6,864 research outputs found
A Reverse-Shock Model for the Early Afterglow of GRB 050525A
The prompt localization of gamma-ray burst (GRB) 050525A by {\em Swift}
allowed the rapid follow-up of the afterglow. The observations revealed that
the optical afterglow had a major rebrightening starting at days
and ending at days, which was followed by an initial power-law
decay. Here we show that this early emission feature can be interpreted as the
reverse shock emission superposed by the forward shock emission in an
interstellar medium environment. By fitting the observed data, we further
constrain some parameters of the standard fireball-shock model: the initial
Lorentz factor of the ejecta , the magnetic energy fraction
, and the medium density . These
limits are consistent with those from the other very-early optical afterglows
observed so far. In principle, a wind environment for GRB 050525A is
disfavored.Comment: 11 pages, 1 figure, accepted for publication in Ap
Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts
The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be
due to shocks sweeping up the circum-stellar medium. However, most GRBs have
been found in dense star-forming regions where a significant fraction of the
prompt X-ray emission can be scattered by dust grains. Here we revisit the
behavior of dust scattering of X-rays in GRBs. We find that the features of
some X-ray afterglows from minutes to days after the gamma-ray triggers are
consistent with the scattering of prompt X-ray emission from GRBs off host dust
grains. This implies that some of the observed X-ray afterglows (especially
those without sharp rising and decaying flares) could be understood with a
dust-scattering--driven emission model.Comment: ApJ, in pres
Electrical Breakdown of Excitonic Insulator
The intrinsic electrical breakdown of normal insulators is usually attributed
to inter-band Zener tunneling. This occurs when the gate voltage reaches the
same level as the bandgap. As a result, the Zener critical field is inversely
proportional to the size of the system. However, when the electrical field is
strong enough to disrupt excitons, an excitonic insulator will also break down,
as its insulation is due to the pairing of free electrons and holes. Unlike the
Zener mechanism, the pair-breaking critical field has little dependence on
system size and has a finite value in the thermodynamic limit. To understand
the relationship between these two mechanisms, a Hartree Fock calculation that
takes into account polarization was performed on a 2D bilayer model. Phase
diagrams were generated as a function of electrical field, system size, and
exciton density. The results showed that, at the large scale (low exciton
density) limit, the breakdown is dominated by Zener tunneling and transitions
smoothly to the pair-breaking case as the system size decreases (exciton
density increases). This provides a straightforward way to distinguish
excitonic insulators from normal insulators.Comment: 11 pages, 5 figure
Transformer with Implicit Edges for Particle-based Physics Simulation
Particle-based systems provide a flexible and unified way to simulate physics
systems with complex dynamics. Most existing data-driven simulators for
particle-based systems adopt graph neural networks (GNNs) as their network
backbones, as particles and their interactions can be naturally represented by
graph nodes and graph edges. However, while particle-based systems usually
contain hundreds even thousands of particles, the explicit modeling of particle
interactions as graph edges inevitably leads to a significant computational
overhead, due to the increased number of particle interactions. Consequently,
in this paper we propose a novel Transformer-based method, dubbed as
Transformer with Implicit Edges (TIE), to capture the rich semantics of
particle interactions in an edge-free manner. The core idea of TIE is to
decentralize the computation involving pair-wise particle interactions into
per-particle updates. This is achieved by adjusting the self-attention module
to resemble the update formula of graph edges in GNN. To improve the
generalization ability of TIE, we further amend TIE with learnable
material-specific abstract particles to disentangle global material-wise
semantics from local particle-wise semantics. We evaluate our model on diverse
domains of varying complexity and materials. Compared with existing GNN-based
methods, without bells and whistles, TIE achieves superior performance and
generalization across all these domains. Codes and models are available at
https://github.com/ftbabi/TIE_ECCV2022.git.Comment: Accepted by ECCV2022, 14 page
Studying newborn neutron stars by the transient emission after stellar collapses and compact binary mergers
The formation of neutron stars (NSs), both from collapses of massive stars
and mergers of compact objects, can be usually indicated by bright transients
emitted from explosively-ejected material. In particular, if the newborn NSs
can rotate at a millisecond period and have a sufficiently high magnetic field,
then the spin-down of the NSs would provide a remarkable amount of energy to
the emitting material. As a result, super-luminous supernovae could be produced
in the massive stellar collapse cases, while some unusual fast evolving and
luminous optical transients could arise from the cases of NS mergers and
accretion-induced collapses of white dwarfs. In all cases, if the dipolar
magnetic fields of the newborn NSs can be amplified to be as high as
G, a relativistic jet could be launched and then a gamma-ray burst can be
produced as the jet successfully breaks out from the surrounding
nearly-isotropic ejected material.Comment: 10 pages, 9 pictures, to appear in the AIP Proceedings of the
Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era
of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, Chin
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