9 research outputs found
Observations of Seyferts by OSSE and parameters of their X-ray/gamma-ray sources
We present a summary of spectra of Seyfert galaxies observed by the OSSE
detector aboard Compton Gamma Ray Observatory. We obtain average spectra of
Seyferts of type 1 and 2, and find they are well fitted by thermal
Comptonization. We present detailed parameter ranges for the plasma temperature
and the Compton parameter in the case of spherical and slab geometries. We find
both the average and individual OSSE spectra of Seyfert 2s are significantly
harder than those of Seyfert 1s, which difference can be due to anisotropy of
Compton reflection and/or Thomson-thick absorption.Comment: ApJ, 10 Nov. 2000, in press, 13 page
Emission of Positron Annihilation Line Radiation by Clusters of Galaxies
Clusters of galaxies are enriched with positrons from jets of active galactic
nuclei (AGNs) or from the interaction of cosmic rays with the intracluster gas.
We follow the cooling of these positrons and show that their eventual
annihilation with cluster electrons yields a narrow annihilation line. Unlike
annihilation in the interstellar medium of galaxies, the line produced in
clusters is not smeared by three-photon decay of positronium, because
positronium formation is suppressed at the high (>~ 1 keV) temperature of the
cluster electrons. We show that if AGN jets are composed of e^+e^- pairs, then
the annihilation line from rich clusters within a distance of 100 Mpc might be
detectable with future space missions, such as INTEGRAL or EXIST.Comment: 39 pages, 11 figures, submitted to Ap
Accretion disk models and their X-ray reflection signatures. I. Local spectra
X-ray illumination of accretion disks is an invaluable diagnostic of the
structure of these disks because of the associated iron K emission.
Here we point out that the resulting reflected spectra depend very sensitively
on the geometry of the X-ray source, and that this fact can be efficiently used
to test these models observationally. In particular, we discuss three different
accretion disk geometries: the ``lamppost model'', accretion disks with
magnetic flares, and the model with a full corona overlying a cold thin disk.
We show that in the case of the lamppost model, unless the X-ray luminosity of
the central source is larger than that of the cold disk by a factor of 10 or
more, a significant fraction of iron in the ionized skin of the disk is in the
hydrogen and helium-like ions. Because these ions have large fluorescence
yields, the resulting reflected spectra look strongly ionized, with Equivalent
Width (EW) of the line {\em increasing} with X-ray luminosity up to the
maximum of eV. This situation contrasts to the magnetic flare model,
where the large X-ray flux near flares completely ionizes the skin of the disk
and thus the resulting spectra appear to be that from a neutral material. The
line EW in this model {\em anti-correlates} with X-ray luminosity, and becomes
arbitrarily small when is a good fraction of the Eddington luminosity.
Finally, in the full corona case, due to the additional pressure and weight of
the corona, the gas pressure (and its density) below the corona is always large
enough to make the gas very cool and effectively neutral. No highly ionized
skin forms in such a model. If the corona is Thomson thin, then EW of the line
does not depend on the accretion disk or corona luminosities for the full
corona model.Comment: submitted to ApJ April 28 2000; 9 pages of text plus 15 figure
Inferring the coronal flaring patterns in AGN from reverberation maps
The relativistically broadened iron K-alpha line at 6.4 keV observed in the
Seyfert 1 galaxy MCG-6-30-15 has provided a probe of the strong-gravity
environment near a black hole, in particular suggesting that it is rapidly
spinning. An important variable in such analyses is the geometry of the
illuminating source. We present a new technique which constrains this geometry
based on the spectral line shape, based on a model of discrete, point-like
flaring regions in the X-ray corona. We apply it to simulated reverberation
maps and give examples of successful reconstructions of complex coronal flaring
patterns. For time-averaged spectral lines the problem is highly degenerate,
and so its inversion more challenging. We quantify this degeneracy and give a
measure of the spatial accuracy of the method in this case, before checking
that it is consistent with the existing picture of MCG-6-30-15 by applying it
to recent data from XMM-Newton.Comment: Submitted to MNRAS. A version with higher-resolution figures is
available for download at http://www.mrao.cam.ac.uk/~rg200 as are colour
versions of some figure
Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni
This document is the Accepted Manuscript version of the following article: Thomas Siegert, et al, âPositron annihilation signatures associated with the outburst of the microquasar V404 Cygniâ, Nature: International Journal of Science, Vol. 531: 341-343, March 2016, DOI: https://doi.org/10.1038/nature16978. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Microquasars1, 2, 3, 4 are stellar-mass black holes accreting matter from a companion star5 and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 106 to 1010 solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares6. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electronâpositron plasma. Transient high-energy spectral features have been reported in two objects7, 8, but their positron interpretation9 remains contentious. Here we report observations of Îł-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity10. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electronâpositron plasma responsible for the bright diffuse emission of annihilation Îł-rays in the bulge region of our Galaxy11. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.Peer reviewe