7,846 research outputs found
GeV-TeV and X-ray flares from gamma-ray bursts
The recent detection of delayed X-ray flares during the afterglow phase of
gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the
deceleration radius characterizing the beginning of the forward shock afterglow
emission. Given the observed temporal overlapping between the flares and
afterglows, there must be inverse Compton (IC) emission arising from such flare
photons scattered by forward shock afterglow electrons. We find that this IC
emission produces GeV-TeV flares, which may be detected by GLAST and
ground-based TeV telescopes. We speculate that this kind of emission may
already have been detected by EGRET from a very strong burst--GRB940217. The
enhanced cooling of the forward shock electrons by the X-ray flare photons may
suppress the synchrotron emission of the afterglows during the flare period.
The detection of GeV-TeV flares combined with low energy observations may help
to constrain the poorly known magnetic field in afterglow shocks. We also
consider the self-IC emission in the context of internal-shock and
external-shock models for X-ray flares. The emission above GeV from internal
shocks is low, while the external shock model can also produce GeV-TeV flares,
but with a different temporal behavior from that caused by IC scattering of
flare photons by afterglow electrons. This suggests a useful approach for
distinguishing whether X-ray flares originate from late central engine activity
or from external shocks.Comment: slightly shortened version, accepted for publication in ApJ Letters,
4 emulateapj pages, no figure
A Multi-Species Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds
We have performed a multi-species hydrodynamical simulation of the formation
and evolution of Lyman alpha clouds in a flat CDM dominated universe with an
external flux of ionizing radiation. We solve the fully coupled non-equilibrium
rate equations for the following species: H, H^+, H^-, H_2, H_2^+, He, He^+,
He^{++}, and e^-. The statistical properties of the distribution and evolution
of both hydrogen and helium absorption lines are extracted and compared to
observed data. We find excellent agreement for the following neutral hydrogen
data: the distribution of column densities is fit well by a power law with
exponent beta=1.55 with a possible deficiency of lines above column density
10^{15} cm^{-2}; the integrated distribution matches observed data over a broad
range of column densities 10^{13} to 10^{17} cm^{-2}; a Gaussian statistical
fit to the Doppler parameter distribution yields a median of 35.6 km s^{-1};
the evolution of the number of clouds with column densities larger than 10^{14}
cm^{-2} follows a power law with exponent gamma=2.22. Analogous calculations
are presented for HeII absorption lines and we find the ratio of Doppler
parameters b_{HeII}/b_{HI} = 0.87. Our data also suggests that Ly
clouds may belong to two morphologically different groups: small column density
clouds which tend to reside in sheets or filamental structures and are very
elongated and/or flattened, and the large column density clouds which are
typically found at the intersections of these caustic structures and are
quasi-spherical.Comment: 14 pages, 4 postscript figure
Collapsed 2-Dimensional Polymers on a Cylinder
Single partially confined collapsed polymers are studied in two dimensions.
They are described by self-avoiding random walks with nearest-neighbour
attractions below the -point, on the surface of an infinitely long
cylinder. For the simulations we employ the pruned-enriched-Rosenbluth method
(PERM). The same model had previously been studied for free polymers (infinite
lattice, no boundaries) and for polymers on finite lattices with periodic
boundary conditions. We verify the previous estimates of bulk densities, bulk
free energies, and surface tensions. We find that the free energy of a polymer
with fixed length has, for , a minimum at a finite cylinder
radius which diverges as . Furthermore, the surface
tension vanishes roughly as for with
. The density in the interior of a globule scales as
with .Comment: 4 pages, 8 figure
Electrodynamics of Amorphous Media at Low Temperatures
Amorphous solids exhibit intrinsic, local structural transitions, that give
rise to the well known quantum-mechanical two-level systems at low
temperatures. We explain the microscopic origin of the electric dipole moment
of these two-level systems: The dipole emerges as a result of polarization
fluctuations between near degenerate local configurations, which have nearly
frozen in at the glass transition. An estimate of the dipole's magnitude, based
on the random first order transition theory, is obtained and is found to be
consistent with experiment. The interaction between the dipoles is estimated
and is shown to contribute significantly to the Gr\"{u}neisen parameter anomaly
in low glasses. In completely amorphous media, the dipole moments are
expected to be modest in size despite their collective origin. In partially
crystalline materials, however, very large dipoles may arise, possibly
explaining the findings of Bauer and Kador, J. Chem. Phys. {\bf 118}, 9069
(2003).Comment: Submitted for publication; April 27, 2005 versio
Dipolar superfluidity in electron-hole bilayer systems
Bilayer electron-hole systems, where the electrons and holes are created via
doping and confined to separate layers, undergo excitonic condensation when the
distance between the layers is smaller than typical distance between particles
within a layer. We argue that the excitonic condensate is a novel dipolar
superfluid in which the phase of the condensate couples to the {\it gradient}
of the vector potential. We predict the existence of dipolar supercurrent which
can be tuned by an in-plane magnetic field and detected by independent contacts
to the layers. Thus the dipolar superfluid offers an example of excitonic
condensate in which the {\it composite} nature of its constituent excitons is
manifest in the macroscopic superfluid state. We also discuss various
properties of this superfluid including the role of vortices.Comment: 5 pages, 1 figure, minor changes and added few references; final
published versio
8x14Gb/s ring WDM modulator array with integrated tungsten heaters and Ge monitor photodetectors
An 8x14Gb/s wavelength-division multiplexed Si ring modulator array is presented with uniform channel performance. Tungsten heaters and Ge monitor photodetectors at the ring modulator drop ports are co-integrated to track and control the modulation quality
Nebular Line Emission During the Epoch of Reionization
Nebular emission lines associated with galactic HII regions carry information
about both physical properties of the ionised gas and the source of ionising
photons as well as providing the opportunity of measuring accurate redshifts
and thus distances once a cosmological model is assumed. While nebular line
emission has been extensively studied at lower redshift there are currently
only few constraints within the epoch of reionisation (EoR, ), chiefly due
to the lack of sensitive near-IR spectrographs. However, this will soon change
with the arrival of the Webb Telescope providing sensitive near-IR spectroscopy
covering the rest-frame UV and optical emission of galaxies in the EoR. In
anticipation of Webb we combine the large cosmological hydrodynamical
simulation Bluetides with photoionisation modelling to predict the nebular
emission line properties of galaxies at . We find good agreement
with the, albeit limited, existing direct and indirect observational
constraints on equivalent widths though poorer agreement with luminosity
function constraints.Comment: 17 pages, accepted to MNRAS, significant modification from v1.0 data
available at https://stephenmwilkins.github.io/BluetidesEmissionLines_Public
Non-substitutional single-atom defects in the Ge_(1-x)Sn_x alloy
Ge_(1-x)Sn_x alloys have proved difficult to form at large x, contrary to
what happens with other group IV semiconductor combinations. However, at low x
they are typical examples of well-behaved substitutional compounds, which is
desirable for harnessing the electronic properties of narrow band
semiconductors. In this paper, we propose the appearance of another kind of
single-site defect (), consisting of a single Sn atom in the center
of a Ge divacancy, that may account for these facts. Accordingly, we examine
the electronic and structural properties of these alloys by performing
extensive numerical ab-initio calculations around local defects. The results
show that the environment of the defect relaxes towards a cubic
octahedral configuration, facilitating the nucleation of metallic white tin and
its segregation, as found in amorphous samples. Using the information stemming
from these local defect calculations, we built a simple statistical model to
investigate at which concentration these defects can be formed in
thermal equilibrium. These results agree remarkably well with experimental
findings, concerning the critical concentration above which the homogeneous
alloys cannot be formed at room temperature. Our model also predicts the
observed fact that at lower temperature the critical concentration increases.
We also performed single site effective-field calculations of the electronic
structure, which further support our hypothesis.Comment: 12 pages, 1 table, 16 figure
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