2,276 research outputs found
Pair loading in Gamma-Ray Burst Fireball And Prompt Emission From Pair-Rich Reverse Shock
Gamma-ray bursts (GRBs) are believed to originate from ultra-relativistic
winds/fireballs to avoid the "compactness problem". However, the most energetic
photons in GRBs may still suffer from absorption leading to
electron/positron pair production in the winds/fireballs. We show here that in
a wide range of model parameters, the resulting pairs may dominate those
electrons associated with baryons. Later on, the pairs would be carried into a
reverse shock so that a shocked pair-rich fireball may produce a strong flash
at lower frequencies, i.e. in the IR band, in contrast with optical/UV emission
from a pair-poor fireball. The IR emission would show a 5/2 spectral index due
to strong self-absorption. Rapid responses to GRB triggers in the IR band would
detect such strong flashes. The future detections of many IR flashes will infer
that the rarity of prompt optical/UV emissions is in fact due to dust
obscuration in the star formation regions.Comment: 8 pages, 2 figures, ApJ accepte
Nonthermal gamma-ray and X-ray flashes from shock breakout in gamma-ray bursts/supernovae
Thermal X-ray emission which is simultaneous with the prompt gamma-rays has
been detected for the first time from a supernova connected with a gamma-ray
burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from
the breakout of a mildly relativistic, radiation-dominated shock from a dense
stellar wind surrounding the progenitor star. There is also evidence for the
presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its
X-ray and radio afterglow. Here we study the process of repeated bulk Compton
scatterings of shock breakout thermal photons by the mildly relativistic
ejecta. During the shock breakout process, a fraction of the thermal photons
would be repeatedly scattered between the pre-shock material and the shocked
material as well as the mildly relativistic ejecta and, as a result, the
thermal photons get boosted to increasingly higher energies. This bulk motion
Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes,
which could account for the prompt gamma-ray burst emission in low-luminosity
supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been
developed to simulate this repeated scattering process, which confirms that a
significant fraction of the thermal photons get "accelerated" to form a
nonthermal component, with a dominant luminosity. This interpretation for the
prompt nonthermal emission of GRB060218 may imply that either the usual
internal shock emission from highly relativistic jets in these low-luminosity
GRBs is weak, or alternatively, that there are no highly relativistic jets in
this peculiar class of bursts.Comment: Accepted for publication in ApJ; Introduction expanded, references
added, conclusions unchanged; total 7 pages including 2 color figures and 1
tabl
Kinetics of catalysis with surface disorder
We study the effects of generalised surface disorder on the monomer-monomer
model of heterogeneous catalysis, where disorder is implemented by allowing
different adsorption rates for each lattice site. By mapping the system in the
reaction-controlled limit onto a kinetic Ising model, we derive the rate
equations for the one and two-spin correlation functions. There is good
agreement between these equations and numerical simulations. We then study the
inclusion of desorption of monomers from the substrate, first by both species
and then by just one, and find exact time-dependent solutions for the one-spin
correlation functions.Comment: LaTex, 19 pages, 1 figure included, requires epsf.st
Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors
We discuss the analysis of mixed-state magnetization data of type-II
superconductors using a recently developed scaling procedure. It is based on
the fact that, if the Ginzburg-Landau parameter kappa does not depend on
temperature, the magnetic susceptibility is a universal function of H/H_c2(T),
leading to a simple relation between magnetizations at different temperatures.
Although this scaling procedure does not provide absolute values of the upper
critical fieldH_c2(T), its temperature variation can be established rather
accurately. This provides an opportunity to validate theoretical models that
are usually employed for the evaluation of H_c2(T) from equilibrium
magnetization data. In the second part of the paper we apply this scaling
procedure for a discussion of the notorious first order phase transition in the
mixed state of high temperature superconductors. Our analysis, based on
experimental magnetization data available in the literature, shows that the
shift of the magnetization accross the transition may adopt either sign,
depending on the particular chosen sample. We argue that this observation is
inconsistent with the interpretation that this transition always represents the
melting transition of the vortex lattice.Comment: 18 pages, 12 figure
Charge qubit dynamics in a double quantum dot coupled to phonons
The dynamics of charge qubit in a double quantum dot coupled to phonons is
investigated theoretically in terms of a perturbation treatment based on a
unitary transformation. The dynamical tunneling current is obtained explicitly.
The result is compared with the standard perturbation theory at Born-Markov
approximation. The decoherence induced by acoustic phonons is analyzed at
length. It is shown that the contribution from deformation potential coupling
is comparable to that from piezoelectric coupling in small dot size and large
tunneling rate case. A possible decoupling mechanism is predicted.Comment: 8 pages, 6 figure
Functional characterization of enhancer activity during a long terminal repeat\u27s evolution
Many transposable elements (TEs) contain transcription factor binding sites and are implicated as potential regulatory elements. However, TEs are rarely functionally tested for regulatory activity, which in turn limits our understanding of how TE regulatory activity has evolved. We systematically tested the human LTR18A subfamily for regulatory activity using massively parallel reporter assay (MPRA) and found AP-1- and CEBP-related binding motifs as drivers of enhancer activity. Functional analysis of evolutionarily reconstructed ancestral sequences revealed that LTR18A elements have generally lost regulatory activity over time through sequence changes, with the largest effects occurring owing to mutations in the AP-1 and CEBP motifs. We observed that the two motifs are conserved at higher rates than expected based on neutral evolution. Finally, we identified LTR18A elements as potential enhancers in the human genome, primarily in epithelial cells. Together, our results provide a model for the origin, evolution, and co-option of TE-derived regulatory elements
Path-integral representation for a stochastic sandpile
We introduce an operator description for a stochastic sandpile model with a
conserved particle density, and develop a path-integral representation for its
evolution. The resulting (exact) expression for the effective action highlights
certain interesting features of the model, for example, that it is nominally
massless, and that the dynamics is via cooperative diffusion. Using the
path-integral formalism, we construct a diagrammatic perturbation theory,
yielding a series expansion for the activity density in powers of the time.Comment: 22 pages, 6 figure
Equilibrium Properties of A Monomer-Monomer Catalytic Reaction on A One-Dimensional Chain
We study the equilibrium properties of a lattice-gas model of an catalytic reaction on a one-dimensional chain in contact with a reservoir
for the particles. The particles of species and are in thermal contact
with their vapor phases acting as reservoirs, i.e., they may adsorb onto empty
lattice sites and may desorb from the lattice. If adsorbed and
particles appear at neighboring lattice sites they instantaneously react and
both desorb. For this model of a catalytic reaction in the
adsorption-controlled limit, we derive analytically the expression of the
pressure and present exact results for the mean densities of particles and for
the compressibilities of the adsorbate as function of the chemical potentials
of the two species.Comment: 19 pages, 5 figures, submitted to Phys. Rev.
Conditions for the freezing phenomena of geometric measure of quantum discord for arbitrary two-qubit X states under non-dissipative dephasing noises
We study the dynamics of geometric measure of quantum discord (GMQD) under
the influences of two local phase damping noises. Consider the two qubits
initially in arbitrary X-states, we find the necessary and sufficient
conditions for which GMQD is unaffected for a finite period. It is further
shown that such results also hold for the non-Markovian dephasing process.Comment: 4 pages, 2 figure
- âŠ