112 research outputs found
Compton-thick AGN in the NuSTAR era III: A systematic study of the torus covering factor
We present the analysis of a sample of 35 candidate Compton thick (CT-)
active galactic nuclei (AGNs) selected in the nearby Universe (average redshift
~0.03) with the Swift-BAT 100-month survey. All sources have available
NuSTAR data, thus allowing us to constrain with unprecedented quality important
spectral parameters such as the obscuring torus line-of-sight column density
(N_{H, z}), the average torus column density (N_{H, tor}) and the torus
covering factor (f_c). We compare the best-fit results obtained with the widely
used MyTorus (Murphy et al. 2009) model with those of the recently published
borus02 model (Balokovic et al. 2018) used in the same geometrical
configuration of MyTorus (i.e., with f_c=0.5). We find a remarkable agreement
between the two, although with increasing dispersion in N_{H, z} moving towards
higher column densities. We then use borus02 to measure f_c. High-f_c sources
have, on average, smaller offset between N_{H, z} and N_{H, tor} than low-f_c
ones. Therefore, low f_c values can be linked to a "patchy torus" scenario,
where the AGN is seen through an over-dense region in the torus, while high-f_c
objects are more likely to be obscured by a more uniform gas distribution.
Finally, we find potential evidence of an inverse trend between f_c and the AGN
2-10 keV luminosity, i.e., sources with higher f_c values have on average lower
luminosities.Comment: 35 Pages, 23 Figures. Accepted for publication in Ap
X-ray selection of Compton Thick AGN at high redshift
Compton Thick (CT) AGN are a key ingredient of Cosmic X-ray Background (CXB)
synthesis models, but are still an elusive component of the AGN population
beyond the local Universe. Multi-wavelength surveys are the only way to find
them at z > 0.1, and a deep X-ray coverage is crucial in order to clearly
identify them among star forming galaxies. As an example, the deep and wide
COSMOS survey allowed us to select a total of 34 CT sources. This number is
computed from the 64 nominal CT candidates, each counted for its N H
probability distribution function. For each of these sources, rich
multi-wavelength information is available, and is used to confirm their
obscured nature, by comparing the expected AGN luminosity from spectral energy
distribution fitting, with the absorption-corrected X-ray luminosity. While
Chandra is more efficient, for a given exposure, in detecting CT candidates in
current surveys (by a factor ~2), deep XMM-Newton pointings of bright sources
are vital to fully characterize their properties: NH distribution above 10^25
cm^-2, reflection intensity etc., all crucial parameters of CXB models. Since
luminous CT AGN at high redshift are extremely rare, the future of CT studies
at high redshift will have to rely on the large area surveys currently
underway, such as XMM-XXL and Stripe82, and will then require dedicated
follow-up with XMM-Newton, while waiting for the advent of the ESA mission
Athena.Comment: 6 pages, 7 figures, to appear in Astronomische Nachrichten, presented
at the XMM-Newton: The Next Decade conference, ESAC, Madrid, Spain, 9 - 11
May 201
The evolution of the Compton thick fraction and the nature of obscuration for AGN in the Chandra Deep Field South
(Abridged) We present the results from the X-ray spectral analysis of high-z
AGN in the CDFS, making use of the new 4Ms data set and new X-ray spectral
models from Brightman & Nandra, which account for Compton scattering and the
geometry of the circumnuclear material. Our goals are to ascertain to what
extent the torus paradigm of local AGN is applicable at earlier epochs and to
evaluate the evolution of the Compton thick fraction (f_CT) with z, important
for XRB synthesis models and understanding the accretion history of the
universe. In addition to the torus models, we measure the fraction of scattered
nuclear light, f_scatt known to be dependant on covering factor of the
circumnuclear materal, and use this to aid in our understanding of its
geometry. We find that the covering factor of the circumnuclear material is
correlated with NH, and as such the most heavily obscured AGN are in fact also
the most geometrically buried. We come to these conclusions from the result
that f_scatt decreases as NH increases and from the prevalence of the torus
model with the smallest opening angle as best fit model in the fits to the most
obscured AGN. We find that a significant fraction of sources (~ 20%) in the
CDFS are likely to be buried in material with close to 4 pi coverage having
been best fit by the torus model with a 0\degree opening angle. Furthermore, we
find 41 CTAGN in the CDFS using the new torus models, 29 of which we report
here for the first time. We bin our sample by z in order to investigate the
evolution of f_CT. Once we have accounted for biases and incompleteness we find
a significant increase in the intrinsic f_CT, normalised to LX= 10^43.5 erg/s,
from \approx 20% in the local universe to \approx 40% at z=1-4.Comment: Accepted for publication in MNRA
On the Lx-L6micron ratio as a diagnostic for Compton-thick AGN
As the mid-IR luminosity represents a good isotropic proxy of the AGN power,
a low X-ray to mid-IR luminosity ratio is often claimed to be a reliable
indicator for selecting Compton-thick (CT) AGN. We assess the efficiency of
this diagnostic by examining the 12mu IRAS AGN sample for which high
signal-to-noise XMM observations have been recently become available. We find
that the vast majority (10/11) of the AGN that have been classified as CT on
the basis the X-ray spectroscopy by Brightman & Nandra present a low Lx/L6
luminosity ratio, i.e. lower than a few percent of the average AGN ratio which
is typical of reflection-dominated CT sources. At low Lx/L6 ratios we also find
a comparable number of AGN, most of which are heavily absorbed but not CT. This
implies that although most Compton-thick AGN present low Lx/L6 ratios, at least
in the local, Universe, the opposite is not necessarily true. Next, we extend
our analysis to higher redshifts. We perform the same analysis in the CDFS
where excellent quality chandra (4 Ms) and xmm (3 Ms) X-ray spectra are
available. We derive accurate X-ray luminosities for chandra sources using
X-ray spectral fits, as well as 6mu luminosities from SED fits. We find 8 AGN
with low Lx/L6 ratios in total, after excluding one source where the 6mu
emission primarily comes from star-formation. One of these sources has been
already demonstrated to host a CT nucleus, while for another one at a redshift
of z=1.22 we argue it is most likely CT on the basis of its combined chandra
and xmm spectrum. We find a large number of non CT contaminant with low Lx/L6
ratios. The above suggest that a low Lx/L6 ratio alone cannot ascertain the
presence of a CT AGN, albeit the majority of low Lx/L6 AGN are heavily
obscured. The two most reliable CT AGN in the high redshift Universe have high
Lx/L6 ratios, showing that this method cannot provide complete CT AGN samples.Comment: 11 pages, to appear to A&
Simultaneous X-ray and optical observations of true type 2 Seyfert galaxies
We present the results of a campaign of simultaneous X-ray and optical observations of ‘true’
type 2 Seyfert galaxies candidates, i.e. active galactic nuclei without a broad-line region (BLR).
Out of the initial sample composed of eight sources, one object, IC 1631, was found to be a
misclassified starburst galaxy, another, Q2130−431, does showbroad optical lines, while other
two, IRAS 01428−0404 and NGC 4698, are very likely absorbed by Compton-thick gas along
the line of sight. Therefore, these four sources are not unabsorbed Seyfert 2s as previously
suggested in the literature. On the other hand, we confirm that NGC 3147, NGC 3660 and
Q2131−427 belong to the class of true type 2 Seyfert galaxies, since they do not show any
evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra.
These three sources have low accretion rates (˙m = Lbol/LEdd<~0.01), in agreement with
theoretical models which predict that the BLR disappears below a critical value of Lbol/LEdd.
The last source, Mrk 273x, would represent an exception even of these accretion-dependent
versions of the Unification Models, due to its high X-ray luminosity and accretion rate, and
no evidence for obscuration. However, its optical classification as a Seyfert 2 is only based on
the absence of a broad component of Hβ, due to the lack of optical spectra encompassing the
Hα band
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.Comment: 9 pages, 7 figures. Accepted for publication in A&
XMM-Newton observations of Seyfert galaxies from the Palomar spectroscopic survey: the X-ray absorption distribution
We present XMM-Newton spectral analysis of all 38 Seyfert galaxies from the
Palomar spectroscopic sample of galaxies. These are found at distances of up to
67 Mpc and cover the absorbed 2-10 keV luminosity range ~10^38-10^43 ergs/s.
Our aim is to determine the distribution of the X-ray absorption in the local
Universe. Three of these are Compton-thick with column densities just above
10^24 cm^-2 and high equivalent width FeKa lines (>700 eV). Five more sources
have low values of the X-ray to [OIII] flux ratio suggesting that they could be
associated with obscured nuclei. Their individual spectra show neither high
absorbing columns nor flat spectral indices. However, their stacked spectrum
reveals an absorbing column density of N_H~10^23 cm^-2. Therefore the fraction
of absorbed sources (>10^22 cm^-2 could be as high as 55+/-12%. A number of
Seyfert-2 appear to host unabsorbed nuclei. These are associated with
low-luminosity sources Lx < 3x10^41 ergs/s. Their stacked spectrum again shows
no absorption while inspection of the \chandra images, where available, shows
that contamination from nearby sources does not affect the {\it XMM-Newton}
spectra in most cases. Nevertheless, such low luminosity sources are not
contributing significantly to the X-ray background flux. When we consider only
the brighter, \lunits, 21 sources, we find that the fraction of
absorbed nuclei rises to % while that of Compton-thick sources to
15-20%. The fraction of Compton-thick AGN is lower than that predicted by the
X-ray background synthesis model in the same luminosity and redshift range.}Comment: 17 pages, to appear in A&
Comparison of pupil diameter and tear production in dogs treated with acepromazine, tramadol and their combination
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