37,589 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
Coupling between quasiparticles and a bosonic mode in the normal state of HgBaCuO
We report a doping dependent study of the quasiparticles dynamics in
HgBaCuO via Electronic Raman Scattering. A well-defined energy
scale is found in the normal state dynamics of the quasiparticles over a broad
doping range. It is interpreted as evidence for coupling between the
quasiparticles and a collective bosonic mode whose energy scale depend only
weakly with doping. We contrast this behavior with that of the superconducting
gap whose amplitude near the node continuously decreases towards the underdoped
regime. We discuss the implications of our findings on the nature of the
collective mode and argue that electron-phonon coupling is the most natural
explanation.Comment: 5 pages, 4 figure
Semileptonic B decays into excited charmed mesons from QCD sum rules
Exclusive semileptonic decays into excited charmed mesons are studied
with QCD sum rules in the leading order of heavy quark effective theory. Two
universal Isgur-Wise functions \tau and \zeta for semileptonic B decays into
four lowest lying excited mesons (, , , and ) are
determined. The decay rates and branching ratios for these processes are
calculated.Comment: RevTeX, 17 pages including 2 figure
The Afterglow of GRB 990123 and a Dense Medium
Recent observations show that the temporal decay of the R-band afterglow from
GRB 990123 steepened about 2.5 days after the burst. We here propose a possible
explanation for such a steepening: a shock expanding in a dense medium has
undergone the transition from a relativistic phase to a nonrelativistic phase.
We find that this model is consistent with the observations if the medium
density is about . By fitting our model to the
observed optical and X-ray afterglow quantitatively, we further infer the
electron and magnetic energy fractions of the shocked medium and find these two
parameters are about 0.1 and respectively. The former
parameter is near the equipartition value while the latter is about six orders
of magnitude smaller than inferred from the GRB 970508 afterglow. We also
discuss possibilities that the dense medium can be produced.Comment: 12 pages, LaTeX, published in ApJ Letter
Gamma-Ray Burst Afterglows from Realistic Fireballs
A GRB afterglow has been commonly thought to be due to continuous
deceleration of a postburst fireball. Many analytical models have made
simplifications for deceleration dynamics of the fireball and its radiation
property, although they are successful at explaining the overall features of
the observed afterglows. We here propose a model for a GRB afterglow in which
the evolution of a postburst fireball is in an intermediate case between the
adiabatic and highly radiative expansion. In our model, the afterglow is both
due to the contribution of the adiabatic electrons behind the external
blastwave of the fireball and due to the contribution of the radiative
electrons. In addition, this model can describe evolution of the fireball from
the extremely relativistic phase to the non-relativistic phase. Our
calculations show that the fireball will go to the adiabatic expansion phase
after about a day if the accelerated electrons are assumed to occupy the total
internal energy. In all cases considered, the fireball will go to the mildly
relativistic phase about seconds later, and to the non-relativistic
phase after several days. These results imply that the relativistic adiabatic
model cannot describe the deceleration dynamics of the several-days-later
fireball. The comparison of the calculated light curves with the observed
results at late times may imply the presence of impulsive events or energy
injection with much longer durations.Comment: 18 pages, 10 figures, plain latex file, submitted to Ap
Environment and Energy Injection Effects in GRB Afterglows
In a recent paper (Dai & Lu 1999), we have proposed a simple model in which
the steepening in the light curve of the R-band afterglow of the gamma-ray
burst (GRB) 990123 is caused by the adiabatic shock which has evolved from an
ultrarelativistic phase to a nonrelativistic phase in a dense medium. We find
that such a model is quite consistent with observations if the medium density
is about . Here we discuss this model in more
details. In particular, we investigate the effects of synchrotron self
absorption and energy injection. A shock in a dense medium becomes
nonrelativistic rapidly after a short relativistic phase. The afterglow from
the shock at the nonrelativistic stage decays more rapidly than at the
relativistic stage. Since some models for GRB energy sources predict that a
strongly magnetic millisecond pulsar may be born during the formation of GRB,
we discuss the effect of such a pulsar on the evolution of the nonrelativistic
shock through magnetic dipole radiation. We find that after the energy which
the shock obtains from the pulsar is much more than the initial energy of the
shock, the afterglow decay will flatten significantly. When the pulsar energy
input effect disappears, the decay will steepen again. These features are in
excellent agreement with the afterglows of GRB 980519, GRB 990510 and GRB
980326. Furthermore, our model fits very well all the observational data of GRB
980519 including the last two detections.Comment: 21 pages, LaTeX, accepted for publication in ApJ, one paragraph adde
The equation of state for two-dimensional hard-sphere gases: Hard-sphere gases as ideal gases with multi-core boundaries
The equation of state for a two-dimensional hard-sphere gas is difficult to
calculate by usual methods. In this paper we develop an approach for
calculating the equation of state of hard-sphere gases, both for two- and
three-dimensional cases. By regarding a hard-sphere gas as an ideal gas
confined in a container with a multi-core (excluded sphere) boundary, we treat
the hard-sphere interaction in an interacting gas as the boundary effect on an
ideal quantum gas; this enables us to treat an interacting gas as an ideal one.
We calculate the equation of state for a three-dimensional hard-sphere gas with
spin , and compare it with the results obtained by other methods. By this
approach the equation of state for a two-dimensional hard-sphere gas can be
calculated directly.Comment: 9 pages, 1 figur
A Hierarchical Emotion Regulated Sensorimotor Model: Case Studies
Inspired by the hierarchical cognitive architecture and the perception-action
model (PAM), we propose that the internal status acts as a kind of
common-coding representation which affects, mediates and even regulates the
sensorimotor behaviours. These regulation can be depicted in the Bayesian
framework, that is why cognitive agents are able to generate behaviours with
subtle differences according to their emotion or recognize the emotion by
perception. A novel recurrent neural network called recurrent neural network
with parametric bias units (RNNPB) runs in three modes, constructing a
two-level emotion regulated learning model, was further applied to testify this
theory in two different cases.Comment: Accepted at The 5th International Conference on Data-Driven Control
and Learning Systems. 201
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