76,414 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
Learjet model 25 drag analysis
Procedures and data for estimating drag were used to calculate the drag characteristics of the Model 25 airplane. Based on cruise flight test data obtained on the Model 25, these methods generally predicted the total drag characteristics within current acceptable and reasonable engineering accuracy
The time-of-flight system on the Goddard medium energy gamma-ray telescope
A scintillation counter time of flight system, incorporated into the Goddard 50 cm by 50 cm spark chamber gamma ray telescope is described. The system, which utilizes constant fractions timing and particle position compensation and digitizes up to 10 ns time differences to six bit accuracy in less than 500 ns is analyzed. The performance of this system during balloon flight is discussed
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
- …