3,298 research outputs found
Absorption of High Energy Gamma-Rays by Low Energy Intergalactic Photons
Following our previously proposed technique, we have used the recent
gamma-ray observations of Mrk 421 to place theoretically significant
constraints on and possible estimates of the intergalactic infrared radiation
field (IIRF) which are consistent with normal galactic IR production by stars
and dust and rule out exotic mechanisms proposed to produce a larger IIRF.
Using models for the low energy intergalactic photon spectrum from microwave to
UV energies, we calculate the opacity of inter- galactic space to gamma-rays as
a function of energy and redshift. These calculations indicate that the GeV
gamma-ray burst recently observed by the EGRET experiment on CGRO originates at
a redshift less than approximately 1.5.Comment: 12 pg., uuencoded, Z-compressed ps file (includes figures), To be
published in Space Sci. Re
A principled approach to the measurement of situation awareness in commercial aviation
The issue of how to support situation awareness among crews of modern commercial aircraft is becoming especially important with the introduction of automation in the form of sophisticated flight management computers and expert systems designed to assist the crew. In this paper, cognitive theories are discussed that have relevance for the definition and measurement of situation awareness. These theories suggest that comprehension of the flow of events is an active process that is limited by the modularity of attention and memory constraints, but can be enhanced by expert knowledge and strategies. Three implications of this perspective for assessing and improving situation awareness are considered: (1) Scenario variations are proposed that tax awareness by placing demands on attention; (2) Experimental tasks and probes are described for assessing the cognitive processes that underlie situation awareness; and (3) The use of computer-based human performance models to augment the measures of situation awareness derived from performance data is explored. Finally, two potential example applications of the proposed assessment techniques are described, one concerning spatial awareness using wide field of view displays and the other emphasizing fault management in aircraft systems
Elliptic Flow and Fixed p_T Suppression in a Final State Interaction Model
It has been shown that a final state interaction model, used to describe
J/psi suppression, can also describe the fixed p_T suppression of the pi^0 (and
charged pions) yield at all values of p_T, with a final state interaction
cross-section sigma close to one milibarn. We propose an extension of the model
to the pion motion in the transverse plane - which introduces a dependence of
the suppression on the azimuthal angle theta_R. Using the same value of sigma,
we obtain values of the elliptic flow v_2 close to the experimental ones, for
all values of p_T, including the soft p_T region.Comment: 21 pages, 6 figure
2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System
We have developed a general purpose dust radiative transfer code for an
axisymmetric system, 2-Dust, motivated by the recent increasing availability of
high-resolution images of circumstellar dust shells at various wavelengths.
This code solves the equation of radiative transfer following the principle of
long characteristic in a 2-D polar grid while considering a 3-D radiation field
at each grid point. A solution is sought through an iterative scheme in which
self-consistency of the solution is achieved by requiring a global luminosity
constancy throughout the shell. The dust opacities are calculated through Mie
theory from the given size distribution and optical properties of the dust
grains. The main focus of the code is to obtain insights on (1) the global
energetics of dust grains in the shell (2) the 2-D projected morphologies that
are strongly dependent on the mixed effects of the axisymmetric dust
distribution and inclination angle of the shell. Here, test models are
presented with discussion of the results. The code can be supplied with a
user-defined density distribution function, and thus, is applicable to a
variety of dusty astronomical objects possessing the axisymmetric geometry.Comment: To be published in ApJ, April 2003 issue; 13 pages, 4 tables, 17
figures, 5-page appendix (no figures for the main text included in this
preprint). For the complete preprint and code distribution, contact the
author
Effective Lagrangian approach to nuclear mu-e conversion and the role of vector mesons
We study nuclear mu-e conversion in the general framework of an effective
Lagrangian approach without referring to any specific realization of the
physics beyond the standard model (SM) responsible for lepton flavor violation
(LFV). We examine the impact of a specific hadronization prescription on the
analysis of new physics in nuclear mu-e conversion and stress the importance of
vector meson exchange between lepton and nucleon currents. A new issue of this
mechanism is the presence of the strange quark vector current contribution
induced by the phi meson. This allows us to extract new limits on the LFV
lepton-quark effective couplings from the existing experimental data.Comment: 19 pages, 3 figures, to be published in Phys Rev
Extragalactic Gamma-ray Absorption and the Intrinsic Spectrum of Mkn 501 During the 1997 Flare
Using the recent models of Malkan & Stecker (2001) for the infrared
background radiation and extrapolating them into the optical and UV range using
recent galaxy count data, we rederive the optical depth of the Universe to high
energy gamma-rays as a function of energy and redshift for energies between 50
GeV and 100 TeV and redshifts between 0.03 and 0.3. We then use these results
to derive the intrinsic gamma-ray spectrum of Mkn 501 during its 1997 high
state. We find that the time averaged spectral energy distribution of Mkn 501
while flaring had a broad, flat peak in the 5 to 10 TeV range which corresponds
to a broad, flat time averaged X-ray peak in the 50 to 100 keV range observed
during the flare. The spectral index of our derived intrinsic differential
photon spectrum for Mkn 501 at energies below about 2 TeV was found to be apx.
1.6 to 1.7. This corresponds to a time averaged spectral index of 1.76 found in
soft X-rays at energies below the X-ray (synchrotron) peak. These results
appear to favor a synchrotron-self Compton origin for the TeV emission together
wuth jet parameters which are consistent with time variability constraints
within the context of a simple SSC model.Comment: Final version, to appear in the Astrophysical Journa
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