17,824 research outputs found
On the afterglow from the receding jet of gamma-ray burst
According to popular progenitor models of gamma-ray bursts, twin jets should
be launched by the central engine, with a forward jet moving toward the
observer and a receding jet (or the counter jet) moving backwardly. However, in
calculating the afterglows, usually only the emission from the forward jet is
considered. Here we present a detailed numerical study on the afterglow from
the receding jet. Our calculation is based on a generic dynamical description,
and includes some delicate ingredients such as the effect of the equal arrival
time surface. It is found that the emission from the receding jet is generally
rather weak. In radio bands, it usually peaks at a time of d,
with the peak flux nearly 4 orders of magnitude lower than the peak flux of the
forward jet. Also, it usually manifests as a short plateau in the total
afterglow light curve, but not as an obvious rebrightening as once expected. In
optical bands, the contribution from the receding jet is even weaker, with the
peak flux being orders of magnitude lower than the peak flux of the
forward jet. We thus argue that the emission from the receding jet is very
difficult to detect. However, in some special cases, i.e., when the
circum-burst medium density is very high, or if the parameters of the receding
jet is quite different from those of the forward jet, the emission from the
receding jet can be significantly enhanced and may still emerge as a marked
rebrightening. We suggest that the search for receding jet emission should
mostly concentrate on nearby gamma-ray bursts, and the observation campaign
should last for at least several hundred days for each event.Comment: A few citations added, together with a few minor revisions, main
conclusions unchanged, accepted for publication in A&A, 7 figures, 10 Page
Exact Moderate Deviation Asymptotics in Streaming Data Transmission
In this paper, a streaming transmission setup is considered where an encoder
observes a new message in the beginning of each block and a decoder
sequentially decodes each message after a delay of blocks. In this
streaming setup, the fundamental interplay between the coding rate, the error
probability, and the blocklength in the moderate deviations regime is studied.
For output symmetric channels, the moderate deviations constant is shown to
improve over the block coding or non-streaming setup by exactly a factor of
for a certain range of moderate deviations scalings. For the converse proof, a
more powerful decoder to which some extra information is fedforward is assumed.
The error probability is bounded first for an auxiliary channel and this result
is translated back to the original channel by using a newly developed
change-of-measure lemma, where the speed of decay of the remainder term in the
exponent is carefully characterized. For the achievability proof, a known
coding technique that involves a joint encoding and decoding of fresh and past
messages is applied with some manipulations in the error analysis.Comment: 23 pages, 1 figure, 1 table, Submitted to IEEE Transactions on
Information Theor
Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation
The exchange interaction among electrons is one of the most fundamental
quantum mechanical interactions in nature and underlies any magnetic phenomena
from ferromagnetic ordering to magnetic storage. The current technology is
built upon a thermal or magnetic field, but a frontier is emerging to directly
control magnetism using ultrashort laser pulses. However, little is known about
the fate of the exchange interaction. Here we report unambiguously that
photoexcitation is capable of quenching the exchange interaction in all three
ferromagnetic metals. The entire process starts with a small number of
photoexcited electrons which build up a new and self-destructive potential that
collapses the system into a new state with a reduced exchange splitting. The
spin moment reduction follows a Bloch-like law as , where is
the absorbed photon energy and is a scaling exponent. A good agreement
is found between the experimental and our theoretical results. Our findings may
have a broader implication for dynamic electron correlation effects in
laser-excited iron-based superconductors, iron borate, rare-earth
orthoferrites, hematites and rare-earth transition metal alloys.Comment: 16 pages, 3 figures, one supplementary material fil
Generating high-order optical and spin harmonics from ferromagnetic monolayers
High-order harmonic generation (HHG) in solids has entered a new phase of
intensive research, with envisioned band-structure mapping on an ultrashort
time scale. This partly benefits from a flurry of new HHG materials discovered,
but so far has missed an important group. HHG in magnetic materials should have
profound impact on future magnetic storage technology advances. Here we
introduce and demonstrate HHG in ferromagnetic monolayers. We find that HHG
carries spin information and sensitively depends on the relativistic spin-orbit
coupling; and if they are dispersed into the crystal momentum space,
harmonics originating from real transitions can be -resolved and carry
the band structure information. Geometrically, the HHG signal is sensitive to
spatial orientations of monolayers. Different from the optical counterpart, the
spin HHG, though probably weak, only appears at even orders, a consequence of
SU(2) symmetry. Our findings open an unexplored frontier -- magneto-high-order
harmonic generation.Comment: 19 pages, 4 figure
Scaling of the magnetic entropy and magnetization in YbRh_2(Si_{0.95}Ge_{0.05})_2
The magnetic entropy of YbRh_2(Si_{0.95}Ge_{0.05})_2 is derived from
low-temperature ( mK) specific heat measurements. Upon field-tuning
the system to its antiferromagnetic quantum critical point unique temperature
over magnetic field scaling is observed indicating the disintegration of heavy
quasiparticles. The field dependence of the entropy equals the temperature
dependence of the dc-magnetization as expected from the Maxwell relation. This
proves that the quantum-critical fluctuations affect the thermal and magnetic
properties in a consistent way.Comment: 6 pages, 2 figures, manuscript submitted to SCES2004 conferenc
Effect of conduction electron interactions on Anderson impurities
The effect of conduction electron interactions for an Anderson impurity is
investigated in one dimension using a scaling approach. The flow diagrams are
obtained by solving the renormalization group equations numerically. It is
found that the Anderson impurity case is different from its counterpart -- the
Kondo impurity case even in the local moment region. The Kondo temperature for
an Anderson impurity shows nonmonotonous behavior, increasing for weak
interactions but decreasing for strong interactions. The implication of the
study to other related impurity models is also discussed.Comment: 10 pages, revtex, 4 figures (the postscript file is included), to
appear in Phys. Rev. B (Rapid Commun.
A grid-based infrastructure for distributed retrieval
In large-scale distributed retrieval, challenges of latency, heterogeneity, and dynamicity emphasise the importance of infrastructural support in reducing the development costs of state-of-the-art solutions. We present a service-based infrastructure for distributed retrieval which blends middleware facilities and a design framework to āliftā the resource sharing approach and the computational services of a European Grid platform into the domain of e-Science applications. In this paper, we give an overview of the DILIGENT Search Framework and illustrate its exploitation in the ļ¬eld of Earth Science
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