26,098 research outputs found
A Reverse-Shock Model for the Early Afterglow of GRB 050525A
The prompt localization of gamma-ray burst (GRB) 050525A by {\em Swift}
allowed the rapid follow-up of the afterglow. The observations revealed that
the optical afterglow had a major rebrightening starting at days
and ending at days, which was followed by an initial power-law
decay. Here we show that this early emission feature can be interpreted as the
reverse shock emission superposed by the forward shock emission in an
interstellar medium environment. By fitting the observed data, we further
constrain some parameters of the standard fireball-shock model: the initial
Lorentz factor of the ejecta , the magnetic energy fraction
, and the medium density . These
limits are consistent with those from the other very-early optical afterglows
observed so far. In principle, a wind environment for GRB 050525A is
disfavored.Comment: 11 pages, 1 figure, accepted for publication in Ap
Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs
For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock
crosses the ejecta, usually the shocked regions are still precipitated by the
prompt MeV \gamma-ray emission. Because of the tight overlapping of the MeV
photon flow with the shocked regions, the optical depth for the GeV photons
produced in the shocks is very large. These high energy photons are absorbed by
the MeV photon flow and generate relativistic e^\pm pairs. These pairs
re-scatter the soft X-ray photons from the forward shock as well as the prompt
\gamma-ray photons and power detectable high energy emission, significant part
of which is in the sub-GeV energy range. Since the total energy contained in
the forward shock region and the reverse shock region are comparable, the
predicted sub-GeV emission is independent on whether the GRB ejecta are
magnetized (in which case the reverse shock IC and synchrotron self-Compton
emission is suppressed). As a result, a sub-GeV flash is a generic signature
for the GRB wind model, and it should be typically detectable by the future
{\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence
neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission
powered by the interaction of the shock accelerated protons with the
synchrotron photons in both the forward and reverse shock regions, there comes
another eV neutrino emission component powered by protons interacting
with the MeV photon flow. This last component has a similar spectrum to the one
generated in the internal shock phase, but the typical energy is slightly
lower.Comment: 7 pages, accepted for publication in Ap
Strong GeV Emission Accompanying TeV Blazar H1426+428
For High frequency BL Lac objects (HBLs) like H1426+428, a significant
fraction of their TeV gamma-rays emitted are likely to be absorbed in
interactions with the diffuse IR background, yielding pairs. The
resulting pairs generate one hitherto undiscovered GeV emission by
inverse Compton scattering with the cosmic microwave background photons
(CMBPs). We study such emission by taking the 1998-2000 CAT data, the
reanalyzed 1999 & 2000 HEGRA data and the corresponding intrinsic spectra
proposed by Aharonian et al. (2003a). We numerically calculate the scattered
photon spectra for different intergalactic magnetic field (IGMF) strengths. If
the IGMF is about or weaker, there comes very strong GeV
emission, whose flux is far above the detection sensitivity of the upcoming
satellite GLAST! Considered its relatively high redshift (), the
detected GeV emission in turn provides us a valuable chance to calibrate the
poor known spectral energy distribution of the intergalactic infrared
background, or provides us some reliable constraints on the poorly known IGMF
strength.Comment: 5 pages, 1 figure. A&A in Pres
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Rheo-diecasting AZ91D magnesium alloy
Magnesium recycling has become more important in todayās environmentally aware society. To prompt the usage of recycled magnesium scrap and further improve casting structure and properties, a novel physical approach, the rheo-diecasting process (RDC), has been applied in the present study to deal with AZ91D Mg-alloy directly from die-casting scrap. The experimental results show that the RDC process can be used to produce recycled AZ91D alloy with fine and uniform microstructure and a very low level of porosity. The intermetallic compounds containing the impurity elements were of fine and of spherical morphology, distributed uniformly in the alloy matrix. No oxide particle clusters or oxide films were found in the RDC microstructure. The tensile properties of the recycled AZ91D alloy were comparable to those produced by RDC from the primary alloy ingots., and much better than those produced by conventional High Pressure Die Casting (HPDC)
Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts
The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be
due to shocks sweeping up the circum-stellar medium. However, most GRBs have
been found in dense star-forming regions where a significant fraction of the
prompt X-ray emission can be scattered by dust grains. Here we revisit the
behavior of dust scattering of X-rays in GRBs. We find that the features of
some X-ray afterglows from minutes to days after the gamma-ray triggers are
consistent with the scattering of prompt X-ray emission from GRBs off host dust
grains. This implies that some of the observed X-ray afterglows (especially
those without sharp rising and decaying flares) could be understood with a
dust-scattering--driven emission model.Comment: ApJ, in pres
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Rheo-processing of an alloy specifically designed for semi-solid metal processing on the Al-Mg-Si system
Semi-solid metal (SSM) processing is a promising technology for forming alloys and composites to near-net shaped products. Alloys currently used for SSM processing are mainly conventional aluminium cast alloys. This is an obstacle to the realisation of full potential of SSM processing, since these alloys were originally designed for liquid state processing and not for semi-solid state processing. Therefore, there is a significant need for designing new alloys specifically for semi-solid state processing to fulfil its potential. In this study, thermodynamic calculations have been carried out to design alloys based on the Al-Mg-Si system for SSM processing via the āRheo-routeā. The suitability of a selected alloy composition has been assessed in terms of the criteria considered by the thermodynamic design process, mechanical properties and heat treatability. The newly designed alloy showed good processability with rheo-processing in terms of good control of solid fraction during processing and a reasonably large processing window. The mechanical property variation was very small and the alloy showed good potential for age hardening by T5 temper heat treatment after rheo-processing
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Microstructural characterisation and thermal stability of an Mg-Al-Sr alloy prepared by rheo-diecasting
A commercial Mg-6Al-2Sr (AJ62) alloy has been prepared by a semisolid rheo-diecasting (RDC) process. The microstructure of the RDC alloy exhibits typical semisolid solidification features, i.e., 8.4 vol% primary Ī±-Mg globules (23 Ī¼m in diameter), formed in the slurry maker at the primary solidification stage, uniformly distributed in the matrix of fine Ī±-Mg grain size (8.2 Ī¼m) and intergranular eutectic Al4Sr lamellae, which resulted from secondary solidification inside the die. A ternary Mg-Al-Sr phase was also observed. Heat treatment revealed the extreme thermal stability of the RDC AJ62 alloy. The hardness showed little change up to 12 hours at 450Ā°C, whilst the Al4Sr eutectic lamellae were broken up, spheroidised and coarsened during the annealing. The RDC alloy offers superior mechanical properties, especially ductility, over the same alloy produced by high pressure die-casting
Advanced operator-splitting-based semi-implicit spectral method to solve the binary phase-field crystal equations with variable coefficients
We present an efficient method to solve numerically the equations of dissipative dynamics of the binary phase-field crystal model proposed by Elder et al. [Phys. Rev. B 75, 064107 (2007)] characterized by variable coefficients. Using the operator splitting method, the problem has been decomposed into sub-problems that can be solved more efficiently. A combination of non-trivial splitting with spectral semi-implicit solution leads to sets of algebraic equations of diagonal matrix form. Extensive testing of the method has been carried out to find the optimum balance among errors associated with time integration, spatial discretization, and splitting. We show that our method speeds up the computations by orders of magnitude relative to the conventional explicit finite difference scheme, while the costs of the pointwise implicit solution per timestep remains low. Also we show that due to its numerical dissipation, finite differencing can not compete with spectral differencing in terms of accuracy. In addition, we demonstrate that our method can efficiently be parallelized for distributed memory systems, where an excellent scalability with the number of CPUs is observed
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