812 research outputs found
The Distribution and Cosmic Density of Relativistic Iron Lines in Active Galactic Nuclei
X-ray observations of several active galactic nuclei show prominent iron
K-shell fluorescence lines that are sculpted due to special and general
relativistic effects. These observations are important because they probe the
space-time geometry close to distant black holes. However, the intrinsic
distribution of Fe line strengths in the cosmos has never been determined. This
uncertainty has contributed to the controversy surrounding the relativistic
interpretation of the emission feature. Now, by making use of the latest
multi-wavelength data, we show theoretical predictions of the cosmic density of
relativistic Fe lines as a function of their equivalent width and line flux. We
are able to show unequivocally that the most common relativistic iron lines in
the universe will be produced by neutral iron fluorescence in Seyfert galaxies
and have equivalent widths < 100 eV. Thus, the handful of very intense lines
that have been discovered are just the bright end of a distribution of line
strengths. In addition to validating the current observations, the predicted
distributions can be used for planning future surveys of relativistic Fe lines.
Finally, the predicted sky density of equivalent widths indicate that the X-ray
source in AGNs can not, on average, lie on the axis of the black hole.Comment: 12 pages, 3 figures, accepted by ApJ Letter
ASCA observations of type-2 Seyfert galaxies: II. The Importance of X-ray Scattering and Reflection
We discuss the importance of X-ray scattering and Compton reflection in
type-2 Seyfert galaxies, based upon the analysis of ASCA observations of 25
such sources. Consideration of the iron Kalpha, [O III] line and X-ray
variability suggest that NGC 1068, NGC 4945, NGC 2992, Mrk 3, Mrk 463E and Mrk
273 are dominated by reprocessed X-rays. We examine the properties of these
sources in more detail.
We find that the iron Kalpha complex contains significant contributions from
neutral and high-ionization species of iron. Compton reflection, hot gas and
starburst emission all appear to make significant contributions to the observed
X-ray spectra.
Mrk 3 is the only source in this subsample which does not have a significant
starburst contamination. The ASCA spectrum below 3 keV is dominated by hot
scattering gas with U_X ~ 5, N_H ~ 4 x 10^23 cm^-2. This material is more
highly ionized than the zone of material comprising the warm absorber seen in
Seyfert~1 galaxies, but may contain a contribution from shock-heated gas
associated with the jet. Estimates of the X-ray scattering fraction cover 0.25
- 5%. The spectrum above 3 keV appears to be dominated by a Compton reflection
component although there is evidence that the primary continuum component
becomes visible close to 10 keV.Comment: 27 pages, 6 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/~george/papers/gnt_s2p2/abstract.htm
High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres
We show that Compton scattering of soft photons with energies near 100 eV in
thermally stratified black-hole accretion plasmas with temperatures in the
range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV.
This could produce the hardening observed in the X-ray spectra of black holes,
which is generally attributed to reflection or partial covering of the incident
continuum source by cold optically thick matter. In addition, we show that the
presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting
high energy tails similar to those observed from Galactic black-hole
candidates.Comment: 11 pages, uuencoded gziped postscript, ApJ Letters in pres
X-ray Line Emitting Objects in XMM-Newton Observations: the Tip of the Iceberg
We present preliminary results from a novel search for X-ray Line Emitting
Objects (XLEOs) in XMM-Newton images. Three sources have been detected in a
test-run analysis of 13 XMM-Newton observations. The three objects found are
most likely extremely absorbed AGN characterized by a column density
NH~10^24cm^-2. Their redshift has been directly determined from the X-ray data,
by interpreting the detected emission line as the 6.4 keV Fe line. The measured
equivalent width of the X-ray line is, in all three cases, several keV. This
pilot study demonstrates the success of our search method and implies that a
large sample of XLEOs can be obtained from the public XMM-Newton data archive.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
X-ray and optical variability of Seyfert 1 galaxies as observed with XMM-Newton
We have examined simultaneous X-ray and optical light curves of a sample of
eight nearby Seyfert 1 galaxies observed using the EPIC X-ray cameras and
Optical Monitor on board XMM. The observations span ~1 day and revealed optical
variability in four of the eight objects studied. In all cases, the X-ray
variability amplitude exceeded that of the optical both in fractional and
absolute luminosity terms. No clearly significant correlations were detected
between wavebands using cross correlation analysis. We conclude that, in three
of the four objects in which optical variability was detected, reprocessing
mechanisms between wavebands do not dominate either the optical or X-ray
variability on the time-scales probed.Comment: 9 pages, 2 figures, accepted for publication in MNRA
Fe K\alpha emission from photoionized slabs: the impact of the iron abundance
Iron K\alpha emission from photoionized and optically thick material is
observed in a variety of astrophysical environments including X-ray binaries,
active galactic nuclei, and possibly gamma-ray bursts. This paper presents
calculations showing how the equivalent width (EW) of the Fe K line depends on
the iron abundance of the illuminated gas and its ionization state -- two
variables subject to significant cosmic scatter. Reflection spectra from a
constant density slab which is illuminated with a power-law spectrum with
photon-index \Gamma are computed using the code of Ross & Fabian. When the Fe K
EW is measured from the reflection spectra alone, we find that it can reach
values greater than 6 keV if the Fe abundance is about 10 times solar and the
illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is
ionized. In contrast, when the EW is measured from the incident+reflected
spectrum, the largest EWs are ~800 keV and are found when the gas is ionized.
When \Gamma is increased, the Fe K line generally weakens, but significant
emission can persist to larger ionization parameters. The iron abundance has
its greatest impact on the EW when it is less than 5 times solar. When the
abundance is further increased, the line strengthens only marginally.
Therefore, we conclude that Fe K lines with EWs much greater than 800 eV are
unlikely to be produced by gas with a supersolar Fe abundance. These results
should be useful in interpreting Fe K emission whenever it arises from
optically thick fluorescence.Comment: 5 pages, 5 figures, accepted by MNRAS Letter
X-ray reflection spectra from ionized slabs
X-ray reflection spectra are an important component in the X-ray spectra of
many active galactic nuclei and Galactic black hole candidates. It is likely
that reflection takes place from highly ionized surfaces of the accretion disc
in some cases. This can lead to strong Comptonization of the emergent iron, and
other, absorption and emission features. We present such reflection spectra
here, computed in a self-consistent manner with the method described by Ross
and Fabian. In particular we emphasise the range where the ionization parameter
(the flux to density ratio) \xi is around and above 10^4. Such spectra may be
relevant to the observed spectral features found in black hole candidates such
as Cygnus X-1 in the low/hard state.Comment: 7 pages with 5 postscript figures. Accepted for publication in MNRA
X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation Mapping
One of the principal scientific objectives of the upcoming Constellation-X
mission is to attempt to map the inner regions of accretion disks around black
holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence
line. This area of the disk is likely radiation pressure dominated and subject
to various dynamical instabilities. Here, we show that density inhomogeneities
in the disk atmosphere resulting from the photon bubble instability (PBI) can
cause rapid changes in the X-ray reflection features, even when the
illuminating flux is constant. Using a simulation of the development of the
PBI, we find that, for the disk parameters chosen, the Fe K and O VIII Ly\alpha
lines vary on timescales as short as a few hundredths of an orbital time. In
response to the changes in accretion disk structure, the Fe K equivalent width
(EW) shows variations as large as ~100 eV. The magnitude and direction
(positive or negative) of the changes depends on the ionization state of the
atmosphere. The largest changes are found when the disk is moderately ionized.
The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid,
low-amplitude changes. This effect provides a natural explanation for some
observed instances of short timescale Fe K variability which was uncorrelated
with the continuum (e.g., Mrk 841). New predictions for Fe K reverberation
mapping should be made which include the effects of this accretion disk driven
line variability and a variable ionization state. Reflection spectra averaged
over the evolution of the instability are well fit by constant density models
in the 2-10 keV region.Comment: 20 pages, 3 figures. Accepted by Ap
Do stellar-mass and super-massive black holes have similar dining habits?
Through the years numerous attempts have been made to connect the
phenomenology and physics of mass accretion onto stellar-mass and super-massive
black holes in a scale-invariant fashion. In this paper, we explore this
connection at the radiatively-efficient (and non-jetted) end of accretion modes
by comparing the relationship between the luminosity of the accretion disk and
corona in the two source classes. We analyse 458 RXTE-PCA archival observations
of the X-ray binary (XRB) GX339-4 focusing on the soft and soft-intermediate
states, which have been suggested to be analogous to radiatively efficient AGN.
The observed scatter in the relationship of
GX339-4 is high (dex) and significantly larger than in a
representative sample of radiatively-efficient, non- or weakly-jetted AGN
(dex). On the face of it, this would appear contrary to the
hypothesis that the systems simply scale with mass. On the other hand we also
find that GX339-4 and our AGN sample show different and
distributions, with the latter being broader in GX339-4 (dispersion of
cf. for AGN). GX339-4 also shows an overall softer slope,
with mean as opposed to for the AGN sample. Remarkably,
once similarly broad and distributions are selected, the AGN
sample overlaps nicely with GX339-4 observations in the mass-normalised plane, with a scatter of dex. This
indicates that a mass-scaling of properties might hold after all, with our
results being consistent with the disk-corona systems in AGN and XRBs
exhibiting the same physical processes, albeit under different conditions for
instance in terms of temperature, optical depth and/or electron energy
distribution in the corona, heating-cooling balance, coronal geometry and/or
black hole spin.Comment: Accepted for publication in Astronomy & Astrophysic
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