27,367 research outputs found
A two component jet model for the X-ray afterglow flat segment in short GRB 051221A
In the double neutron star merger or neutron star-black hole merger model for
short GRBs, the outflow launched might be mildly magnetized and neutron rich.
The magnetized neutron-rich outflow will be accelerated by the magnetic and
thermal pressure and may form a two component jet finally, as suggested by
Vlahakis, Peng & K\"{o}nigl (2003). We show in this work that such a two
component jet model could well reproduce the multi-wavelength afterglow
lightcurves, in particular the X-ray flat segment, of short GRB 051221A. In
this model, the central engine need not to be active much longer than the
prompt ray emission.Comment: 11 pages, 2 figure; Accepted for publication by ApJ
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
A super-ductile alloy for the die-casting of aluminium automotive body structural components
Super-ductile die-cast aluminium alloys are critical to future light-weighting of automotive body structures. This paper introduces a die-cast aluminium alloy that can satisfy the requirements of these applications. After a review of currently available alloys, the requirement of a die-cast aluminium alloy for automotive body structural parts is proposed and an Al-Mg-Si system is suggested. The effect of the alloying elements, in the composition, has been investigated on the microstructure and mechanical properties, in particular the yield strength, the ultimate tensile strength and elongation. © (2014) Trans Tech Publications, Switzerland.The EPSRC and JLR U
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Development of a super ductile diecast Al-Mg-Si alloy
This is the post-print version of the final paper published in Materials Science and Engineering A. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2012 Elsevier B.V.The super ductile diecast aluminium alloys have been developed particularly for application in automotive body structure. On the basis of the reviewing aluminium alloys currently available, the requirement of diecast aluminium alloys is summarized and the Al-Mg-Si system is focused in the development. The effect of various alloying elements on the microstructure and the mechanical properties, such as yield strength, ultimate tensile strength and elongation is assessed. The optimized composition of the super ductile Al-Mg-Si alloy has been found to be at 5.0-5.5wt% Mg, 1.5-2.0wt% Si, 0.5-0.7wt% Mn, 0.15-0.2wt% Ti with Fe <0.25wt% for the best combination of strength and ductility, which shows 150MPa of yield strength, 300MPa of ultimate tensile strength, and 15% of elongation under as-cast condition. The paint baking hardenability of the optimized alloy is found to be insignificant. Less than a 10% increase in the yield strength was achieved, with a slight decrease in the elongation after aging at 180°C for 30min, which is a simulated process of paint baking. Cu is found to slightly increase the yield strength under the as-cast condition and after the heat treatment, but with a significant reduction in the ductility. Therefore, Cu should be limited in the super ductile aluminium alloy. The microstructure of diecast aluminium alloys at the optimized composition consists of the primary α-Al phase, the α-AlFeMnSi intermetallics and the Al-Mg Si eutectics. There are two types of primary α-Al phase: dendritic or fragmented dendritic α-Al phase solidified in the shot sleeve and globular α-Al particles solidified in the die cavity. The α-AlFeMnSi intermetallics is in the form of compact morphology and with a size of less than 3μm. The eutectic cells are at size of 10μm with a typical lamellar morphology of α-Al phase and Mg Si phase.EPSRC and JLRUK
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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