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