32,607 research outputs found
Spectrum and Duration of Delayed MeV-GeV Emission of Gamma-Ray Bursts in Cosmic Background Radiation Fields
We generally analyze prompt high-energy emission above a few hundreds of GeV
due to synchrotron self-Compton scattering in internal shocks. However, such
photons cannot be detected because they may collide with cosmic infrared
background photons, leading to electron/positron pair production.
Inverse-Compton scattering of the resulting electron/positron pairs off cosmic
microwave background photons will produce delayed MeV-GeV emission, which may
be much stronger than a typical high-energy afterglow in the external shock
model. We expand on the Cheng & Cheng model by deriving the emission spectrum
and duration in the standard fireball shock model. A typical duration of the
emission is ~ 10^3 seconds, and the time-integrated scattered photon spectrum
is nu^{-(p+6)/4}, where p is the index of the electron energy distribution
behind internal shocks. This is slightly harder than the synchrotron photon
spectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photon
spectrum is dependent on the spectral energy distribution of the cosmic
infrared background radiation. Therefore, future observations on such delayed
MeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray Large
Area Space Telescope (GLAST) would provide a probe of the cosmic infrared
background radiation.Comment: 5 pages, accepted for publication in Ap
Comparison of predictive scores of symptomatic intracerebral haemorrhage after stroke thrombolysis in a single centre
Peer reviewedPublisher PD
Neutralino dark matter stars can not exist
Motivated by the recent "Cosmos Project" observation of dark-matter
concentrations with no ordinary matter in the same place, we study the question
of the existence of compact objects made of pure dark matter. We assume that
the dark matter is neutralino, and compare its elastic and annihilation cross
sections. We find that the two cross sections are of the same order of
magnitude. This result has a straightforward and important consequence that
neutralinos comprising a compact object can not achieve thermal equilibrium. To
substantiate our arguments, by solving Oppenheimer-Volkoff equation we
constructed a model of the star made of pure neutralinos. We explicitly showed
that the condition for the thermal equilibrium supported by the Fermi pressure
is never fulfilled inside the star. This neutralino state can not be described
by the Fermi-Dirac distribution. Thus, a stable neutralino star, which is
supported by the Fermi pressure, can not exist. We also estimated that a stable
star can not contain more than a few percents of neutralinos, most of the mass
must be in the form of the standard model particles.Comment: published in JHE
An exactly solvable phase transition model: generalized statistics and generalized Bose-Einstein condensation
In this paper, we present an exactly solvable phase transition model in which
the phase transition is purely statistically derived. The phase transition in
this model is a generalized Bose-Einstein condensation. The exact expression of
the thermodynamic quantity which can simultaneously describe both gas phase and
condensed phase is solved with the help of the homogeneous Riemann-Hilbert
problem, so one can judge whether there exists a phase transition and determine
the phase transition point mathematically rigorously. A generalized statistics
in which the maximum occupation numbers of different quantum states can take on
different values is introduced, as a generalization of Bose-Einstein and
Fermi-Dirac statistics.Comment: 17 pages, 2 figure
Heavy surface state in a possible topological Kondo insulator: Magneto-thermoelectric transport on the (011)-plane of SmB
Motivated by the high sensitivity to Fermi surface topology and scattering
mechanisms in magneto-thermoelectric transport, we have measured the
thermopower and Nernst effect on the (011)-plane of the proposed topological
Kondo insulator SmB. These experiments, together with electrical
resistivity and Hall effect measurements, demonstrate that the (011)-plane also
harbors a metallic surface with the effective mass in the order of 10-10
. The surface and bulk conductances are well distinguished in these
measurements and are categorized into metallic and non-degenerate
semiconducting regimes, respectively. Electronic correlations play an important
role in enhancing scattering and also contribute to the heavy surface state.Comment: 4 figures, 1 tabl
Smoothed Particle Magnetohydrodynamics II. Variational principles and variable smoothing length terms
In this paper we show how a Lagrangian variational principle can be used to
derive the SPMHD (smoothed particle magnetohydrodynamics) equations for ideal
MHD. We also consider the effect of a variable smoothing length in the SPH
kernels after which we demonstrate by numerical tests that the consistent
treatment of terms relating to the gradient of the smoothing length in the
SPMHD equations significantly improves the accuracy of the algorithm. Our
results complement those obtained in a companion paper (Price and Monaghan
2003a, paper I) for non ideal MHD where artificial dissipative terms were
included to handle shocks.Comment: 14 pages, 4 figures, accepted to MNRA
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