70,411 research outputs found
GRB afterglows: deep Newtonian phase and its application
Gamma-ray burst afterglows have been observed for months or even years in a
few cases. It deserves noting that at such late stages, the remnants should
have entered the deep Newtonian phase, during which the majority of
shock-accelerated electrons will no longer be highly relativistic. To calculate
the afterglows, we must assume that the electrons obey a power-law distribution
according to their kinetic energy, not simply the Lorentz factor.Comment: Poster at the 4th workshop "Gamma-Ray Bursts in the Afterglow Era"
(Rome, 2004), accepted for publication in the proceedings. 4 pages, with 3
figures inserte
Radiation transport equations in non-Riemannian space-times
The transport equations for polarized radiation transfer in non-Riemannian,
Weyl-Cartan type space-times are derived, with the effects of both torsion and
non-metricity included. To obtain the basic propagation equations we use the
tangent bundle approach. The equations describing the time evolution of the
Stokes parameters, of the photon distribution function and of the total
polarization degree can be formulated as a system of coupled first order
partial differential equations. As an application of our results we consider
the propagation of the cosmological gamma ray bursts in spatially homogeneous
and isotropic spaces with torsion and non-metricity. For this case the exact
general solution of the equation for the polarization degree is obtained, with
the effects of the torsion and non-metricity included. The presence of a
non-Riemannian geometrical background in which the electromagnetic fields
couple to torsion and/or non-metricity affect the polarization of photon beams.
Consequently, we suggest that the observed polarization of prompt cosmological
gamma ray bursts and of their optical afterglows may have a propagation effect
component, due to a torsion/non-metricity induced birefringence of the vacuum.
A cosmological redshift and frequency dependence of the polarization degree of
gamma ray bursts also follows from the model, thus providing a clear
observational signature of the torsional/non-metric effects. On the other hand,
observations of the polarization of the gamma ray bursts can impose strong
constraints on the torsion and non-metricity and discriminate between different
theoretical models.Comment: 12 pages, 3 figures, accepted for publication in PR
Beaming effects in GRBs and orphan afterglows
The overall dynamical evolution and radiation mechanism of -ray burst
jets are briefly introduced. Various interesting topics concerning beaming in
-ray bursts are discussed, including jet structures, orphan afterglows
and cylindrical jets. The possible connection between -ray bursts and
neutron star kicks is also addressed.Comment: 10 Pages, 4 figures, to appear in a special issue of ApSS. Oral
report presented at "The Multiwavelength Approach to Unidentified Gamma-Ray
Sources" (Hong Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and
G.E. Romero
Recent advancement of turbulent flow measurement techniques
Advancements of the fluctuating density gradient cross beam laser Schlieren technique, the fluctuating line-reversal temperature measurement and the development of the two-dimensional drag-sensing probe to a three-dimensional drag-sensing probe are discussed. The three-dimensionality of the instantaneous momentum vector can shed some light on the nature of turbulence especially with swirling flow. All three measured fluctuating quantities (density, temperature, and momentum) can provide valuable information for theoreticians
Dominant moving species in the formation of amorphous NiZr by solid-state reaction
The displacements of W and Hf markers have been monitored by backscattering of MeV He to study the growth of the amorphous NiZr phase by solid-state reaction. We find that the Ni is the dominant moving species in this reaction
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