9,013 research outputs found
NuSTAR observations of heavily obscured Swift/BAT AGN: constraints on the Compton-thick AGN fraction
The all-sky hard X-ray survey performed by Swift/BAT allowed the detection of
many heavily obscured Compton-thick AGN. In our previous work, we have
identified more than 50 candidate Compton-thick AGN in the local Universe,
corresponding to an observed fraction of about 7% of the total AGN population.
This number can be converted to the intrinsic Compton-thick AGN number density,
only if we know the form of the Compton-thick AGN spectrum, that is the energy
of their absorption turnover, photon-index and its cut-off energy at high
energies, as well as the strength of the reflection component on the matter
surrounding the nucleus. In order to constrain their number density, we analyse
the spectra of 19 Compton-thick AGN which have been detected with Swift/BAT and
have been subsequently observed with NuSTAR in the 3-80 keV band. We analyse
their X-ray spectra using the MYTORUS models of Murphy and Yaqoob which
properly take into account the Compton scattering effects. These are combined
with physically motivated Comptonisation models which accurately describe the
primary coronal X-ray emission. We derive absorbing column densities which are
consistent with those derived by the previous Swift/BAT analyses. We estimate
the coronal temperatures to be roughly between 25 and 80 keV corresponding to
high energy cut-offs roughly between 75 and 250 keV. We find that the majority
of our AGN lacks a strong reflection component in the 20-40 keV band placing
tighter constraints on the intrinsic Compton-thick AGN fraction. Combining
these results with our X-ray background synthesis models, we estimate a
Compton-thick AGN fraction in the local Universe of ~20 +/-3 % relative to the
type-II AGN population.Comment: Accepted for publication in Astronomy and Astrophysic
X-ray observations of highly obscured τ_(9.7 μm) > 1 sources: an efficient method for selecting Compton-thick AGN?
Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ_(9.7 μm) > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine)
in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si
absorption, with the most notable example being NGC1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey.
All these have been selected to have no PAH features (EW_(6.2 μm) 10^(42) erg s^(−1)) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L_(X)/L_(6 μm) luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN
Suzaku and SWIFT-BAT observations of a newly discovered Compton-thick AGN
Obscured AGN are fundamental to understand the history of Super Massive Black
Hole growth and their influence on galaxy formation. However, the Compton-thick
AGN (NH>1e24 cm^-2) population is basically unconstrained, with less than few
dozen confirmed Compton-thick AGN found and studied so far. A way to select
heavily obscured AGN is to compare the X-ray emission below 10 keV (which is
strongly depressed) with the emission from other bands less affected by the
absorption, i.e. the IR band. To this end, we have cross-correlated the 2XMM
catalogue with the IRAS Point Source catalogue and, by using the X-ray to
infrared flux ratio and X-ray colors, we selected a well defined sample of
Compton-thick AGN candidates at z<0.1. The aim of this work is to confirm the
nature and to study one of these local Compton-thick AGN candidates, the nearby
(z=0.029) Seyfert 2 galaxy IRAS 04507+0358, by constraining the amount of
intrinsic absorption (NH) and thus the intrinsic luminosity. To this end we
obtained deep (100 ks) Suzaku observations (AO4 call) and performed a joint fit
with SWIFT-BAT data. We analyzed XMM-Newton, Suzaku and SWIFT-BAT data and we
present here the results of this broad-band (0.4-100 keV) spectral analysis. We
found that the broad-band X-ray emission of IRAS 04507+0358 requires a large
amount of absorption (larger than 1e24 cm^-2) to be well reproduced, thus
confirming the Compton-thick nature of this source. In particular, the most
probable scenario is that of a mildly (NH (1.3-1.5)x1e24 cm^-2, L(2-10 keV)
(5-7)x1e43 erg s^-1) Compton-thick AGN.Comment: Accepted for publication in A&A; 8 pages, 7 figure
Unveiling obscured accretion in the Chandra Deep Field South
A large population of heavily obscured, Compton Thick AGNs is predicted by
models of galaxy formation, models of Cosmic X-ray Background and by the
``relic'' super-massive black-hole mass function measured from local bulges.
However, so far only a handful of Compton thick AGNs have been possibly
detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can
be recovered thanks to the reprocessing of the AGN UV emission in the infrared
by selecting sources with AGN luminosity's in the mid-infrared and faint
near-infrared and optical emission. To this purpose, we make use of deep HST,
VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the
number of Compton thick AGN in this field. We show that sources with high
24m to optical flux ratios and red colors form a distinct source
population, and that their infrared luminosity is dominated by AGN emission.
Analysis of the X-ray properties of these extreme sources shows that most of
them (80) are indeed likely to be highly obscured, Compton thick AGNs.
The number of infrared selected, Compton thick AGNs with 5.8m luminosity
higher than erg s turns out to be similar to that of X-ray
selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity
higher than erg s in the redshift bin 1.2-2.6. This ``factor
of 2'' source population is exactly what it is needed to solve the
discrepancies between model predictions and X-ray AGN selection.Comment: Revised version, to be published by The Astrophysical Journa
Infrared Excess Sources: Compton Thick QSOs, low luminosity Seyferts or starbursts?
We explore the nature of Infrared Excess sources (IRX), which are proposed as
candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}z\approx1z\approx2$ IRX population are not Compton
Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/s], possibly Compton Thin,
AGN or dusty starbursts. It is shown that the decomposition of the AGN and
starburst contribution to the mid-IR is essential for interpreting the nature
of this population, as star-formation may dominate this wavelength regime.Comment: Accepted by MNRA
Heavily obscured AGN with SIMBOL-X
By comparing an optically selected sample of narrow lines AGN with an X-ray
selected sample of AGN we have recently derived an estimate of the intrinsic
(i.e. before absorption) 2-10 keV luminosity function (XLF) of Compton Thick
AGNs. We will use this XLF to derive the number of Compton Thick AGN that will
be found in the SIMBOL-X survey(s).Comment: Talk at the Simbol-X symposium held in Paris, 2-5 December, 2008. 6
pages, 1 figure with three panel
Molecular lines as tracers of Compton-thick AGN ?
Recently, Papadopoulos et al., 2010 using sub-mm CO molecular line
observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs, have found that
exceptionally large gas column densities (N_H > 10^25 cm-2) can be present
across some of the very dense gaseous disks that are typically found in these
objects. They also proposed a diagnostic for finding such sources using CO and
HCN molecular lines. Given that such high column densities are expected to
absorb any X-ray luminous AGN, yielding Compton-thick sources, we set out
toexplore whether this can be discerned using X-ray observations. More
specifically we examine X-ray spectral observations of 14 sources in their
sample, using public Chandra observations (0.5-10 keV) for eleven sources as
well as BeppoSAX results (2-100 keV) from the literature for another three
sources. Our goal is to find candidate Compton-thick AGN and to check whether
the molecular line selection criterion is successful in selecting such systems.
X-ray spectroscopy reveals four candidate Compton-thick AGN of which half fall
within the high obscuration region in the molecular line ratio diagnostics. Of
the remaining five sources falling into the `high dust obscuration' box, one
(Mrk273) is highly obscured (N_H ~4x10^23 cm-2) while in the other four the
X-ray emission is most probably associated with star-forming processes rather
than an AGN on the basis of their X-ray and mid-infrared properties. Overall,
we argue that although this method as expected cannot recover all Compton-thick
AGN, there are no examples of X-ray luminous AGN inside that region that have
low obscuration, suggesting that this method is efficient in finding heavily
obscured AGN in dust-enshrouded star-forming galaxies. The above results bear
important implications for future joint ALMA and X-ray observations for the
detection of Compton-thick AGN.Comment: To appear in A&A Letter
Compton Thick AGN in the 70 Month Swift-BAT All-Sky Hard X-ray Survey: a Bayesian approach
The 70-month Swift/BAT catalogue provides a sensitive view of the
extragalactic X-ray sky at hard energies (>10 keV) containing about 800 Active
Galactic Nuclei. We explore its content in heavily obscured, Compton-thick AGN
by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data.
We apply a Bayesian methodology using Markov chains to estimate the exact
probability distribution of the column density for each source. We find 53
possible Compton-thick sources (with probability 3 to 100%) translating to a
~7% fraction of the AGN in our sample. We derive the first parametric
luminosity function of Compton-thick AGN. The unabsorbed luminosity function
can be represented by a double power-law with a break at in the 20-40 keV band.Comment: 13 pages, 9 figure
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