77,493 research outputs found

### Iron K-alpha Emission from X-ray Reflection: Predictions for Gamma-Ray Burst Models

Recent observations of several gamma-ray burst (GRB) afterglows have shown
evidence for a large amount of X-ray line emitting material, possibly arising
from ionized iron. A significant detection of an X-ray spectral feature, such
as that found in the Chandra observation of GRB 991216, may provide important
constraints on the immediate environment of the burst and hence on progenitor
models. The large Fe K-alpha equivalent widths inferred from the X-ray
observations favor models in which the line is produced when the primary X-ray
emission from the source strikes Thomson-thick material and Compton scatters
into our line of sight. We present such reflection spectra here, computed in a
fully self-consistent manner, and discuss the range of ionization parameters
that may be relevant to different models of GRBs. We argue that the presence of
a strong hydrogen-like K-alpha line is unlikely, because Fe-XXVI photons would
be trapped resonantly and removed from the line core by Compton scattering. In
contrast, a strong narrow emission line from He-like Fe-XXV is prominent in the
model spectra. We briefly discuss how these constraints may affect the line
energy determination in GRB 991216.Comment: 8 pages, 3 figures, Ap.J. Letters accepte

### X-ray reflection in accreting stellar-mass black hole systems

The X-ray spectra of accreting stellar-mass black hole systems exhibit
spectral features due to reflection, especially broad iron K alpha emission
lines. We investigate the reflection by the accretion disc that can be expected
in the high/soft state of such a system. First, we perform a self-consistent
calculation of the reflection that results from illumination of a hot, inner
portion of the disc with its atmosphere in hydrostatic equilibrium. Then we
present reflection spectra for a range of illumination strengths and disc
temperatures under the assumption of a constant-density atmosphere. Reflection
by a hot accretion disc differs in important ways from that of a much cooler
disc, such as that expected in an active galactic nucleus.Comment: 5 pages with 9 figures, accepted for publication in MNRA

### On the interpretation of the multicolour disc model for black hole candidates

We present a critical analysis of the usual interpretation of the multicolour
disc model parameters for black hole candidates in terms of the inner radius
and temperature of the accretion disc. Using a self-consistent model for the
radiative transfer and the vertical temperature structure in a Shakura-Sunyaev
disc, we simulate the observed disc spectra, taking into account doppler
blurring and gravitational redshift, and fit them with multicolour models. We
show not only that such a model systematically underestimates the value of the
inner disc radius, but that when the accretion rate and/or the energy
dissipated in the corona are allowed to change the inner edge of the disc, as
inferred from the multicolour model, appears to move even when it is in fact
fixed at the innermost stable orbit.Comment: 4 pages including 2 figures, accepted for publication in MNRA

### Spin Effects in Long Range Gravitational Scattering

We study the gravitational scattering of massive particles with and without
spin in the effective theory of gravity at one loop level. Our focus is on long
distance effects arising from nonanalytic components of the scattering
amplitude and we show that the spin-independent and the spin-dependent long
range components exhibit a universal form. Both classical and quantum
corrections are obtained, and the definition of a proper second order potential
is discussed.Comment: 51 pages, 8 figure

### Topological properties of spaces admitting free group actions

In 1992, David Wright proved a remarkable theorem about which contractible
open manifolds are covering spaces. He showed that if a one-ended open manifold
M has pro-monomorphic fundamental group at infinity which is not pro-trivial
and is not stably Z, then M does not cover any manifold (except itself). In the
non-manifold case, Wright's method showed that when a one-ended, simply
connected, locally compact ANR X with pro-monomorphic fundamental group at
infinity admits an action of Z by covering transformations then the fundamental
group at infinity of X is (up to pro-isomorphism) an inverse sequence of
finitely generated free groups. We improve upon this latter result, by showing
that X must have a stable finitely generated free fundamental group at
infinity. Simple examples show that a free group of any finite rank is
possible. We also prove that if X (as above), admits a non-cocompact action of
Z+Z by covering transformations, then X is simply connected at infinity.
Corollary: Every finitely presented one-ended group G which contains an element
of infinite order satisfies exactly one of the following: 1) G is simply
connected at infinity; 2) G is virtually a surface group; 3) The fundamental
group at infinity of G is not pro-monomorphic. Our methods also provide a quick
new proof of Wright's open manifold theorem.Comment: Revised version with a shorter proof of the main theorem, plus
numerous small corrections. To appear in the Journal of Topology. 31 pages, 4
figure

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