2,392 research outputs found
The Enigmatic Radio Afterglow of GRB 991216
We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of
the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The
optical and X-ray afterglow of this burst were fairly typical and are explained
by a jet fireball. In contrast, the radio light curve is unusual in two
respects: (a) the radio light curve does not show the usual rise to maximum
flux on timescales of weeks and instead appears to be declining already on day
1 and (b) the power law indices show significant steepening from the radio
through the X-ray bands. We show that the standard fireball model, in which the
afterglow is from a forward shock, is unable to account for (b) and we conclude
that the bulk of the radio emission must arise from a different source. We
consider two models, neither of which can be ruled out with the existing data.
In the first (conventional) model, the early radio emission is attributed to
emission from the reverse shock as in the case of GRB 990123. We predict that
the prompt optical emission would have been as bright (or brighter) than 8th
magnitude. In the second (exotic) model, the radio emission originates from the
forward shock of an isotropically energetic fireball (10^54 erg) expanding into
a tenuous medium (10^-4 cm^-3). The resulting fireball would remain
relativistic for months and is potentially resolvable with VLBI techniques.
Finally, we note that the near-IR bump of the afterglow is similar to that seen
in GRB 971214 and no fireball model can explain this bump.Comment: ApJ, submitte
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
Vortex Dynamics in Selfdual Maxwell-Higgs Systems with Uniform Background Electric Charge Density
We introduce selfdual Maxwell-Higgs systems with uniform background electric
charge density and show that the selfdual equations satisfied by topological
vortices can be reduced to the original Bogomol'nyi equations without any
background. These vortices are shown to carry no spin but to feel the Magnus
force due to the shielding charge carried by the Higgs field. We also study the
dynamics of slowly moving vortices and show that the spin-statistics theorem
holds to our vortices.Comment: 24 pages + 2 figures ( not included), Cu-TP-611, IASSNS-HEP-93/33,
NSF-ITP-93-13
Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment
Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla, in the River Fre´mur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996
The Broadband Afterglow of GRB980329
We present radio observations of the afterglow of the bright gamma-ray burst
GRB980329 made between one month and several years after the burst, a
re-analysis of previously published submillimeter data, and late-time optical
and near-infrared (NIR) observations of the host galaxy. From the absence of a
spectral break in the optical/NIR colors of the host galaxy, we exclude the
earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our
data with the numerous multi-wavelength observations of the early afterglow,
fit a comprehensive afterglow model to the entire broadband dataset, and derive
fundamental physical parameters of the blast-wave and its host environment.
Models for which the ejecta expand isotropically require both a high
circumburst density and extreme radiative losses from the shock. No low density
model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}
erg, with the ejecta narrowly collimated to an opening angle of a few degrees,
driven into a surrounding medium with density ~ 20 cm^{-3}, provides a
satisfactory fit to the lightcurves over a range of redshifts.Comment: 27 pages, incl. 6 figures, minor revisions (e.g. added/updated
references) Accepted by Ap
Discovery of Early Optical Emission from GRB 021211
We report our discovery and early time optical, near-infrared, and radio
wavelength follow-up observations of the afterglow of the gamma-ray burst GRB
021211. Our optical observations, beginning 21 min after the burst trigger,
demonstrate that the early afterglow of this burst is roughly three magnitudes
fainter than the afterglow of GRB 990123 at similar epochs, and fainter than
almost all known afterglows at an epoch of 1d after the GRB. Our near-infrared
and optical observations indicate that this is not due to extinction. Combining
our observations with data reported by other groups, we identify the signature
of a reverse shock. This reverse shock is not detected to a 3-sigma limit of
110 uJy in an 8.46-GHz VLA observation at t=0.10d, implying either that the
Lorentz factor of the burst gamma <~ 200, or that synchrotron self-absorption
effects dominate the radio emission at this time. Our early optical
observations, near the peak of the optical afterglow (forward shock), allow us
to characterize the afterglow in detail. Comparing our model to flux upper
limits from the VLA at later times, t >~ 1 week, we find that the late-time
radio flux is suppressed by a factor of two relative to the >~ 80 uJy peak flux
at optical wavelengths. This suppression is not likely to be due to synchrotron
self-absorption or an early jet break, and we suggest instead that the burst
may have suffered substantial radiative corrections.Comment: 13 pages, 2 figures, ApJL accepted; edits for lengt
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Curie: Constraining Solar System Bombardment Using In Situ Radiometric Dating
The Curie mission would constrain the existence of the putative cataclysm by determining the age of samples directly sourced from the impact melt sheet of a major pre-Imbrium lunar basin. The measurements would also enable further understanding of lunar evolution by characterizing new lunar lithologies far from the Apollo and Luna landing sites, including the very low-Ti basalts in Mare Crisium and potential olivine rich lithologies in the margins of both Mare Nectaris and Mars Crisium. Equipped with a mass spectrometer and a LIBS, Curie would also be well-placed to survey volatile components of the lunar regolith, including surface-bound hydrogen
The Computational Complexity of the Lorentz Lattice Gas
The Lorentz lattice gas is studied from the perspective of computational
complexity theory. It is shown that using massive parallelism, particle
trajectories can be simulated in a time that scales logarithmically in the
length of the trajectory. This result characterizes the ``logical depth" of the
Lorentz lattice gas and allows us to compare it to other models in statistical
physics.Comment: 9 pages, LaTeX, to appear in J. Stat. Phy
The Discovery and Broad-band Follow-up of the Transient Afterglow of GRB 980703
We report on the discovery of the radio, infrared and optical transient
coincident with an X-ray transient proposed to be the afterglow of GRB 980703.
At later times when the transient has faded below detection, we see an
underlying galaxy with R=22.6; this galaxy is the brightest host galaxy (by
nearly 2 magnitudes) of any cosmological GRB thus far. In keeping with an
established trend, the GRB is not significantly offset from the host galaxy.
Interpreting the multi-wavelength data in the framework of the popular fireball
model requires that the synchrotron cooling break was between the optical and
X-ray bands on July 8.5 UT and that the intrinsic extinction of the transient
is Av=0.9. This is somewhat higher than the extinction for the galaxy as a
whole, as estimated from spectroscopy.Comment: 5 pages, 3 figures, and 2 tables. Submitted to the Astrophysical
Journal Letters on 27 August 199
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