1,538 research outputs found
Potential Barrier Classification by Short-Time Measurement
We investigate the short-time dynamics of a delta-function potential barrier
on an initially confined wave-packet. There are mainly two conclusions: A) At
short times the probability density of the first particles that passed through
the barrier is unaffected by it. B) When the barrier is absorptive (i.e., its
potential is imaginary) it affects the transmitted wave function at shorter
times than a real potential barrier. Therefore, it is possible to distinguish
between an imaginary and a real potential barrier by measuring its effect at
short times only on the transmitting wavefunction.Comment: 6 pages, 5 figure
Neutrinos from Gamma-Ray Bursts in Pulsar Wind Bubbles: \sim 10^{16} eV
The supranova model for Gamma-Ray Bursts (GRBs) is becoming increasingly more
popular. In this scenario the GRB occurs weeks to years after a supernova
explosion, and is located inside a pulsar wind bubble (PWB). Protons
accelerated in the internal shocks that emit the GRB may interact with the
external PWB photons producing pions which decay into \sim 10^{16} eV
neutrinos. A km^2 neutrino detector would observe several events per year
correlated with the GRBs.Comment: Accepted for publication in PRL. 4 pages, 3 figures, minor change
VLBI and Archival VLA and WSRT Observations of the GRB 030329 Radio Afterglow
We present VLBI and archival Karl G. Jansky Very Large Array (VLA) and
Westerbork Synthesis Radio Telescope (WSRT) observations of the radio afterglow
from the gamma-ray burst (GRB) of 2003 March 29 (GRB 030329) taken between 672
and 2032 days after the burst. The EVLA and WSRT data suggest a simple power
law decay in the flux at 5 GHz, with no clear signature of any rebrightening
from the counter jet. We report an unresolved source at day 2032 of size
mas, which we use in conjunction with the expansion rate of the
burst to argue for the presence of a uniform, ISM-like circumburst medium.
We develop a semi-analytic method to model gamma-ray burst afterglows, and
apply it to the 5 GHz light curve to perform burst calorimetry. A limit of mas yr is placed on the proper motion, supporting the standard
afterglow model for gamma-ray bursts.Comment: 24 pages, 5 figure
Gamma-Ray Burst afterglow scaling coefficients for general density profile
Gamma-ray burst (GRB) afterglows are well described by synchrotron emission
originating from the interaction between a relativistic blast wave and the
external medium surrounding the GRB progenitor. We introduce a code to
reconstruct spectra and light curves from arbitrary fluid configurations,
making it especially suited to study the effects of fluid flows beyond those
that can be described using analytical approximations. As a check and first
application of our code we use it to fit the scaling coefficients of
theoretical models of afterglow spectra. We extend earlier results of other
authors to general circumburst density profiles. We rederive the physical
parameters of GRB 970508 and compare with other authorsComment: 11 pages, 5 figures. Revised edition removes references to unphysical
chromatic break and adds appendix on hot region directly behind shoc
The polarisation of afterglow emission reveals GRB jet structure
We numerically compute light and polarisation curves of gamma-ray burst
afterglows for various configurations of the jet luminosity structure and for
different dynamical evolutions. We especially consider the standard homogeneous
``top hat'' jet and the ``universal structured jet'' with power-law wings. We
also investigate a possible more physical variation of the ``top hat'' model:
the ``Gaussian jet''. The polarisation curves for the last two jet types are
shown here for the first time together with the computation of X-ray and radio
polarised fluxes. We show that the lightcurves of the total flux from these
configurations are very similar to each other, and therefore only very high
quality data could allow us to pin down the underlying jet structure. We
demonstrate instead that polarisation curves are a powerful means to solve the
jet structure, since the predicted behaviour of polarisation and its position
angle at times around the jet break are very different if not opposite. We
conclude that the afterglow polarisation measurements provide clear footprints
of any outflow energy distribution (unlike the lightcurves of the total flux)
and the joint analysis of the total and polarised flux should reveal GRBs jet
structure.Comment: 16 pages, 18 figures, MNRAS, temp, 321. Light curves and polarisation
curves for a Gaussian jet added. Cartoon of the three jet structures adde
Synchrotron Self Absorption in GRB Afterglow
GRB afterglow is reasonably described by synchrotron emission from
relativistic blast waves at cosmological distances. We perform detailed
calculations taking into account the effect of synchrotron self absorption. We
consider emission from the whole region behind the shock front, and use the
Blandford McKee self similar solution to describe the fluid behind the shock.
We calculate the spectra and the observed image of a GRB afterglow near the
self absorption frequency and derive an accurate expression for
. We show that the image is rather homogeneous for , as
opposed to the bright ring at the outer edge and dim center, which appear at
higher frequencies. We compare the spectra we obtain to radio observations of
GRB970508. We combine the calculations of the spectra near the self absorption
frequency with other parts of the spectra and obtain revised estimates for the
physical parameters of the burst: , ,
, . These estimates are different by up to two
orders of magnitude than the estimates based on an approximate spectrum.Comment: 19 page latex file including 6 figures and 1 tabl
Multiple synchrotron self-Compton modeling of gamma-ray flares in 3C 279
The correlation often observed in blazars between optical-to-radio outbursts
and gamma-ray flares suggests that the high-energy emission region shall be
co-spatial with the radio knots, several parsecs away from the central engine.
This would prevent the important contribution at high-energies from the Compton
scattering of seed photons from the accretion disk and the broad-line region
that is generally used to model the spectral energy distribution of
low-frequency peaking blazars. While a pure synchrotron self-Compton model has
so far failed to explain the observed gamma-ray emission of a flat spectrum
radio quasar like 3C 279, the inclusion of the effect of multiple
inverse-Compton scattering might solve the apparent paradox. Here, we present
for the first time a physical, self-consistent SSC modeling of a series of
shock-waves in the jet of 3C 279. We show that the analytic description of the
high-energy emission from multiple inverse-Compton scatterings in the
Klein-Nishina limit can fairly well account for the observed gamma-ray spectrum
of 3C 279 in flaring states.Comment: 6 pages, 3 figures, proceedings of "Beamed and Unbeamed Gamma-rays
from Galaxies", 11-15 April 2011, Finland. To be published in the Journal of
Physics: Conference Serie
VLBI Observations of SN 2008D
We report on two epochs of very-long-baseline interferometry (VLBI)
observations of the Type Ib/c supernova SN 2008D, which was associated with the
X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the
explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days,
at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the
source's size and position at each epoch, and thus constrain its expansion
velocity and proper motion. We find the source at best marginally resolved at
both epochs, allowing us to place a 3sigma upper limit of ~0.75c on the
expansion velocity of a circular source. For an elongated source, our
measurements are compatible with mildly relativistic expansion. However, our
3sigma upper limit on the proper motion is 4 micro-arcsec/day, corresponding to
an apparent velocity of <0.6c, and is consistent with a stationary flux
centroid. This limit rules out a relativistic jet such as an gamma-ray burst
jet away from the line of sight, which would be expected to show apparent
proper motion of >c. Taken together, our measurements argue against the
presence of any long-lived relativistic outflow in SN 2008D. On the other hand,
our measurements are consistent with the nonrelativistic expansion velocities
of <30,000 km/s and small proper motions (<500 km/s) seen in typical
supernovae.Comment: Accepted for publication in the Astrophysical Journal Letter
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