1,326 research outputs found
Analysis of X-ray spectral variability and black hole mass determination of the NLS1 galaxy Mrk 766
We present an XMM-Newton time-resolved spectral analysis of the NLS1 galaxy
Mrk 766. We analyse eight available observations of the EPIC-pn camera taken
between May 2000 and June 2005 to investigate the X-ray spectral variability as
produced by changes in the mass accretion rate. The 0.2-10 keV spectra are
extracted in time bins longer than 3 ks to accurately trace the variations of
the best fit parameters of our adopted Comptonisation spectral model. We test a
bulk-motion Comptonisation (BMC) model which is in general applicable to any
physical system powered by accretion onto a compact object, and assumes that
soft seed photons are efficiently up-scattered via inverse Compton scattering
in a hot and dense electron corona. The Comptonised spectrum has a
characteristic power-law shape, whose slope was found to increase for large
values of the normalisation of the seed component, that is proportional to the
mass accretion rate (in Eddington units). Our baseline spectral model also
includes a warm absorber lying on the line of sight and radiation reprocessing
from the accretion disk or from outflowing matter in proximity of the central
compact object. Our study reveals that the normalisation-slope correlation,
observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766:
variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to
be related to the variations of m-dot, as observed in X-ray binary systems. We
finally applied a scaling technique based on the observed correlation to
estimate the BH mass in Mrk 766. This technique is commonly and successfully
applied to measure masses of GBHs, and this is the first time it is applied in
detail to estimate the BH mass in an AGN. We obtain a value of
M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with
that estimated by the reverberation mappingComment: 26 pages, 7 figures, 4 tables to be published in Astronomy and
Astrophysic
Compton-thick AGN and the Synthesis of the Cosmic X-ray Background: the Suzaku Perspective
We discuss the abundance of Compton-thick AGN as estimated by the most recent
population synthesis models of the cosmic X-ray background. Only a small
fraction of these elusive objects have been detected so far, in line with the
model expectations. The advances expected by the broad band detectors on board
Suzaku are briefly reviewed.Comment: proceedings of "The Extreme Universe in the Suzaku Era", Kyoto 4-8
December 2006, to be published in Progress of Theoretical Physics, Supplemen
The contribution of AGN to the X-ray background: the effect of iron features
The contribution of the iron emission line, commonly detected in the X-ray
spectra of Seyfert (Sey) galaxies, to the cosmic X-ray background (XRB)
spectrum is evaluated in the framework of the XRB synthesis models based on AGN
unification schemes. To derive the mean line properties, we have carried out a
search in the literature covering a sample of about 70 AGN. When adopting line
parameters in agreement with the observations, it turns out that the maximum
contribution of the iron line to the XRB is less than 7% at a few keV. This is
still below the present uncertainties in the XRB spectrum measurements.Comment: 21 LaTeX pages with 5 Postscript figures. Accepted for publication in
New Astronom
The evolution of obscured accretion
Our current understanding of the evolution of obscured accretion onto
supermassive black holes is reviewed. We consider the literature results on the
relation between the fraction of moderately obscured, Compton-thin AGN and
redshift, and discuss the biases which possibly affect the various
measurements. Then, we discuss a number of methods - from ultradeep X-ray
observations to the detection of high-ionization optical emission lines - to
select the population of the most heavily obscured, Compton-thick AGN, whose
cosmological evolution is basically unknown. The space density of heavily
obscured AGN measured through different techniques is discussed and compared
with the predictions by current synthesis models of the X-ray background.
Preliminary results from the first half of the 3 Ms XMM observation of the
Chandra Deep Field South (CDFS) are also presented. The prospects for
population studies of heavily obscured AGN with future planned or proposed
X-ray missions are finally discussed.Comment: 6 pages, 2 figures. Invited talk at the conference "X-ray Astronomy
2009: Present status, multiwavelength approach and future perspectives",
September 2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri,
M. Cappi, L. Angelini)
The dust content of QSO hosts at high redshift
Infrared observations of high-z quasar (QSO) hosts indicate the presence of
large masses of dust in the early universe. When combined with other
observables, such as neutral gas masses and star formation rates, the dust
content of z~6 QSO hosts may help constraining their star formation history. We
have collected a database of 58 sources from the literature discovered by
various surveys and observed in the FIR. We have interpreted the available data
by means of chemical evolution models for forming proto-spheroids,
investigating the role of the major parameters regulating star formation and
dust production. For a few systems, given the derived small dynamical masses,
the observed dust content can be explained only assuming a top-heavy initial
mass function, an enhanced star formation efficiency and an increased rate of
dust accretion. However, the possibility that, for some systems, the dynamical
mass has been underestimated cannot be excluded. If this were the case, the
dust mass can be accounted for by standard model assumptions. We provide
predictions regarding the abundance of the descendants of QSO hosts; albeit
rare, such systems should be present and detectable by future deep surveys such
as Euclid already at z>4.Comment: 22 pages, 8 figures, MNRAS, accepte
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