46 research outputs found
Gamma-Ray Bursts Observed with the Spectrometer SPI Onboard INTEGRAL
The spectrometer SPI is one of the main detectors of ESA's INTEGRAL mission.
The instrument offers two interesting and valuable capabilities for the
detection of the prompt emission of Gamma-ray bursts. Within a field of view of
16 degrees, SPI is able to localize Gamma-ray bursts with an accuracy of 10
arcmin. The large anticoincidence shield, ACS, consisting of 512 kg of BGO
crystals, detects Gamma-ray bursts quasi omnidirectionally above ~70 keV. Burst
alerts from SPI/ACS are distributed to the interested community via the
INTEGRAL Burst Alert System. The ACS data have been implemented into the 3rd
Interplanetary Network and have proven valuable for the localization of bursts
using the triangulation method. During the first 8 months of the mission
approximately one Gamma-ray burst per month was localized within the field of
fiew of SPI and 145 Gamma-ray burst candidates were detected by the ACS from
which 40 % have been confirmed by other instruments.Comment: 4 pages, 2 figures, to appear in the Proceedings of the Conference
"30 Years of GRB Discovery", Santa Fe, New Mexico, USA, September 8-12, 200
Spectral analysis of a large sample of BeppoSAX Seyfert spectra with Comptonization models: Preliminary results
We present preliminary results of the spectral analysis of a large sample of
Seyfert galaxies observed by BeppoSAX. The only selection criterium was a
sufficiently large S/N ratio (>10) in the PDS band (12-200 keV) to allow good
detection up to the highest energy. The resulting sample is composed of 28
objects (17 Seyfert 1, 11 Seyfert 2) and 50 observations. Our main effort here
is to adopt Comptonization models to fit the different spectra on a truly broad
band basis (0.1-200 keV). We use two distinct disc-corona configurations, an
anisotropic slab and an isotropic spherical one. We discuss the distributions
of the physical parameters, like temperature and optical depth of the corona
and the reflection component, among this sample. We also discussed the
existence (or inexistence) of correlations between these parameters.Comment: 4 pages, 2 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
X-ray power law spectra in active galactic nuclei
X-ray spectra of active galactic nuclei (AGN) are usually described as power
law spectra, characterized by the spectral slope or photon index
. Here we discuss the X-ray spectral properties within the framework of
clumpy accretion flows, and estimate the power law slope as a function of the
source parameters. We expect harder spectra in massive objects than in less
massive sources, and steeper spectra in higher accretion rate systems. The
predicted values of the photon index cover the range of spectral slopes
typically observed in Seyfert galaxies and quasars. The overall trends are
consistent with observations, and may account for the positive correlation of
the photon index with Eddington ratio (and the possible anticorrelation with
black hole mass) observed in different AGN samples. Spectral properties are
also closely related to variability properties. We obtain that shorter
characteristic time scales are associated with steeper spectra. This agrees
with the observed `spectral-timing' correlation.Comment: 6 pages, 1 figure, Astronomy and Astrophysics, accepte
Restless Quasar Activity: From BeppoSAX to Chandra and XMM-Newton
We briefly review some of the progress made in the last decade in the study
of the X-ray properties of the quasar population from the luminous, local
objects observed by BeppoSAX to the large, rapidly increasing population of z>4
quasars detected by Chandra and XMM-Newton in recent years.Comment: 4 pages, 4 figures. To appear in the Proceedings of the Symposium
"The Restless High-Energy Universe", 5-8 May 2003, Amsterdam, The
Netherlands, E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers
Ed
Comptonization in the vicinity of black hole horizon
Using a Monte Carlo method, we derive spectra arising from Comptonization
taking place close to a Kerr black hole. We consider a model consisting of a
hot thermal corona Comptonizing seed photons emitted by a cold accretion disc.
We find that general relativistic effects are crucial for the emerging spectra
in models which involve significant contribution of radiation produced in the
black hole ergosphere. Due to this contribution, spectra of hard X-ray emission
produced in the vicinity of a rapidly rotating black hole strongly depend on
the inclination of the line of sight, with larger inclinations corresponding to
harder spectra. Remarkably, such anisotropy could be responsible for properties
of the X-ray spectra of Seyfert galaxies, which appear to be intrinsically
harder in type 2 objects than in type 1, as reported recently.Comment: 13 pages, 15 figures. Accepted for publication in MNRA
Investigating the central engine of Seyfert 2 galaxies with and without Polarized Broad Lines
We study the hard X-ray emission of two samples of Seyfert 2 galaxies with
and without Polarized Broad Lines (PBL). In the hard X-ray domain, absorption
effects do not significantly modify the intrinsic emission allowing us a direct
access to the central engine. The purpose of this study is to compare the
primary emission of the two Seyfert 2 subclasses in order to investigate the
nature of their central engine and to test unified models according to which
they both have a hidden Seyfert 1 nucleus. We compute the average hard X-ray
spectra of Seyfert 2 galaxies with and without PBL observed with
BeppoSAX/PDS (15-136 keV).
The two spectra have a common general behavior at first sight, but
investigating deeper we find differences in the intrinsic properties of the two
categories of Seyfert 2 galaxies. Sy 2 with polarized broad lines have physical
conditions close to those of Sy 1 galaxies whereas Sy 2 without PBL differ
substantially, suggesting that they may have a particular place in the scheme
of Seyfert galaxies.Comment: 10 pages, accepted for publication in A&
Observation of GRB 030131 with the INTEGRAL satellite
A long Gamma-Ray Burst (GRB) was detected with the instruments on board the
INTEGRAL satellite on January 31 2003. Although most of the GRB, which lasted
150 seconds, occurred during a satellite slew, the automatic software of
the INTEGRAL Burst Alert System was able to detect it in near-real time. Here
we report the results obtained with the IBIS instrument, which detected GRB
030131 in the 15 keV - 200 keV energy range, and ESO/VLT observations of its
optical transient. The burst displays a complex time profile with numerous
peaks. The peak spectrum can be described by a single power law with photon
index 1.7 and has a flux of 2 photons cm s in
the 20-200 keV energy band. The high sensitivity of IBIS has made it possible
for the first time to perform detailed time-resolved spectroscopy of a GRB with
a fluence of 7 erg cm (20-200 keV).Comment: Accepted for publication in A&A, 5 pages, 4 figures, late
Reflection in Seyfert Galaxies and the Unified Model of AGN
We present a deep study of the average hard X-ray spectra of Seyfert
galaxies. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the
165 Seyfert galaxies detected at z<0.2. Our final sample consists of 44 Seyfert
1's, 29 Seyfert 1.5's, 78 Seyfert 2's, and 14 Narrow Line Seyfert 1's. We
derived the average hard X-ray spectrum of each subsample in the 17-250keV
energy range. All classes of Seyfert galaxies show on average the same nuclear
continuum, as foreseen by the zeroth order unified model, with a cut-off energy
of Ec>200keV, and a photon index of Gamma ~1.8. Compton-thin Seyfert 2's show a
reflection component stronger than Seyfert 1's and Seyfert 1.5's. Most of this
reflection is due to mildly obscured (10^23 cm^-2 < NH < 10^24 cm^-2) Seyfert
2's, which have a significantly stronger reflection component
(R=2.2^{+4.5}_{-1.1}) than Seyfert 1's (R<=0.4), Seyfert 1.5's (R<= 0.4) and
lightly obscured (NH < 10^23 cm^-2) Seyfert 2's (R<=0.5). This cannot be
explained easily by the unified model. The absorber/reflector in mildly
obscured Seyfert 2's might cover a large fraction of the X-ray source, and have
clumps of Compton-thick material. The large reflection found in the spectrum of
mildly obscured Seyfert 2's reduces the amount of Compton-thick objects needed
to explain the peak of the cosmic X-ray background. Our results are consistent
with the fraction of Compton-thick sources being ~10%. The spectra of Seyfert
2's with and without polarized broad lines do not show significant differences,
the only difference between the two samples being the higher hard X-ray and
bolometric luminosity of Seyfert 2's with polarized broad lines. The average
hard X-ray spectrum of Narrow line Seyfert 1's is steeper than those of Seyfert
1's and Seyfert 1.5's, probably due to a lower energy of the cutoff.Comment: 19 pages, accepted for publication in Astronomy and Astrophysics,
final versio
Scientific Performance of the ISDC Quick Look Analysis
The INTEGRAL Science Data Centre (ISDC) routinely monitors the Near Real Time
data (NRT) from the INTEGRAL satellite. A first scientific analysis is made in
order to check for the detection of new, transient or highly variable sources
in the data. Of primary importance for this work is the Interactive Quick Look
Analysis (IQLA), which produces JEM-X and ISGRI images and monitors them for
interesting astrophysical eventsComment: 4 pages, 3 figures. Proceedings of 5th INTEGRAL Workshop: The
INTEGRAL Universe, Munich, 16-20 February 2004. Accepted for publication in
European Space Agency Special Publication 552. See paper for institute
affiliation