955 research outputs found
Galaxy Clusters in the Swift/BAT era II: 10 more Clusters detected above 15 keV
We report on the discovery of 10 additional galaxy clusters detected in the
ongoing Swift/BAT all-sky survey. Among the newly BAT-discovered clusters there
are: Bullet, Abell 85, Norma, and PKS 0745-19. Norma is the only cluster, among
those presented here, which is resolved by BAT. For all the clusters we perform
a detailed spectral analysis using XMM-Newton and Swift/BAT data to investigate
the presence of a hard (non-thermal) X-ray excess. We find that in most cases
the clusters' emission in the 0.3-200keV band can be explained by a
multi-temperature thermal model confirming our previous results. For two
clusters (Bullet and Abell 3667) we find evidence for the presence of a hard
X-ray excess. In the case of the Bullet cluster, our analysis confirms the
presence of a non-thermal, power-law like, component with a 20-100 keV flux of
3.4 \times 10-12 erg cm-2 s-1 as detected in previous studies. For Abell 3667
the excess emission can be successfully modeled as a hot component (kT=~13keV).
We thus conclude that the hard X-ray emission from galaxy clusters (except the
Bullet) has most likely thermal origin.Comment: Accepted for publication by Ap
The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey
We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by
analysing data acquired in the first 54 months of the Swift mission. Using our
software dedicated to the analysis of data from coded mask telescopes, we
analysed the BAT survey data in three energy bands (15-30 keV, 15-70 keV,
15-150 keV), obtaining a list of 1256 detections above a significance threshold
of 4.8 standard deviations. The identification of the source counterparts is
pursued using two strategies: the analysis of field observations of soft X-ray
instruments and cross-correlation of our catalogue with source databases.The
survey covers 50% of the sky to a 15--150 keV flux limit of 1.0 x 10^-11 erg
s^-1 cm^-2 and 9.2 x 10^-12 erg s^-1 cm^-2 for |b| 10
degrees, respectively. The Second Palermo Swift-BAT hard X-ray catalogue
includes 1079 (86%) hard X-ray sources with an associated counterpart (26 with
a double association and 2 with a triple association) and 177 BAT excesses
(14%) that still lack a counterpart. The distribution of the BAT sources among
the different object classes consists of 19% Galactic sources, 57%
extragalactic sources, and 10% sources with a counterpart at softer energies
whose nature has not yet been determined. About half of the BAT associated
sources lack a counterpart in the ROSAT catalogues. This suggests that either
moderate or strong absorption may be preventing their detection in the ROSAT
energy band. The comparison of our BAT catalogue with the Fermi Large Area
Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48
blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries,
and 4 pulsars/supernova remnants. This small number of correspondences
indicates that different populations make the sky shine in these two different
energy bands
BAT X-ray Survey - III: X-ray Spectra and Statistical Properties
In this concluding part of the series of three papers dedicated to the
Swift/BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and
statistical properties of the source sample. Using a dedicated method to
extract time-averaged spectra of BAT sources we show that Galactic sources
have, generally, softer spectra than extragalactic objects and that Seyfert 2
galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert
galaxies is consistent with a power-law with photon index of 2.00 (+/-0.07).
The cumulative flux-number relation for the extragalactic sources in the 14-170
keV band is best described by a power-law with a slope alpha=1.55 (+/-0.20) and
a normalization of 9.6 AGN deg (or 396(+/-80) AGN
all-sky) above a flux level of 2erg cm s (~0.85
mCrab). The integration of the cumulative flux per unit area indicates that BAT
resolves 1-2% of the X-ray background emission in the 14-170 keV band. A
sub-sample of 24 extragalactic sources above the 4.5 sigma detection limit is
used to study the statistical properties of AGN. This sample comprises local
Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of
the Seyfert galaxies are absorbed by column densities of Log(N_H)>22, but that
none is a bona fide Compton-thick. This study shows the capabilities of BAT to
probe the hard X-ray sky to the mCrab level.Comment: Accepted for publication in The Astrophysical Journal; 42 pages, 4
tables, 51 figure
PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe
We present the results of a multifrequency campaign on the high-redshift (z =
3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006
to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based
near-IR/optical telescopes. The SEDs are compatible with a model based on
synchrotron radiation and external inverse Compton scattering. The campaign
gives an insight into the physical environment of the blazar.Comment: 8 pages, 5 figures, accepted for publication in A&
The properties of the AGN torus as revealed from a set of unbiased NuSTAR observations
The obscuration observed in active galactic nuclei (AGN) is mainly caused by
dust and gas distributed in a torus-like structure surrounding the supermassive
black hole (SMBH). However, properties of the obscuring torus of the AGN in
X-ray have not been fully investigated yet due to the lack of high-quality data
and proper models. In this work, we perform a broadband X-ray spectral analysis
of a large, unbiased sample of obscured AGN (with line-of-sight column density
23log(NH)24) in the nearby universe which has high-quality archival
NuSTAR data. The source spectra are analyzed using the recently developed
borus02 model, which enables us to accurately characterize the physical and
geometrical properties of AGN obscuring tori. We also compare our results
obtained from the unbiased Compton thin AGN with those of Compton-thick AGN. We
find that Compton thin and Compton-thick AGN may possess similar tori, whose
average column density is Compton thick (N
1.410 cm), but they are observed through different
(under-dense or over-dense) regions of the tori. We also find that the
obscuring torus medium is significantly inhomogeneous, with the torus average
column densities significantly different from their line-of-sight column
densities (for most of the sources in the sample). The average torus covering
factor of sources in our unbiased sample is c=0.67, suggesting that the
fraction of unobscured AGN is 33%. We develop a new method to measure the
intrinsic line-of-sight column density distribution of AGN in the nearby
universe, which we find the result is in good agreement with the constraints
from recent population synthesis models.Comment: 16 pages, 14 figures, 7 tables; accepted by A&
Cosmic X-ray background and Earth albedo Spectra with Swift/BAT
We use Swift/BAT Earth occultation data at different geomagnetic latitudes to
derive a sensitive measurement of the Cosmic X-ray background (CXB) and of the
Earth albedo emission in the 15--200 keV band. We compare our CXB spectrum with
recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement.
Using an independent measurement of the CXB spectrum we are able to confirm our
results. This study shows that the BAT CXB spectrum has a normalization
~8(+/-3)% larger than the HEAO-1 measurement. The BAT accurate Earth albedo
spectrum can be used to predict the level of photon background for satellites
in low Earth and mid inclination orbits.Comment: Accepted for publication in the Astrophysical Journal. 38 Pages, 16
Figures, 2 Table
Hydrogen Column Density Variability in a Sample of Local Compton-Thin AGN II
We present the multi-epoch analysis of 13 variable, nearby (z<0.1),
Compton-thin (22<logN_H<24) active galactic nuclei (AGN) selected from the
105-month BAT catalog. Analyzing all available archival soft and hard X-ray
observations, we investigate the line-of-sight hydrogen column density (N_H)
variability on timescales ranging from a few days to approximately 20 years.
Each source is analyzed by simultaneously modeling the data with three physical
torus models, providing tight constraints on torus properties, including the
covering factor, the cloud dispersion, and the torus average hydrogen column
density (N_H,av). For each epoch, we measure the N_H and categorize the source
as `N_H Variable', `Non-variable in N_H', or `Undetermined' based on the degree
of variability. Our final sample includes 27 variable, Compton-thin AGN after
implementing another 14 AGN analyzed in our previous work. We find that all
sources require either flux or N_H variability. We classify 37% of them as `N_H
Variable', 44% as `Non-variable in N_H', and 19% as `Undetermined'. Noticeably,
there is no discernible difference between geometrical and intrinsic properties
among the three variability classes, suggesting no intrinsic differences
between the N_H-variable and non-variable sources. We measure the median
variation in N_H between any observation pair of the same source to be 25% with
respect to the lowest N_H measure in the pair. Furthermore, 48% of the analyzed
sources require the inclusion of a Compton-thick reflector in the spectral
fitting. Among these, the 30% exhibits recorded 22 GHz water megamaser
emission, suggesting a potential shared nature between the two structures.Comment: Submitted to Ap
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