12,398 research outputs found
A Compact Fireball Model of Gamma Ray Bursts
It is proposed that the gamma ray burst photons near the peak of the spectrum
at several hundred KeV are produced on very compact scales, where photon
production is limited by blackbody effects and/or the requirement of energetic
quanta () for efficient further production. The fast variation of
order milliseconds in the time profile is then a natural expectation, given the
other observed GRB parameters. Analytic calculations are presented to show that
the escape of non-thermal, energetic gamma rays can emerge within a second of
the thermal photons from a gammasphere of below cm. The minimum
asymptotic bulk Lorentz factor in this model is found to be of order several
hundred if the photosphere is of order cm and greater for
larger or smaller photospheric radii. It is suggested that prompt UHE gamma
rays might provide a new constraint on the asymptotic Lorentz factor of the
outflow.Comment: To appear in ApJ, revisions requested by the refere
The laboratory of clinical virology in monitoring patients undergoing monoclonal antibody therapy
AbstractThe relevant efficacy of monoclonal antibodies (mAbs) has resulted in the successful treatment of several diseases, although susceptibility to infections remains a major problem. This review summarizes aspects of the literature regarding viral infections and mAbs, specifically addressing the risk of infection/reactivation, the measures that can reduce this risk, and the role played by the laboratory of clinical virology in monitoring patients undergoing mAb therapy
Preliminary Abundance Analysis of Galactic Bulge Main Sequence, Subgiant, and Giant Branch Stars Observed During Microlensing with Keck/HIRES
We present an abundance analysis of six main sequence turnoff, subgiant, and
giant branch stars toward the Galactic bulge that were observed with Keck/HIRES
during microlensing events. This is an early look at the first detailed
chemical analysis of main sequence stars in the Galactic bulge. Lensing events
allow the effective aperture of Keck to be increased beyond its current
dimensions; although, some events still stretched its spectroscopic
capabilities. Future large telescopes with high resolution and high throughput
spectrometers will allow the study of abundances in distant stellar populations
and in less evolved stars with greater ease.Comment: 8 pages including 2 figures. To appear in SPIE proceedings on
Astronomical Telescopes and Instrumentation. Uses spie.cl
The Theory Behind TheoryMine
Abstract. We describe the technology behind the TheoryMine novelty gift company, which sells the rights to name novel mathematical theorems. A tower of four computer systems is used to generate recursive theories, then to speculate conjectures in those theories and then to prove these conjectures. All stages of the process are entirely automatic. The process guarantees large numbers of sound, novel theorems of some intrinsic merit.
CFD Analysis of Helicopter Wakes in Ground Effect
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios.
The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground
changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical
analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the
need for detailed surveys of helicopter wakes using full-scale physical experiments
Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?
Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV
emission from the galactic bulge and the high-energy positron excess inferred
from the ATIC, PAMELA, and Fermi data. While independent explanations can be
responsible for these phenomena, we explore the possibility of their common
GRB-related origin by modeling the GRB distribution and estimating the rates.
For an expected Milky Way long GRB rate, neither of the two signals is generic;
the local excess requires a 2% coincidence, while the signal from the galactic
center requires a 20% coincidence with respect to the timing of the latest GRB.
The simultaneous explanation requires a 0.4% coincidence. Considering the large
number of statistical "trials" created by multiple searches for new physics,
the coincidences of a few per cent cannot be dismissed as unlikely.
Alternatively, both phenomena can be explained by GRBs if the galactic rate is
higher than expected. We also show that a similar result is difficult to obtain
assuming a simplified short GRB distribution.Comment: 4 pages; version accepted for publicatio
The Expected Duration of Gamma-Ray Bursts in the Impulsive Hydrodynamic Models
Depending upon the various models and assumptions, the existing literature on
Gamma Ray Bursts (GRBs) mentions that the gross theoretical value of the
duration of the burst in the hydrodynamical models is tau~r^2/(eta^2 c), where
r is the radius at which the blastwave associated with the fireball (FB)
becomes radiative and sufficiently strong. Here eta = E/Mc^2, c is the speed of
light, E is initial lab frame energy of the FB, and M is the baryonic mass of
the same (Rees and Meszaros 1992). However, within the same basic framework,
some authors (like Katz and Piran) have given tau ~ r^2 /(eta c). We intend to
remove this confusion by considering this problem at a level deeper than what
has been considered so far. Our analysis shows that none of the previously
quoted expressions are exactly correct and in case the FB is produced
impulsively and the radiative processes responsible for the generation of the
GRB are sufficiently fast, its expected duration would be tau ~ar^2/(eta^2 c),
where a~O(10^1). We further discuss the probable change, if any, of this
expression, in case the FB propagates in an anisotropic fashion. We also
discuss some associated points in the context of the Meszaros and Rees
scenario.Comment: 21 pages, LATEX (AAMS4.STY -enclosed), 1 ps. Fig. Accepted in
Astrophysical Journa
A two-rigid block model for sliding gravity retaining walls
This paper presents a new two rigid block model for sliding gravity retaining walls. Some conceptual limitations of a direct application of Newmark's sliding block method to the case of retaining walls are discussed with reference to a simple scheme of two interacting rigid blocks on an inclined plane. In particular, it is shown that both the internal force between the blocks and their absolute acceleration are not constant during sliding, and must be computed by direct consideration of the dynamic equilibrium and kinematic constraints for the whole system. The same concepts are extended to the analysis of the active soil wedge-wall system, leading to an extremely simple procedure to compute the relative displacements of the wall when subjected to base accelerations exceeding the critical value. A comparison with the results of numerical analyses demonstrates that the proposed method is capable of describing fully the kinematics of the soil wedge-wall system under dynamic loading. On the contrary, direct application of Newmark's method may lead to inaccurate predictions of the final displacements, in excess or in defect depending on a coefficient, which emerges from direct consideration of the dynamic equilibrium of the whole system. This coefficient can be viewed as a corrective factor for the horizontal relative acceleration of the wall, related to the mechanical and geometrical properties of the soil-wall system
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