11,900 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
Recommended from our members
Rheo-processing of an alloy specifically designed for semi-solid metal processing on the Al-Mg-Si system
Semi-solid metal (SSM) processing is a promising technology for forming alloys and composites to near-net shaped products. Alloys currently used for SSM processing are mainly conventional aluminium cast alloys. This is an obstacle to the realisation of full potential of SSM processing, since these alloys were originally designed for liquid state processing and not for semi-solid state processing. Therefore, there is a significant need for designing new alloys specifically for semi-solid state processing to fulfil its potential. In this study, thermodynamic calculations have been carried out to design alloys based on the Al-Mg-Si system for SSM processing via the ‘Rheo-route’. The suitability of a selected alloy composition has been assessed in terms of the criteria considered by the thermodynamic design process, mechanical properties and heat treatability. The newly designed alloy showed good processability with rheo-processing in terms of good control of solid fraction during processing and a reasonably large processing window. The mechanical property variation was very small and the alloy showed good potential for age hardening by T5 temper heat treatment after rheo-processing
Analysis of the X(1576) as a tetraquark state with the QCD sum rules
In this letter, we take the point of view that the X(1576) be tetraquark
state which consists of a scalar-diquark and an anti-scalar-diquark in relative
-wave, and calculate its mass in the framework of the QCD sum rules
approach. The numerical value of the mass is
consistent with the experimental data, there may be some tetraquark component
in the vector meson X(1576).Comment: 6 pages, 1 figure, second version, typos correcte
Rate-dependent morphology of Li2O2 growth in Li-O2 batteries
Compact solid discharge products enable energy storage devices with high
gravimetric and volumetric energy densities, but solid deposits on active
surfaces can disturb charge transport and induce mechanical stress. In this
Letter we develop a nanoscale continuum model for the growth of Li2O2 crystals
in lithium-oxygen batteries with organic electrolytes, based on a theory of
electrochemical non-equilibrium thermodynamics originally applied to Li-ion
batteries. As in the case of lithium insertion in phase-separating LiFePO4
nanoparticles, the theory predicts a transition from complex to uniform
morphologies of Li2O2 with increasing current. Discrete particle growth at low
discharge rates becomes suppressed at high rates, resulting in a film of
electronically insulating Li2O2 that limits cell performance. We predict that
the transition between these surface growth modes occurs at current densities
close to the exchange current density of the cathode reaction, consistent with
experimental observations.Comment: 8 pages, 6 fig
Production rates for hadrons, pentaquarks and , and di-baryon in relativistic heavy ion collisions by a quark combination model
The hadron production in relativistic heavy ion collisions is well described
by the quark combination model. The mixed ratios for various hadrons and the
transverse momentum spectra for long-life hadrons are predicted and agree with
recent RHIC data. The production rates for the pentaquarks , and the di-baryon are estimated, neglecting
the effect from the transition amplitude for constituent quarks to form an
exotic state.Comment: The difference between our model and other combination models is
clarified. The scaled transverse momentum spectra for pions, kaons and
protoms at both 130 AGeV and 200 AGeV are given, replacing the previous
results in transverse momentum spectr
The Norm of the Learning-Disability Checklist for Elementary and Middle School Children (in Chinese)
The Learning-Disabilities Checklist for Elementary and Middle School Children was administered to 1067 subjects in Shanghai. The data analyses brought us the following: 1) The mean and standard deviation of each variable of raw data; 2) There were significant age differences and sex differences for each variable; 3) The reliability and validity of this test was up to the criteria of psychometrology; 4) A Shanghai norm was made
Note on a new fundamental length scale instead of the Newtonian constant
The newly proposed entropic gravity suggests gravity as an emergent force
rather than a fundamental one. In this approach, the Newtonian constant
does not play a fundamental role any more, and a new fundamental constant is
required to replace its position. This request also arises from some
philosophical considerations to contemplate the physical foundations for the
unification of theories. We here consider the suggestion to derive from
more fundamental quantities in the presence of a new fundamental length scale
, which is suspected to originate from the structure of quantum space-time,
and can be measured directly from Lorentz-violating observations. Our results
are relevant to the fundamental understanding of physics, and more practically,
of natural units, as well as explanations of experimental constraints in
searching for Lorentz violation.Comment: 10 latex pages, final version for journal publicatio
- …