950 research outputs found
Iron K line emission in AGN: observations
Iron K-alpha lines are key diagnostics of the central regions of AGN. Their
profiles indicate that they are formed deep in the potential well of the
central black hole, where extreme broadening and red shift occur. The profiles
are most easily reproducible in an accretion disk: the lack of significant
emission blue-ward of the rest energy is difficult produce in other geometries.
In one source an apparent (and perhaps variable) absorption feature in the red
wing of the line may represent rare evidence for inflow onto the black hole.
Sample analysis has defined the mean properties, showing a strong concentration
of the emission in the central regions and face-on accretion disks, at least in
Seyfert 1 galaxies. Surprising results have been obtained from examination of
the line variability. Strong profile changes may be accounted for by changes in
the illumination pattern of the central, relativistic part of the disk. In at
least the case of MCG-6-30-15, there is evidence for emission from within 6
R_g, possibly indicating a spinning black hole. Developing an understanding of
these complex changes has the potential to reveal the geometry and kinematics
of the inner few gravitational radii around extragalactic black holes.Comment: 10 pages, 3 figures, to appear in "Proceedings of X-ray Astronomy '99
- Stellar Endpoints, AGN and the Diffuse Background". Also available via
http://lheawww.gsfc.nasa.gov/users/nandra/home.htm
A Compton Thick AGN Powering the Hyperluminous Infrared Galaxy IRAS 00182--7112
We present X-ray observations of the Hyperluminous Infrared Galaxy (HLIRG)
IRAS 00182--7112 (F00183--7111) obtained using the XMM-Newton EPIC camera. A
luminous hard X-ray source co-incident with the nucleus is revealed, along with
weaker soft X-ray emission which may be extended or offset from the hard. The
EPIC spectrum is extremely flat and shows Fe K emission with very high
equivalent width: both are typical characteristics of a buried, Compton--thick
AGN which is seen only in scattered light. Perhaps the most remarkable
characteristic of the spectrum is that the Fe K line energy is that of
He-like iron, making IRAS 00182--7112 the first hidden AGN known to be
dominated by ionized, Compton thick reflection. Taking an appropriate
bolometric correction we find that this AGN could easily dominate the FIR
energetics. The nuclear reflection spectrum is seen through a relatively cold
absorber with column density consistent with recent Spitzer observations. The
soft X-ray emission, which may be thermal in nature and associated with
star-forming activity, is seen unabsorbed. The soft X-rays and weak PAH
features both give estimates of the star formation rate
yr, insufficient to power the FIR emission and supportive of the idea
that this HLIRG is AGN-dominated.Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letter
ASCA and ROSAT observations of NGC5548: discrepant spectral indices
We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy
NGC5548 made during the ASCA Performance Verification phase. Spectral features
due to a warm absorber and reflection are clearly seen in the X-ray spectra. We
find that the continuum spectral shape differs between the ASCA and ROSAT
datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the
ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band
over which both detectors are sensitive. The spectra cannot be made consistent
by choosing a more complex model. The problem likely lies in the response curve
(estimated effective area) of one, or both, detectors. There may be important
consequences for a wide range of published results.Comment: 9 pages, accepted for publication in MNRA
The Number Density of Intermediate and High Luminosity Active Galactic Nuclei at z~2-3
We use the combination of the 2 Ms Chandra X-ray image, new J and H band
images, and the Spitzer IRAC and MIPS images of the Chandra Deep Field-North to
obtain high spectroscopic and photometric redshift completeness of high and
intermediate X-ray luminosity sources in the redshift interval z=2-3. We
measure the number densities of z=2-3 active galactic nuclei (AGNs) and
broad-line AGNs in the rest-frame 2-8 keV luminosity intervals 10^44-10^45 and
10^43-10^44 ergs/s and compare with previous lower redshift results. We confirm
a decline in the number densities of intermediate-luminosity sources at z>1. We
also measure the number density of z=2-3 AGNs in the luminosity interval
10^43-10^44.5 ergs/s and compare with previous low and high-redshift results.
Again, we find a decline in the number densities at z>1. In both cases, we can
rule out the hypothesis that the number densities remain flat to z=2-3 at above
the 5-sigma level.Comment: 8 pages, Accepted by The Astrophysical Journal (scheduled for 10 Dec
2005
Constraining the fraction of Compton-thick AGN in the Universe by modelling the diffuse X-ray background spectrum
This paper investigates what constraints can be placed on the fraction of
Compton-thick (CT) AGN in the Universe from the modeling of the spectrum of the
diffuse X-ray background (XRB). We present a model for the synthesis of the XRB
that uses as input a library of AGN X-ray spectra generated by the Monte Carlo
simulations described by Brightman & Nandra. This is essential to account for
the Compton scattering of X-ray photons in a dense medium and the impact of
that process on the spectra of obscured AGN. We identify a small number of
input parameters to the XRB synthesis code which encapsulate the minimum level
of uncertainty in reconstructing the XRB spectrum. These are the power-law
index and high energy cutoff of the intrinsic X-ray spectra of AGN, the level
of the reflection component in AGN spectra and the fraction of CT AGN in the
Universe. We then map the volume of the space allowed to these parameters by
current observations of the XRB spectrum in the range 3-100 keV. One of the
least constrained parameters is the fraction of CT AGN. Statistically
acceptable fits to the XRB spectrum at the 68% confidence level can be obtained
for CT fractions in the range 5-50%. This is because of degeneracies among
input parameters to the XRB synthesis code and uncertainties in the modeling of
AGN spectra (e.g. reflection). The most promising route for constraining the
fraction of CT AGN in the Universe is via the direct detection of those sources
in high energy (>10keV) surveys. It is shown that the observed fraction of CT
sources identified in the SWIFT/BAT survey, limits the intrinsic fraction of CT
AGN, at least at low redshift, to 10-20% (68% confidence level). We also make
predictions on the number density of CT sources that current and future X-ray
missions are expected to discover. Testing those predictions will constrain the
intrinsic fraction of CT AGN as a function of redshift.Comment: To appear in A&
X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results
We present followup observations of five hard X-ray sources from the ASCA
5-10 keV SHEEP survey, which has a limiting flux of erg
cm s. Chandra data have been obtained to improve the X-ray
positions from a few arcmin to , which allows unambiguous optical
identification. While the objects almost certainly house AGN based on their
X-ray luminosity, optical spectroscopy reveals a variety of properties. The
identifications indicate that the SHEEP survey samples the same populations as
deeper surveys which probe the origin of the X-ray background, but because the
SHEEP sources are far brighter, they are more amenable to detailed followup
work. We find a variety of classifications and properties, including a type II
QSO, a galaxy undergoing star formation, and a broad-line AGN which has a very
hard X-ray spectrum, indicating substantial absorption in the X-ray but none in
the optical. Two objects have X-ray/optical flux ratios which, were they at an
X-ray flux level typical of objects in Chandra deep surveys, would place them
in the ``optically faint'' category. They are both identified with broad line
QSOs at z. Clearly this survey - which is relatively unbiased against
obscured objects - is revealing a set of remarkable objects quite different to
the familiar classes of AGN found in previous optical and soft X-ray surveys.Comment: 5 pages, 3 figures. MNRAS, in pres
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