19,429 research outputs found
The Discovery of Vibrationally-Excited H_2 in the Molecular Cloud near GRB 080607
GRB 080607 has provided the first strong observational signatures of
molecular absorption bands toward any galaxy hosting a gamma-ray burst. Despite
the identification of dozens of features as belonging to various atomic and
molecular (H_2 and CO) carriers, many more absorption features remained
unidentified. Here we report on a search among these features for absorption
from vibrationally-excited H_2, a species that was predicted to be produced by
the UV flash of a GRB impinging on a molecular cloud. Following a detailed
comparison between our spectroscopy and static, as well as dynamic, models of
H_2* absorption, we conclude that a column density of 10^{17.5+-0.2} cm^{-2} of
H_2* was produced along the line of sight toward GRB 080607. Depending on the
assumed amount of dust extinction between the molecular cloud and the GRB, the
model distance between the two is found to be in the range 230--940 pc. Such a
range is consistent with a conservative lower limit of 100 pc estimated from
the presence of Mg I in the same data. These distances show that substantial
molecular material is found within hundreds of pc from GRB 080607, part of the
distribution of clouds within the GRB host galaxy.Comment: Submitted to ApJL, 6 pages emulate
Deposition of general ellipsoidal particles
We present a systematic overview of granular deposits composed of ellipsoidal
particles with different particle shapes and size polydispersities. We study
the density and anisotropy of such deposits as functions of size polydispersity
and two shape parameters that fully describe the shape of a general ellipsoid.
Our results show that, while shape influences significantly the macroscopic
properties of the deposits, polydispersity plays apparently a secondary role.
The density attains a maximum for a particular family of non-symmetrical
ellipsoids, larger than the density observed for prolate or oblate ellipsoids.
As for anisotropy measures, the contact forces show are increasingly preferred
along the vertical direction as the shape of the particles deviates for a
sphere. The deposits are constructed by means of an efficient molecular
dynamics method, where the contact forces are efficiently and accurately
computed. The main results are discussed in the light of applications for
porous media models and sedimentation processes.Comment: 7 pages, 8 figure
Expected characteristics of the subclass of Supernova Gamma-ray Bursts (S-GRBs)
The spatial and temporal coincidence between the gamma-ray burst (GRB) 980425
and supernova (SN) 1998bw has prompted speculation that there exists a class of
GRBs produced by SNe (``S-GRBs''). Robust arguments for the existence of a
relativistic shock have been presented on the basis of radio observations. A
physical model based on the radio observations lead us to propose the following
characteristics of supernovae GRBs (S-GRBs): 1) prompt radio emission and
implied brightness temperature near or below the inverse Compton limit, 2) high
expansion velocity of the optical photosphere as derived from lines widths and
energy release larger than usual, 3) no long-lived X-ray afterglow, and 4) a
single pulse (SP) GRB profile. Radio studies of previous SNe show that only
type Ib and Ic potentially satisfy the first condition. Accordingly we have
investigated proposed associations of GRBs and SNe finding no convincing
evidence (mainly to paucity of data) to confirm any single connection of a SN
with a GRB. If there is a more constraining physical basis for the burst
time-history of S-GRBs beyond that of the SP requirement, we suggest the 1% of
light curves in the BATSE catalogue similar to that of GRB 980425 may
constitute the subclass. Future optical follow-up of bursts with similar
profiles should confirm if such GRBs originate from some fraction of SN type
Ib/Ic.Comment: 11 pages of LaTeX with 1 figure. Submitted to the Astrophysical
Journal Letter
Convergence of random zeros on complex manifolds
We show that the zeros of random sequences of Gaussian systems of polynomials
of increasing degree almost surely converge to the expected limit distribution
under very general hypotheses. In particular, the normalized distribution of
zeros of systems of m polynomials of degree N, orthonormalized on a regular
compact subset K of C^m, almost surely converge to the equilibrium measure on K
as the degree N goes to infinity.Comment: 16 page
Undirected Graphs of Entanglement Two
Entanglement is a complexity measure of directed graphs that origins in fixed
point theory. This measure has shown its use in designing efficient algorithms
to verify logical properties of transition systems. We are interested in the
problem of deciding whether a graph has entanglement at most k. As this measure
is defined by means of games, game theoretic ideas naturally lead to design
polynomial algorithms that, for fixed k, decide the problem. Known
characterizations of directed graphs of entanglement at most 1 lead, for k = 1,
to design even faster algorithms. In this paper we present an explicit
characterization of undirected graphs of entanglement at most 2. With such a
characterization at hand, we devise a linear time algorithm to decide whether
an undirected graph has this property
Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators
A density perturbation produced in an underdense plasma was used to improve
the quality of electron bunches produced in the laser-plasma wakefield
acceleration scheme. Quasi-monoenergetic electrons were generated by controlled
injection in the longitudinal density gradients of the density perturbation. By
tuning the position of the density perturbation along the laser propagation
axis, a fine control of the electron energy from a mean value of 60 MeV to 120
MeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%,
divergence of 4 +/- 0.8 mrad and charge of 6 +/- 1.8 pC.Comment: 7 pages, 8 figure
Making mentoring work: The need for rewiring epistemology
To help produce expert coaches at both participation and performance levels, a number of governing bodies have established coach mentoring systems. In light of the limited literature on coach mentoring, as well as the risks of superficial treatment by coach education systems, this paper therefore critically discusses the role of the mentor in coach development, the nature of the mentor-mentee relationship and, most specifically, how expertise in the mentee may best be developed. If mentors are to be effective in developing expert coaches then we consequently argue that a focus on personal epistemology is required. On this basis, we present a framework that conceptualizes mentee development on this level through a step by step progression, rather than unrealistic and unachievable leap toward expertise. Finally, we consider the resulting implications for practice and research with respect to one-on-one mentoring, communities of practice, and formal coach education
Multiwaveband Observations of Quasars with Flat Radio Spectra and Strong Millimeter Emission
We present multiwaveband observations of a well selected sample of 28 quasars
and two radio galaxies with flat radio spectra and strong millimeter wave
emission (referred to here as FSRQ's). The observations include multifrequency
VLBI measurements, X-ray observations with ROSAT and submillimeter observations
with the JCMT. Particularly interesting among many findings is a correlation
between the X-ray to millimeter spectral index and fraction of flux density
contained in the VLBI core. This tendency toward higher X-ray fluxes from
sources with stronger jet emission implies that the knots in the jet are the
prominent source of X-rays.Comment: 38 pages, 17 figures, 12 tables, accepted for publication in Ap J
Suppl, May 199
GRB 000418: A Hidden Jet Revealed?
We report on optical, near-infrared and centimeter radio observations of
GRB000418 which allow us to follow the evolution of the afterglow from 2 to 200
days after the gamma-ray burst. In modeling these broad-band data, we find that
an isotropic explosion in a constant density medium is unable to simultaneously
fit both the radio and optical data. However, a jet-like outflow with an
opening angle of 10-20 degress provides a good description of the data. The
evidence in favor of a jet interpretation is based on the behavior of the radio
light curves, since the expected jet break is masked at optical wavelengths by
the light of the host galaxy. We also find evidence for extinction, presumably
arising from within the host galaxy, with A(V)=0.4 mag, and host flux densities
of F_R=1.1 uJy and F_K=1.7 uJy. These values supercede previous work on this
burst due to the availability of a broad-band data set allowing a global
fitting approach. A model in which the GRB explodes into a wind-stratified
circumburst medium cannot be ruled out by these data. However, in examining a
sample of other bursts (e.g. GRB990510, GRB000301C) we favor the jet
interpretation for GRB000418.Comment: ApJ, submitte
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