical emission line type 1 AGN, and steep X-ray spectrum, narrow optical emission line type 1 AGN (NLSy1) are dramatically different. In particolar, we find that both the absorption features seen in many Seyfert 1 galaxies between 0.7 and 1 keV and in several NLSy1 between 1 and 2 keV can be explained by our model, without requiring relativistic outflowing velocities of the gas.

astro-ph↗

arXiv1998

1H0419-577: A "two-state" soft X-ray Seyfert galaxy

In this paper we report on the first simultaneous optical and X-ray (Beppo-SAX) observations of the radio-quiet AGN 1H0419-577. The optical spectrum clearly leads us to classify this source as a Seyfert 1. The X-ray spectrum is, however, somewhat at odds with this classification: a simple flat (Gamma ~ 1.55) and featureless power--law is a good description of the whole 1.8-40 keV spectrum, even if the upper limit to a broad iron line is not very tight. An analysis of a still unpublished ROSAT observation of the same target reveals that the soft X-ray spectrum has undergone a transition from a steep (Gamma ~ 2.5) to a flat (Gamma ~ 1.55) state, at least in the 0.7-2 keV band. If this difference is due to a remarkably variable soft excess, it is unlikely that a single component is responsible for the optical/UV/soft X-ray spectral distribution. The hypothesis that the difference is due to a change in the primary X-ray continuum and its implications for current Comptonization models are discussed.

astro-ph↗

arXiv1998

The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory

BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times fainter than its historical average value, and remained steady along the whole observation (~2.3 days). The observed flat spectrum (Γ~ 0.8) and intense iron line (EW ~600 eV) are best explained assuming that the active nucleus has switched off, leaving only a residual reflection component visible.

astro-ph↗

arXiv1998

High resolution imaging of the X-ray afterglow of GRB970228 with ROSAT

We report results of a ROSAT High-Resolution Imager (HRI) observation of the X-ray error box given by the BeppoSAX Wide Field Camera for the gamma-ray burst that occurred on 1997 February 28. The observation started 10 days after the burst and ended three days later, with a total exposure of 34.3 ks. An X-ray source was detected within the 3' WFC error box and its position determined with a 10'' radius accuracy. The source position is in the BeppoSAX Narrow Field Instrument source error box and is coincident (to within 2'') with the optical transient associated with GRB970228. This is the most precise position obtained for an X-ray afterglow and confirms that the X-ray and optical afterglows have the same origin. We present the 0.1--2.4 keV combined HRI and BeppoSAX Low-Energy Concentrator Spectrometer decay light curve which can be well fit with a power-law. The decay is consistent with that measured at higher energies (2--10 keV) with the BeppoSAX Medium-Energy Concentrator Spectrometer.

astro-ph↗

arXiv1998

ASCA observations of two steep soft X-ray quasars

Steep soft X-ray (0.1-2 keV) quasars share several unusual properties: narrow Balmer lines, strong FeII emission, large and fast X-ray variability, rather steep 2-10 keV spectrum. These intriguing objects have been suggested to be the analogs of Galactic black hole candidates in the high, soft state. We present here results from ASCA observations for two of these quasars: NAB0205+024 and PG1244+026. Both objects show similar variations (factor of about 2 in 10 ks), despite a factor of about ten difference in the 0.5-10 keV luminosity (7.3E43 erg/s for PG1244+026 and 6.4E44 erg/s for NAB0205+024, assuming isotropic emission, H_0 = 50.0 and q_0 = 0.0). The X-ray continuum of the two quasars flattens by 0.5-1 going from the 0.1-2 keV band toward higher energies, strengthening recent results on another half dozen steep soft X-ray AGN. PG1244+026 shows a significant feature in the `1 keV' region, which can be described by either as a broad emission line centered at 0.95 keV (quasar frame) or as edge or line absorption at 1.17 (1.22) keV. The line emission could be due to reflection from an highly ionized accretion disk, in line with the view that steep soft X-ray quasars are emitting close to the Eddington luminosity. Photoelectric edge absorption or resonant line absorption could be produced by gas outflowing at a large velocity (0.3-0.6 c).

astro-ph↗

arXiv1998

The X-ray spectrum and variability of the Seyfert 2 Galaxy NGC7172

We present evidence of flux variability, on both short (hours) and long (months) time scales of the Seyfert 2 galaxy NGC 7172. These results are based on the ASCA observation of NGC 7172 performed on May 1996. The source was detected at a rather low flux level, about 3 times fainter than its usual state (including one year before, when it was also observed by ASCA). The source also varied by about 30 percent during the observation, confirming the presence of a type 1 nucleus in its center. However, its spectrum appears to be flatter than the typical Seyfert 1 spectrum (in agreement with findings on other Seyfert 2's), posing problems for the unification model unless complex absorption is invoked.

astro-ph↗

arXiv1998

How Abundant is Iron in the Core of the Perseus Cluster?

The analysis of Perseus data collected with the Medium Energy Concentrator Spectrometer (MECS) on board Beppo-SAX shows that the ratio of the flux of the 8 keV line complex (dominated by Fe K$_β$ emission) over the 6.8 keV line complex (dominated by Fe K$_α$ emission) is significantly larger than predicted by standard thermal emission codes. Moreover the analysis of spatially resolved spectra shows that the above ratio decreases with increasing cluster radius. We find that, amongst the various explanations we consider, the most likely requires the plasma to be optically thick for resonant scattering at the energy of the Fe K$_α$ line. We argue that if this is the case, then measures of the iron abundance made using standard thermal emission codes, that assume optically thin emission, can significantly underestimate the true iron abundance. In the case of the core of Perseus we estimate the true abundance to be $\sim$ 0.9 solar in a circular region with radius of $\sim 60$ kpc and centered on NGC 1275. Finally we speculate that similar results may hold for the core of other rich clusters.

astro-ph↗