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G. C. Perola

Publications and source records attributed to G. C. Perola.

71 records · Page 4Linked to original sources
arXiv1999

X-ray spectral components from a broad band BeppoSAX observation of the Seyfert galaxy IC 4329A

From the spectral analysis of a broadband (0.1-200 keV) BeppoSAX observation of the Seyfert 1 galaxy IC 4329A, the main results obtained are: a) the amount of reflection, together with the intensity of the iron K line, indicate a geometry with a solid angle substantially less than 2pi; b) the power law is affected by a cut off with e-folding energy about 270 keV, the fourth individual object so far where this property has been firmly detected; c) two absorption features at about 0.7 and 1 keV are found, the first corresponding to a blend of O vi and O vii, the other to a combination of FeL and NeK edges. Compared to an earlier ASCA observation, when the source was 30% fainter, the values of both the relative amount of reflection and the warm absorber ionization degree are significantly lower: the comparison is suggestive of sizeable delay effects in this object, due to geometrical factors in the reflection, and to relaxation to equilibrium states in the ionization of the absorber.

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arXiv1999

BeppoSAX unveils the nuclear component in NGC 6240

The direct nuclear X-ray emission of the ultraluminous IRAS galaxy NGC 6240 has been observed for the first time by BeppoSAX. It is seen through an absorber with N_H=2*10^{24} cm^{-2}. The 2-10 keV X-ray luminosity >10^{44} erg s^{-1} definitely proves that a powerful AGN is hosted in this galaxy, despite the LINER optical appearance. Assuming the typical L_X/L_bol ratio for quasars, our result implies that it is the AGN and not the starburst that dominates the energy output of NGC 6240.

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arXiv1999

Iron line signatures in X-ray afterglows of GRB by BeppoSAX

We report the possible detection (99.3% of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material ($>0.5 \Ms$), consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase. We did not find evidence of Fe line in two other GRB with known redshift (GB971214 and GB980613), but we note that the upper limits are of the same order of the intensity measured in GB97508 and that none of these afterglows shows rebursting activity.

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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.

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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.

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arXiv1999

The X-ray afterglow of the Gamma-ray burst of May 8, 1997: spectral variability and possible evidence of an iron line

We report the possible detection (99.3% of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. The line disappeared about 1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray afterglow of this GRB. The spectrum gets harder during the flare, turning to steep when the flux decreases. The variability, intensity and width of the line indicate that the emitting region should have a mass approximately greater than 0.5 solar masses (assuming the iron abundance similar to its solar value), a size of about 3 times 10^15 cm, be distributed anisotropically, and be moving with sub-relativistic speed. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material, consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase.

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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.

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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.

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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).

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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.

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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.

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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.

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arXiv1997

A BeppoSAX observation of 3C273: broadband spectrum and detection of a low-energy absorption feature

We report the results of a 3C273 observation performed during the Science Verification Phase (SVP) of the BeppoSax satellite. The broad-band spectrum is well represented by a power-law between 1 keV and 200 keV. The spectral slope is flat (Γ~1.6) with a weak emission line at ~ 6.4 keV (rest frame) of EW ~ 30 eV. Below 1 keV, a deviation from a power-law due to an absorption feature plus a soft component is present. This is the first time that a feature in absorption at ~ 0.5 keV (observer frame) is unambiguously detected in 3C273.

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arXiv1997

Hard X--ray detection of NGC 1068 with BeppoSAX

We report on the first detection above ~10 keV of the archetypal Compton-thick Seyfert 2 galaxy NGC 1068. This detection, obtained with the PDS instrument onboard BeppoSAX in the 20-100 keV range, confirms the hardness of the X--ray spectrum above a few keV (as indicated by ASCA observations and supports models envisaging a mixture of both neutral and ionized reflections of an otherwise invisible nuclear continuum, the neutral reflection component being the dominant one in hard X-rays.

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arXiv1997

First Results from the MECS on board BeppoSAX

In this contribution we discuss briefly a few calibration items relevant to the data analysis and present some preliminary scientific results. The discussion on instrumental topics focuses on the response matrix and Point Spread Function (PSF). In the scientific results section we discuss a first analysis of the two Seyferts MCG 6-30-15 and NGC 4151 and of the Cosmic X-ray Background.

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arXiv1994

A ROSAT Hri Obsrevation of the Cooling Flow Cluster MS0839.9+2938

A ROSAT HRI observation of the cluster MS0839.9+2938 at z=0.194 is presented. It confirms the earlier suggestion, based on the detection of extended H$α$ emission, that the inner regions of this cluster are dominated by a cooling flow. The surface brightness distribution within the cooling radius shows structures which can be interpreted as evidence of a highly inhomogeneous space distribution of the cooling process. We note that the brightness at the barycentre of its distribution, which falls right on top of the central giant elliptical galaxy, is lower than the peaks in the structures around: we suggest that this situation is likely to arise as a consequence of photoelectric absorption by cold gas within the cooling flow, with an equivalent column density in the order of 5$\times$10$^{21}$ cm$^{-2}$ within $\sim$10" from the centre.

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