85,590 research outputs found
Constraints on extra-dimensions and variable constants from cosmological gamma ray bursts
The observation of the time delay between the soft emission and the
high-energy radiation from cosmological gamma ray bursts can be used as an
important observational test of multi-dimensional physical theories. The main
source of the time delay is the variation of the electromagnetic coupling, due
to dimensional reduction, which induces an energy dependence of the speed of
light. For photons with energies around 1 TeV, the time delay could range from
a few seconds in the case of Kaluza-Klein models to a few days for models with
large extra-dimensions. Based on these results we suggest that the detection of
the 18-GeV photon 4500 s after the keV/MeV burst in GRB 940217 provides
a strong evidence for the existence of extra-dimensions. The time delay of
photons, if observed by the next generation of high energy detectors, like, for
example, the SWIFT and GLAST satellite based detectors, or the VERITAS
ground-based TeV gamma-ray instrument, could differentiate between the
different models with extra-dimensions.Comment: 8 pages, 4 figures, contribution to the proceedings of the II
Workshop on Unidentified Gamma-Ray Sources, Hong Kong, June 1-4, 200
Electron-positron energy deposition rate from neutrino pair annihilation on the rotation axis of neutron and quark stars
We investigate the deposition of energy due to the annihilations of neutrinos
and antineutrinos on the rotation axis of rotating neutron and quark stars,
respectively. The source of the neutrinos is assumed to be a neutrino-cooled
accretion disk around the compact object. Under the assumption of the
separability of the neutrino null geodesic equation of motion we obtain the
general relativistic expression of the energy deposition rate for arbitrary
stationary and axisymmetric space-times. The neutrino trajectories are obtained
by using a ray tracing algorithm, based on numerically solving the
Hamilton-Jacobi equation for neutrinos by reversing the proper time evolution.
We obtain the energy deposition rates for several classes of rotating neutron
stars, described by different equations of state of the neutron matter, and for
quark stars, described by the MIT bag model equation of state and in the CFL
(Color-Flavor-Locked) phase, respectively. The electron-positron energy
deposition rate on the rotation axis of rotating neutron and quark stars is
studied for two accretion disk models (isothermal disk and accretion disk in
thermodynamical equilibrium). Rotation and general relativistic effects modify
the total annihilation rate of the neutrino-antineutrino pairs on the rotation
axis of compact stellar, as measured by an observer at infinity. The
differences in the equations of state for neutron and quark matter also have
important effects on the spatial distribution of the energy deposition rate by
neutrino-antineutrino annihilation.Comment: 38 pages, 9 figures, accepted for publication in MNRA
A COMPARISON OF EXTENDED SOURCE-FILTER MODELS FOR MUSICAL SIGNAL RECONSTRUCTION
China Scholarship Council (CSC)/
Queen Mary Joint PhD scholarship;
Royal Academy of Engineering Research Fellowshi
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
Reply to [arXiv:1105.5147] "Are GRB 090423 and Similar Bursts due to Superconducting Cosmic Strings?"
The GRB outflow driven by superconducting cosmic strings is likely to be an
arc rather than a usually-considered spherical cap. In such a case, the
afterglows of the cosmic string GRBs could be basically consistent with the
observation of the high-redshift GRBs.Comment: 2 pages, 1 figure, to appear in Phys. Rev. Let
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
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