Gamma-ray burst (GRB) afterglows have provided important clues to the nature
of these massive explosive events, providing direct information on the nearby
environment and indirect information on the central engine that powers the
burst. We report the discovery of two bright X-ray flares in GRB afterglows,
including a giant flare comparable in total energy to the burst itself, each
peaking minutes after the burst. These strong, rapid X-ray flares imply that
the central engines of the bursts have long periods of activity, with strong
internal shocks continuing for hundreds of seconds after the gamma-ray emission
has ended.Comment: 12 pages, 1 table, 2 figures. Originally submitted to Nature on
6/6/05. Declined on 6/9/05. Revised and submitted to Science on 6/14/05.
Accepted for publication in Science on 7/29/05 (this version
We examine the propagation of 2-dimensional relativistic jets through the
stellar progenitor in the collapsar model for gamma-ray bursts. In agreement
with previous studies, we find that relativistic jets are collimated by their
passage through the stellar mantle. Interaction of these jets with the star and
their own cocoons also causes mixing that sporadically decelerates the flow. We
speculate that this mixing instability is chiefly responsible for the variable
Lorentz factor needed in the internal shock model and for the complex light
curves seen in many GRBs. In all cases studied, the jet is shocked deep inside
the star following a brief period of adiabatic expansion. The jet that finally
emerges from the star thus has a moderate Lorentz factor, modulated by mixing,
and a very large internal energy. In a second series of calculations, we follow
the escape of that sort of jet. Because of the large ratio of internal to
kinetic energy in both the jet and its cocoon, the opening angle of the final
jet is significantly greater than at breakout. A small amount of material
emerges at large angles, but with a Lorentz factor still sufficiently large to
make a weak GRB. This leads us to propose a "unified model" in which a variety
of high energy transients, ranging from x-ray flashes to "classic" GRBs, may be
seen depending upon the angle at which a standard collapsar is observed. We
also speculate that the breakout of a relativistic jet and its collision with
the stellar wind will produce a brief transient with properties similar to the
class of "short-hard" GRBs. Implications of our calculations for GRB light
curves, the luminosity-variability relation, and the GRB-supernova association
are also discussed. (Abridged)Comment: 40 pages, 16 figures, To appear in vol. 586, ApJ, March 20, 200
High resolution computer simulations of two-dimensional convection using the anelastic approximation are presented. These calculations span Rayleigh numbers from 10(8)-10(12) for Prandtl number equal to unity, with the fluid density decreasing by a factor of 12 from the bottom to the top of the convection region. This range covers several decades in the "hard" turbulent regime. While many studies of this sort have been conducted for the Boussinesq approximation (i.e., no density stratification), we use the anelastic approximation with a significant density stratification in this turbulent regime. The convection is dominated by a large-scale coherent flow composed of ascending and descending superplumes. We find a power law exponent of 0.28 for the Nusselt-Rayleigh number scaling and a power law with exponent of 0.50 for the Reynolds-Rayleigh number scaling for the entire parameter space studied. These values are very similar to those determined experimentally and analytically for convection with no density stratification.
A deep all-sky imaging hard x-ray survey and wide-field monitor is needed to extend soft (ROSAT) and medium (ABRIXAS2) x-ray surveys into the 10-100 keV band (and beyond) at comparable sensitivity (∼0.05 mCrab). This would enable discovery and study of > ∼ 3000 obscured AGN, which probably dominate the hard x-ray background; detailed study of spectra and variability of accreting black holes and a census of BHs in the Galaxy; Gamma-ray bursts and associated massive star formation (PopIII) at very high redshift and Soft Gamma-ray Repeaters throughout the Local Group; and a full galactic survey for obscured supernova remnants. The Energetic Xray Imaging Survey Telescope (EXIST) is a proposed array of 8 × 1m 2 coded aperture telescopes fixed on the International Space Station (ISS) with 160 • × 40 • field of view which images the full sky each 90 min orbit. EXIST has been included in the most recent NASA Strategic Plan as a candidate mission for the next decade. An overview of the science goals and mission concept is presented.
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