1,052,381 research outputs found
A Late-Time Flattening of Afterglow Light Curves
We present a sample of radio afterglow light curves with measured decay
slopes which show evidence for a flattening at late times compared to optical
and X-ray decay indices. The simplest origin for this behavior is that the
change in slope is due to a jet-like outflow making a transition to
sub-relativistic expansion. This can explain the late-time radio light curves
for many but not all of the bursts in the sample. We investigate several
possible modifications to the standard fireball model which can flatten
late-time light curves. Changes to the shock microphysics which govern particle
acceleration, or energy injection to the shock (either radially or azimuthally)
can reproduce the observed behavior. Distinguishing between these different
possibilities will require simultaneous optical/radio monitoring of afterglows
at late times.Comment: ApJ, submitte
Spatial Relationship between Solar Flares and Coronal Mass Ejections
We report on the spatial relationship between solar flares and coronal mass
ejections (CMEs) observed during 1996-2005 inclusive. We identified 496
flare-CME pairs considering limb flares (distance from central meridian > 45
deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large
Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric
Observatory (SOHO). We investigated the flare positions with respect to the CME
span for the events with X-class, M-class, and C-class flares separately. It is
found that the most frequent flare site is at the center of the CME span for
all the three classes, but that frequency is different for the different
classes. Many X-class flares often lie at the center of the associated CME,
while C-class flares widely spread to the outside of the CME span. The former
is different from previous studies, which concluded that no preferred flare
site exists. We compared our result with the previous studies and conclude that
the long-term LASCO observation enabled us to obtain the detailed spatial
relation between flares and CMEs. Our finding calls for a closer flare-CME
relationship and supports eruption models typified by the CSHKP magnetic
reconnection model.Comment: 7 pages; 4 figures; Accepted by the Astrophysical Journa
No Radio Afterglow from the Gamma-Ray Burst of February 28, 1997
We present radio observations of the gamma-ray burster GRB 970228 made with
the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO)
spanning a range of postburst timescales from one to 300 days. A search for a
time-variable radio source was conducted covering an area which included a
fading X-ray source and an optical transient, both of which are thought to be
the long wavelength counterparts to the gamma-ray burst. At the position of the
optical transient sensitive limits between 10 uJy and 1 mJy can be placed on
the absence of a radio counterpart to GRB 970228 between 1.4 and 240 GHz. We
apply a simple formulation of a fireball model which has been used with some
success to reproduce the behavior of the optical and X-ray light curves. Using
this model we conclude that the radio non-detections are consistent with the
peak flux density of the afterglow lying between 20-40 uJy and it requires that
the optical flux peaked between 4 and 16 hours after the burst.Comment: ApJ Let (submitted
A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor
We present radio observations of GRB 020405 starting 1.2 days after the
burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and
spectral properties, we interpret the radio flare as emission from the reverse
shock. This scenario rules out a circumburst medium with a radial density
profile \rho ~ r^{-2} expected around a mass-losing massive star, since in that
case the reverse shock emission decays on the timescale of the burst duration
t~100 s. Using published optical and X-ray data, along with the radio data
presented here, we further show that a self-consistent model requires
collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a
consequence of the early jet break, the late-time (t>10 days) emission measured
with the Hubble Space Telescope significantly deviates from an extrapolation of
the early, ground-based data. This, along with an unusually red spectrum, F_\nu
\~ \nu^{-3.9}, strengthens the case for a supernova that exploded at about the
same time as GRB 020405, thus pointing to a massive stellar progenitor for this
burst. This is the first clear association of a massive progenitor with a
uniform medium, indicating that a \rho ~ r^{-2} profile is not a required
signature, and in fact may not be present on the lengthscales probed by the
afterglow in the majority of bursts.Comment: Submitted to ApJ; 14 pages, 2 tables, 3 figure
Tracing Galaxy Assembly: Tadpole Galaxies in the Hubble Ultra Deep Field
In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a
knot at one end plus an extended tail, resembling a tadpole. These "tadpole
galaxies" appear dynamically unrelaxed--presumably in an early merging
state--where tidal interactions likely created the distorted knot-plus-tail
morphology. Here we systematically select tadpole galaxies from the HUDF and
study their properties as a function of their photometric redshifts. In a
companion HUDF variability study, Cohen et al. (2005) revealed a total of 45
variable objects believed to be Active Galactic Nuclei (AGN). Here we show that
this faint AGN sample has no overlap with the tadpole galaxy sample, as
predicted by theoretical work. The tadpole morphology--combined with the lack
of overlap with the variable objects--supports the idea that these galaxies are
in the process of an early-stage merger event, i.e., at a stage that likely
precedes the "turn-on" of any AGN component and the onset of any point-source
variability.Comment: 7 pages, 4 figures. Accepted for publication by Astrophysical Journa
Meson PVV Interactions are determined by Quark Loops
We show that all abnormal parity three-body meson interactions can be
adequately described by quark loops, evaluated at zero external momentum, with
couplings determined by symmetry. We focus primarily on radiative
meson decays which involve one pseudoscalar. The agreement with experiment for
non-rare decays is surprisingly good and requires very few parameters, namely
the coupling constants and and some mixing angles.
This agreement extends to some three-body decays that are dominated by pion
pairs in a P-wave state.Comment: 21 pages, Revtex, one figur
Recent Experiments with Bose-Condensed Gases at JILA
We consider a binary mixture of two overlapping Bose-Einstein condensates in
two different hyperfine states of \Rb87 with nearly identical magnetic moments.
Such a system has been simply realized through application of radiofrequency
and microwave radiation which drives a two-photon transition between the two
states. The nearly identical magnetic moments afford a high degree of spatial
overlap, permitting a variety of new experiments. We discuss some of the
conditions under which the magnetic moments are identical, with particular
emphasis placed on the requirements for a time-averaged orbiting potential
(TOP) magnetic trap.Comment: 9 pages, 5 figures; corrected post-publication editio
Analytical and Experimental Evaluation of the Heat Transfer Distribution over the Surfaces of Turbine Vanes
Three airfoil data sets were selected for use in evaluating currently available analytical models for predicting airfoil surface heat transfer distributions in a 2-D flow field. Two additional airfoils, representative of highly loaded, low solidity airfoils currently being designed, were selected for cascade testing at simulated engine conditions. Some 2-D analytical methods were examined and a version of the STAN5 boundary layer code was chosen for modification. The final form of the method utilized a time dependent, transonic inviscid cascade code coupled to a modified version of the STAN5 boundary layer code featuring zero order turbulence modeling. The boundary layer code is structured to accommodate a full spectrum of empirical correlations addressing the coupled influences of pressure gradient, airfoil curvature, and free-stream turbulence on airfoil surface heat transfer distribution and boundary layer transitional behavior. Comparison of pedictions made with the model to the data base indicates a significant improvement in predictive capability
Turbulence in Clusters of Galaxies and X-Ray Line Profiles
Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas
inside clusters of galaxies significantly broaden X-ray emission lines. For
lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the
hydrodynamic broadening is significantly larger than the thermal broadening.
Since cluster of galaxies have a negligible optical depth for resonant
scattering in forbidden and intercombination lines of these ions, these lines
are not additionally broadened. At the same time, they are very intense, which
allows deviations of the spectrum from the Gaussian spectrum in the line wings
to be investigated. The line shape becomes an important indicator of bulk
hydrodynamic processes because the cryogenic detectors of new generation of
X-ray observatories will have a high energy resolution (from 5 eV for ASTRO-E2
to 1-2 eV for Constellation-X and XEUS). We use the spectral representation of
a Kolmogorov cascade in the inertial range to calculate the characteristic
shapes of X-ray lines. Significant deviations in the line profiles from the
Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the
exponential tails) are expected for large-scale turbulence. The kinematic SZ
effect and the X-ray line profile carry different information about the
hydrodynamic velocity distribution in clusters of galaxies and complement each
other, allowing the redshift, the peculiar velocity of the cluster, and the
bulk velocity dispersion to be measured and separatedComment: 29 pages, 12 figures, Astronomy Letters 2003, v.29, p.79
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