64,398 research outputs found
Reverse Shock Emission as a Probe of GRB Ejecta
We calculate the reverse shock (RS) synchrotron emission in the optical and
the radio wavelength bands from electron-positron pair enriched gamma-ray burst
ejecta with the goal of determining the pair content of GRBs using early time
observations. We take into account an extensive number of physical effects that
influence radiation from the reverse-shock heated GRB ejecta. We find that
optical/IR flux depends very weakly on the number of pairs in the ejecta, and
there is no unique signature of ejecta pair enrichment if observations are
confined to a single wavelength band. It may be possible to determine if the
number of pairs per proton in the ejecta is > 100 by using observations in
optical and radio bands; the ratio of flux in the optical and radio at the peak
of each respective reverse-shock light curve is dependent on the number of
pairs per proton. We also find that over a large parameter space, RS emission
is expected to be very weak; GRB 990123 seems to have been an exceptional burst
in that only a very small fraction of the parameter space produces optical
flashes this bright. Also, it is often the case that the optical flux from the
forward shock is brighter than the reverse shock flux at deceleration. This
could be another possible reason for the paucity of prompt optical flashes with
a rapidly declining light curve at early times as was seen in 990123 and
021211. Some of these results are a generalization of similar results reported
in Nakar & Piran (2004).Comment: 12 pages, 6 figures, 2 tables, accepted to MNRA
Can re-entrance be observed in force induced transitions?
A large conformational change in the reaction co-ordinate and the role of the
solvent in the formation of base-pairing are combined to settle a long standing
issue {\it i.e.} prediction of re-entrance in the force induced transition of
DNA. A direct way to observe the re-entrance, i.e a strand goes to the closed
state from the open state and again to the open state with temperature, appears
difficult to be achieved in the laboratory. An experimental protocol (in direct
way) in the constant force ensemble is being proposed for the first time that
will enable the observation of the re-entrance behavior in the
force-temperature plane. Our exact results for small oligonucleotide that forms
a hairpin structure provide the evidence that re-entrance can be observed.Comment: 12 pages and 5 figures (RevTex4). Accepted in Europhys Lett. (2009
Giant Tunneling Magnetoresistance, Glassiness, and the Energy Landscape at Nanoscale Cluster Coexistence
We present microscopic results on the giant tunneling magnetoresistance that
arises from the nanoscale coexistence of ferromagnetic metallic (FMM) and
antiferromagnetic insulating (AFI) clusters in a disordered two dimensional
electron system with competing double exchange and superexchange interactions.
Our Monte Carlo study allows us to map out the different field regimes in
magnetotransport and correlate it with the evolution of spatial structures. At
coexistence, the isotropic O(3) model shows signs of slow relaxation, and has a
high density of low energy metastable states, but no genuine glassiness.
However, in the presence of weak magnetic anisotropy, and below a field
dependent irreversibility temperature , the response on field cooling
(FC) differs distinctly from that on zero field cooling (ZFC). We map out the
phase diagram of this `phase coexistence glass', highlight how its response
differs from that of a standard spin glass, and compare our results with data
on the manganites.Comment: Final published versio
A New Superconformal Mechanics
In this paper we propose a new supersymmetric extension of conformal
mechanics. The Grassmannian variables that we introduce are the basis of the
forms and of the vector-fields built over the symplectic space of the original
system. Our supersymmetric Hamiltonian itself turns out to have a clear
geometrical meaning being the Lie-derivative of the Hamiltonian flow of
conformal mechanics. Using superfields we derive a constraint which gives the
exact solution of the supersymmetric system in a way analogous to the
constraint in configuration space which solved the original non-supersymmetric
model. Besides the supersymmetric extension of the original Hamiltonian, we
also provide the extension of the other conformal generators present in the
original system. These extensions have also a supersymmetric character being
the square of some Grassmannian charge. We build the whole superalgebra of
these charges and analyze their closure. The representation of the even part of
this superalgebra on the odd part turns out to be integer and not spinorial in
character.Comment: Superfield re-define
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