1,220 research outputs found
A comprehensive analysis of the hard X-ray spectra of bright Seyfert galaxies
Hard X-ray spectra of 28 bright Seyfert galaxies observed with INTEGRAL were
analyzed together with the X-ray spectra from XMM-Newton, Suzaku and RXTE.
These broad-band data were fitted with a model assuming a thermal
Comptonization as a primary continuum component. We tested several model
options through a fitting of the Comptonized continuum accompanied by a complex
absorption and a Compton reflection. Both the large data set used and the model
space explored allowed us to accurately determine a mean temperature kTe of the
electron plasma, the Compton parameter y and the Compton reflection strength R
for the majority of objects in the sample. Our main finding is that a vast
majority of the sample (20 objects) is characterized by kTe < 100 keV, and only
for two objects we found kTe > 200 keV. The median kTe for entire sample is
48(-14,+57) keV. The distribution of the y parameter is bimodal, with a broad
component centered at ~0.8 and a narrow peak at ~1.1. A complex, dual absorber
model improved the fit for all data sets, compared to a simple absorption
model, reducing the fitted strength of Compton reflection by a factor of about
2. Modest reflection (median R ~0.32) together with a high ratio of Comptonized
to seed photon fluxes point towards a geometry with a compact hard X-ray
emitting region well separated from the accretion disc. Our results imply that
the template Seyferts spectra used in AGN population synthesis models should be
revised.Comment: 26 pages, 12 figures, accepted for publication in MNRA
Long-term variability of AGN at hard X-rays
Variability at all observed wavelengths is a distinctive property of AGN.
Hard X-rays provide us with a view of the innermost regions of AGN, mostly
unbiased by absorption along the line of sight. Swift/BAT offers the unique
opportunity to follow, on time scales of days to years and with a regular
sampling, the 14-195 keV emission of the largest AGN sample available up to
date for this kind of investigation. We study the amplitude of the variations,
and their dependence on sub-class and on energy, for a sample of 110 radio
quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of
the AGN in the sample are found to exhibit significant variability on months to
years time scales, radio loud sources being the most variable. The amplitude of
the variations and their energy dependence are incompatible with variability
being driven at hard X-rays by changes of the absorption column density. In
general, the variations in the 14-24 and 35-100 keV bands are well correlated,
suggesting a common origin of the variability across the BAT energy band.
However, radio quiet AGN display on average 10% larger variations at 14-24 keV
than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert
galaxies with detectable spectral variability on month time scale. In addition,
sources with harder spectra are found to be more variable than softer ones.
These properties are generally consistent with a variable power law continuum,
in flux and shape, pivoting at energies >~ 50 keV, to which a constant
reflection component is superposed. When the same time scales are considered,
the timing properties of AGN at hard X-rays are comparable to those at lower
energies, with at least some of the differences possibly ascribable to
components contributing differently in the two energy domains (e.g.,
reflection, absorption).Comment: 17 pages, 11 figures, accepted for publication in A&
The Second INTEGRAL AGN Catalogue
The INTEGRAL mission provides a large data set for studying the hard X-ray
properties of AGN and allows testing of the unified scheme for AGN. We present
analysis of INTEGRAL IBIS/ISGRI, JEM-X, and OMC data for 199 AGN supposedly
detected by INTEGRAL above 20 keV. The data analysed here allow a significant
spectral extraction on 148 objects and an optical variability study of 57 AGN.
The slopes of the hard X-ray spectra of Seyfert 1 and Seyfert~2 galaxies are
found to be consistent within the uncertainties, whereas higher cut-off
energies and lower luminosities are measured for the more absorbed / type 2
AGN. The intermediate Seyfert 1.5 objects exhibit hard X-ray spectra consistent
with those of Seyfert 1. When applying a Compton reflection model, the
underlying continua appear the same in Seyfert 1 and 2 with photon index 2, and
the reflection strength is about R = 1, when assuming different inclination
angles. A significant correlation is found between the hard X-ray and optical
luminosity and the mass of the central black hole in the sense that the more
luminous objects appear to be more massive. There is also a general trend
toward the absorbed sources and type 2 AGN having lower Eddington ratios. The
black holemass appears to form a fundamental plane together with the optical
and X-ray luminosity of the form Lv being proportional to Lx^0.6 M^0.2, similar
to that found between radio luminosity Lr, Lx, and M. The unified model for
Seyfert galaxies seems to hold, showing in hard X-rays that the central engine
is the same in Seyfert 1 and 2, but seen under different inclination angles and
absorption. (Abridged)Comment: 26 pages, 16 figures, accepted for publication in A&A. Corrections by
language editor included in version
MAXI J1659-152: the shortest orbital period black-hole binary
Following the detection of a bright new X-ray source, MAXI J1659-152, a
series of observations was triggered with almost all currently flying
high-energy missions. We report here on XMM-Newton, INTEGRAL and RXTE
observations during the early phase of the X-ray outburst of this transient
black-hole candidate. We confirm the dipping nature in the X-ray light curves.
We find that the dips recur on a period of 2.4139+/-0.0005 hrs, and interpret
this as the orbital period of the system. It is thus the shortest period
black-hole X-ray binary known to date. Using the various observables, we derive
the properties of the source. The inclination of the accretion disk with
respect to the line of sight is estimated to be 60-75 degrees. The companion
star to the black hole is possibly a M5 dwarf star, with a mass and radius of
about 0.15 M_sun and 0.23 R_sun, respectively. The system is rather compact
(orbital separation is about 1.35 R_sun) and is located at a distance of
roughly 7 kpc. In quiescence, MAXI J1659-152 is expected to be optically faint,
about 28 mag in the V-band.Comment: 5 pages, 4 figures, to be published in the proceedings of the 4th
International MAXI Workshop `The First Year of MAXI: Monitoring variable
X-ray sources', 2010 Nov 30 - Dec 2, Tokyo, Japa
The High Energy Spectrum of NGC 4151
We present the first INTEGRAL observations of the type 1.5 Seyfert galaxy NGC
4151. Combining several INTEGRAL observations performed during 2003, totaling
about 400 ks of exposure time, allows us to study the spectrum in the 2 - 300
keV range. The measurements presented here reveal an overall spectrum from
X-rays up to soft gamma-rays that can be described by an absorbed (NH = 6.9e22
1/cm**2) model based on a Compton continuum from a hot electron population (kT
= 94 keV) from an optically thick (tau = 1.3) corona, reflected on cold
material (R=0.7), consistent with earlier claims. The time resolved analysis
shows little variation of the spectral parameters over the duration of the
INTEGRAL observations. The comparison with CGRO/OSSE data shows that the same
spectral model can be applied over a time span of 15 years, with flux
variations of the order of a factor of 2 and changes in the underlying
continuum reflected by the temperature of the electron population (kT = 50 -
100 keV). When modeled with an exponentially cut-off power law plus Compton
reflection this results in photon indices ranging from Gamma = 1.5 to Gamma =
1.9 and a cut-off energy in the range 100 - 500 keV.Comment: 13 pages, 7 figures, accepted for publication in Ap
Multiwavelength campaign on Mrk 509 XII. Broad band spectral analysis
(Abridged) The simultaneous UV to X-rays/gamma rays data obtained during the
multi-wavelength XMM/INTEGRAL campaign on the Seyfert 1 Mrk 509 are used in
this paper and tested against physically motivated broad band models. Each
observation has been fitted with a realistic thermal comptonisation model for
the continuum emission. Prompted by the correlation between the UV and soft
X-ray flux, we use a thermal comptonisation component for the soft X-ray
excess. The UV to X-rays/gamma-rays emission of Mrk 509 can be well fitted by
these components. The presence of a relatively hard high-energy spectrum points
to the existence of a hot (kT~100 keV), optically-thin (tau~0.5) corona
producing the primary continuum. On the contrary, the soft X-ray component
requires a warm (kT~1 keV), optically-thick (tau~15) plasma. Estimates of the
amplification ratio for this warm plasma support a configuration close to the
"theoretical" configuration of a slab corona above a passive disk. An
interesting consequence is the weak luminosity-dependence of its emission, a
possible explanation of the roughly constant spectral shape of the soft X-ray
excess seen in AGNs. The temperature (~ 3 eV) and flux of the soft-photon field
entering and cooling the warm plasma suggests that it covers the accretion disk
down to a transition radius of 10-20 . This plasma could be the
warm upper layer of the accretion disk. On the contrary the hot corona has a
more photon-starved geometry. The high temperature ( 100 eV) of the
soft-photon field entering and cooling it favors a localization of the hot
corona in the inner flow. This soft-photon field could be part of the
comptonised emission produced by the warm plasma. In this framework, the change
in the geometry (i.e. ) could explain most of the observed flux and
spectral variability.Comment: 19 pages, 14 figures. Accepted for publication in A&
Multiwavelength campaign on Mrk 509. I. Variability and spectral energy distribution
(Abridged) Active galactic nuclei show a wealth of interesting physical
processes, some of which are poorly understood. We want to address a number of
open questions, including the location and physics of the outflow from AGN, the
nature of the continuum emission, the geometry and physical state of the X-ray
broad emission line region, the Fe-K line complex, the metal abundances of the
nucleus and finally the interstellar medium of our own Galaxy. We study one of
the best targets for these aims, the Seyfert 1 galaxy Mrk 509 with a
multiwavelength campaign using five satellites (XMM-Newton, INTEGRAL, Chandra,
HST and Swift) and two ground-based facilities (WHT and PAIRITEL). Our
observations cover more than five decades in frequency, from 2 um to 200 keV.
The combination of high-resolution spectroscopy and time variability allows us
to disentangle and study the different components. Our campaign covers 100 days
from September to December 2009, and is centred on a simultaneous set of deep
XMM-Newton and INTEGRAL observations with regular time intervals, spanning
seven weeks. We obtain a continuous light curve in the X-ray and UV band,
showing a strong, up to 60% flux increase in the soft X-ray band during the
three weeks in the middle of our deepest monitoring campaign, and which is
correlated with an enhancement of the UV flux. This allows us to study the time
evolution of the continuum and the outflow. By stacking the observations, we
have also obtained one of the best X-ray and UV spectra of a Seyfert galaxy
ever obtained. In this paper we also study the effects of the spectral energy
distribution (SED) that we obtained on the photo-ionisation equilibrium. Thanks
to our broad-band coverage, uncertainties on the SED do not strongly affect the
determination of this equilibrium.Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysic
BeppoSAX view of NGC 526A: a Seyfert 1.9 galaxy with a flat spectrum
In the present work we report the BeppoSAX observation of the Seyfert 1.9
galaxy NGC 526A in the band 0.1-150 keV. The high energy instrument onboard,
PDS, has succeeded in measuring for the first time the spectrum of this source
in the 13-150 keV range. The combined analysis of all Narrow Field Instruments
provides a power law spectral index of ~ 1.6 and confirms the flat spectral
nature of this source. Although NGC 526A varies strongly in the 2-10 keV over
period of months/years, its spectral shape remains constant over these
timescales. An Fe K-alpha line, characterized by a complex structure, has been
detected in the 6-7 keV range. The line, which has an equivalent width of 120
eV, is not compatible with being produced in an absorbing torus with N_H ~
10^22 cm^-2, but most likely originates by reflection in an accretion disk
viewed at an intermediate inclination angle of ~ 42 deg. The reflection
component is however small (R < 0.7) and so it is not sufficient to steepen the
spectrum to photon index values more typical of AGNs. Instead, we find that the
data are more consistent with a flat power law spectrum cut-off at around 100
keV plus a small reflection component which could explain the observed iron
line. Thus NGC 526A is the only bona-fide Seyfert 2 galaxy which maintains a
"flat spectrum" even when broad band data are considered: in this sense its
properties, with respect to the general class of Seyfert 2's, are analogous to
those of NGC 4151 with respect to the vast majority of Seyfert 1's.Comment: 8 pages, 6 PostScript figures, Latex manuscript, new A&A file style
included, accepted for publication on Astronomy and Astrophysic
Comprehensive genetic analyses reveal evolutionary distinction of a mouse (\u3ci\u3eZapus hudsonius preblei\u3c/i\u3e) proposed for delisting from the US Endangered Species Act
Zapus hudsonius preblei, listed as threatened under the US Endangered Species Act (ESA), is one of 12 recognized subspecies of meadow jumping mice found in North America. Recent morphometric and phylogenetic comparisons among Z. h. preblei and neighbouring conspecifics questioned the taxonomic status of selected subspecies, resulting in a proposal to delist the Z. h. preblei from the ESA. We present additional analyses of the phylogeographic structure within Z. hudsonius that calls into question previously published data (and conclusions) and confirms the original taxonomic designations. A survey of 21 microsatellite DNA loci and 1380 base pairs from two mitochondrial DNA (mtDNA) regions (control region and cytochrome b) revealed that each Z. hudsonius subspecies is genetically distinct. These data do not support the null hypothesis of a homogeneous gene pool among the five subspecies found within the southwestern portion of the species’ range. The magnitude of the observed differentiation was considerable and supported by significant findings for nearly every statistical comparison made, regardless of the genome or the taxa under consideration. Structuring of nuclear multilocus genotypes and subspecies-specific mtDNA haplotypes corresponded directly with the disjunct distributions of the subspecies investigated. Given the level of correspondence between the observed genetic population structure and previously proposed taxonomic classification of subspecies (based on the geographic separation and surveys of morphological variation), we conclude that the nominal subspecies surveyed in this study do not warrant synonymy, as has been proposed for Z. h. preblei , Z. h. campestris , and Z. h. intermedius
4U 1344-60: a bright intermediate Seyfert galaxy at z=0.012 with a relativistic Fe Kalpha emission line
We present analysis of the optical and X-ray spectra of the low Galactic
latitude bright (F(2-10) = 3.6 10^{-11} erg/cm2/s) source 4U 1344-60. On the
basis of the optical data we propose to classify 4U 1344-60 as an intermediate
type Seyfert galaxy and we measure a value of z=0.012+/-0.001 for its redshift.
From the XMM-Newton observation we find that the overall X-ray spectral shape
of 4U 1344-60 is complex and can be described by a power-law continuum (Gamma ~
1.55) obscured by two neutral absorption components (Nh(f) ~ 10^{22} cm^{-2}
and Nh(p) ~ 4 10^{22} cm^{-2}), the latter covering only the ~50% of the
primary X-ray source. The X-ray data therefore lend support to our
classification of 4U 1344-60. It exhibits a broad and skewed Fe Kalpha line at
6.4 keV, which suggests the existence of an accretion disk that is able to
reprocess the primary continuum down to a few gravitational radii. Such a line
represents one of the clearest examples of a relativistic line observed by
XMM-Newton so far. Our analysis has also revealed the marginal presence of two
narrow line-like emission features at ~4.9 and ~5.2 keV.Comment: Accepted for publication in Astronomy & Astrophysic
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