23,822 research outputs found
The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s
We have obtained the average 1--500 keV spectrum of radio-quiet Seyfert 1s
using data from EXOSAT, Ginga, HEAO, and GRO/OSSE. The spectral fit to the
combined average EXOSAT and OSSE data is fully consistent with that for Ginga
and OSSE, confirming results from an earlier Ginga/OSSE sample. The average
spectrum is well-fitted by a power-law X-ray continuum with an energy spectral
index of moderately absorbed by an ionized medium and with
a Compton reflection component. A high-energy cutoff (or a break) in the the
power-law component at a few hundred keV or more is required by the data. We
also show that the corresponding average spectrum from HEAO A1 and A4 is fully
compatible with that obtained from EXOSAT, Ginga and OSSE. These results
confirm that the apparent discrepancy between the results of Ginga (with
) and the previous results of EXOSAT and HEAO (with ) is indeed due to ionized absorption and Compton reflection first
taken into account for Ginga but not for the previous missions. Also, our
results confirm that the Seyfert-1 spectra are on average cut off in gamma-rays
at energies of at least a few hundred keV, not at keV (as suggested
earlier by OSSE data alone). The average spectrum is compatible with emission
from either an optically-thin relativistic thermal plasma in a disk corona, or
with a nonthermal plasma with a power-law injection of relativistic electrons.Comment: 7 pages, 3 Postscript figures, MNRAS accepte
Distinguishing fractional and white noise in one and two dimensions
We discuss the link between uncorrelated noise and Hurst exponent for one and
two-dimensional interfaces. We show that long range correlations cannot be
observed using one-dimensional cuts through two-dimensional self-affine
surfaces whose height distributions are characterized by a Hurst exponent lower
than -1/2. In this domain, fractional and white noise are not distinguishable.
A method analysing the correlations in two dimensions is necessary. For Hurst
exponents larger than -1/2, a crossover regime leads to a systematic over
estimate of the Hurst exponent.Comment: 3 pages RevTeX, 4 Postscript figure
Nature of eclipsing pulsars
We present a model for pulsar radio eclipses in some binary systems, and test
this model for PSRs B1957+20 and J2051-0827. We suggest that in these binaries
the companion stars are degenerate dwarfs with strong surface magnetic fields.
The magnetospheres of these stars are permanently infused by the relativistic
particles of the pulsar wind. We argue that the radio waves emitted by the
pulsar split into the eigenmodes of the electron-positron plasma as they enter
the companion's magnetosphere and are then strongly damped due to cyclotron
resonance with the ambient plasma particles. Our model explains in a natural
way the anomalous duration and behavior of radio eclipses observed in such
systems. In particular, it provides stable, continuous, and frequency-dependent
eclipses, in agreement with the observations. We predict a significant
variation of linear polarization both at eclipse ingress and egress. In this
paper we also suggest several possible mechanisms of generation of the optical
and -ray emission observed from these binary systems.Comment: 12 pages, 5 figures, submitted to Ap
Spherical Formulation for Diagramatic Evaluations on a Manifold with Boundary
The mathematical formalism necessary for the diagramatic evaluation of
quantum corrections to a conformally invariant field theory for a
self-interacting scalar field on a curved manifold with boundary is considered.
The evaluation of quantum corrections to the effective action past one-loop
necessitates diagramatic techniques. Diagramatic evaluations and higher
loop-order renormalisation can be best accomplished on a Riemannian manifold of
constant curvature accommodating a boundary of constant extrinsic curvature. In
such a context the stated evaluations can be accomplished through a consistent
interpretation of the Feynman rules within the spherical formulation of the
theory for which the method of images allows. To this effect, the mathematical
consequences of such an interpretation are analyzed and the spherical
formulation of the Feynman rules on the bounded manifold is, as a result,
developed.Comment: 12 pages, references added. To appear in Classical and Quantum
Gravit
Perturbative Evaluation of Interacting Scalar Fields on a Curved Manifold with Boundary
The effects of quantum corrections to a conformally invariant scalar field
theory on a curved manifold of positive constant curvature with boundary are
considered in the context of a renormalisation procedure. The renormalisation
of the theory to second order in the scalar self-coupling pursued herein
involves explicit calculations of up to third loop-order and reveals that, in
addition to the renormalisation of the scalar self-coupling and scalar field,
the removal of all divergences necessitates the introduction of conformally
non-invariant counterterms proportional to and in the
bare scalar action as well as counterterms proportional to , and
in the gravitational action. The substantial backreaction effects and
their relevance to the renormalisation procedure are analysed.Comment: 25 pages, 1 figure. Minor elucidations in the Appendix regarding the
cut-off and in p.4 regarding the gravitational action. Certain
reference-related ommission corrected. To appear in Classical and Quantum
Gravit
Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer
We report the detection with the Rossi X-ray Timing Explorer of a Compton
reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the
Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement
provides the first independent confirmation of ASCA detections in Seyfert
galaxies of broad Fe K-alpha lines that are thought to be the signature of
emission from the inner regions of an accretion disk orbiting a black hole.
Under the assumption that reflection arises from an isotropic source located
above a neutral accretion disk, and using a theoretical model that accounts for
the dependence of the reflected spectrum on inclination angle, we derive a 90%
confidence range for the disk inclination of i = 50 to 81 degrees. The large
inclination is consistent with that expected from the unified model for
MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the
high-energy cutoff in the incident spectrum lies at energies larger than a few
hundred keV, then the equivalent width of the Fe K-alpha line is much larger
than predicted for the amount of reflection. This implies either an enhanced
iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in
the incident spectrum at energies between ~60 and ~200 keV.Comment: Accepted for publication in ApJ, LaTeX. 14 pages including 3 figures,
with 1 table as a separate postscript file. Typo corrected in abstrac
Are the Nuclei of Seyfert 2 Galaxies Viewed Face-On?
We show from modeling the Fe Kalpha line in the ASCA spectra of four X-ray
bright narrow emission line galaxies (Seyfert types 1.9 and 2) that two equally
viable physical models can describe the observed line profile. The first is
discussed by Turner et al. (1998) and consists of emission from a nearly
pole-on accretion disk. The second, which is statistically preferred, is a
superposition of emission from an accretion disk viewed at an intermediate
inclination of about 48 degrees and a distinct, unresolved feature that
presumably originates some distance from the galaxy nucleus. The intermediate
inclination is entirely consistent with unified schemes and our findings
challenge recent assertions that Seyfert 2 galaxies are preferentially viewed
with their inner regions face-on. We derive mean equivalent widths for the
narrow and disk lines of =60 eV and = 213 eV, respectively. The
X-ray data are well described by a geometry in which our view of the active
nucleus intersects and is blocked by the outer edges of the obscuring torus,
and therefore do not require severe misalignments between the accretion disk
and the torus.Comment: 19 pages, 3 postscript figures. Accepted for publication in ApJ
The lack of variability of the iron line in MCG-6-30-15: general relativistic effects
The spectrum and variability of the Seyfert galaxy MCG-6-30-15 can be
decomposed into two apparently disconnected components: a highly variable power
law and an almost constant component which contains a broad and strong iron
line. We explore a possible explanation of the puzzling lack of variability of
the iron line, by assuming that the variations of the power law component are
due to changes in the height of the primary source in the near vicinity of a
rotating black hole. Due to the bending of light in the strong field of the
central black hole, the apparent brightness of the power-law component can vary
by about a factor 4 according to its position, while the total iron line flux
variability is less than 20 percent. This behaviour is obtained if the primary
source is located within 3-4 gravitational radii () from the
rotation axis with a variable height of between 3 and 8 .
These results revive the possibility that future X-ray observations of
MCG-6-30-15 can map out the strong gravity regime of accreting black holes.Comment: accepted for publication in MNRAS Letter
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