Evidence For and Against Collimation of Gamma Ray Bursts

The degree to which gamma ray bursts are collimated is now the dominant uncertainty in their energy requirements and event rates. In this review I begin with the reasons for studying GRB collimation, then discuss existing tests for collimation and their applications to date, and finally outline some possible future tests. The most important conclusions are that (1) mean collimation angles much tighter than 1 degree appear ruled out; (2) the collimation angle appears to vary from burst to burst (like most other GRB properties). Some alternative explanations of apparent collimation signatures remain, but it should be possible to distinguish them from true collimation with future data sets and may be possible already. New satellites, improved followup observations, and new tests for collimation all promise continued rapid progress in coming years.Comment: Invited review at Ninth Marcel Grossmann Meeting. 8 pages, uses World Scientific macros (included

Gamma Ray Burst Beaming Constraints from Afterglow Light Curves: GRB 970508

The beaming angle (zeta) is the main uncertainty in gamma ray burst energy requirements today. We summarize predictions for the light curves of beamed bursts, and model the R band light curve of GRB 970508 to derive zeta > 30 degrees. This yields an irreducible minimum energy requirement of 3.4 times 10^{49} ergs to power the afterglow alone.Comment: 2 pages, one embedded postscript figure. To appear in Astronomy and Astrophysics Supplement series (Rome conference proceedings issue). Uses "aa" documentclas

Taking Sides In Peacekeeping: Impartiality And The Future Of The United Nations

United Nations peacekeeping has undergone radical transformation in the new millennium. \u27Taking Sides in Peacekeeping\u27 explores this transformation and its implications, in what is the first conceptual and empirical study of impartiality in UN peacekeeping. The book challenges dominant scholarly approaches that conceive of norms as linear and static, conceptualizing impartiality as a \u27composite\u27 norm, one that is not free-standing but an aggregate of other principles-each of which can change and is open to contestation. Drawing on a large body of primary evidence, it uses the composite norm to trace the evolution of impartiality, and to illuminate the macro-level politics surrounding its institutionalization at the UN, as well as the micro-level politics surrounding its implementation in the Democratic Republic of the Congo, site of the largest and costliest peacekeeping mission in UN history. This book reveals that, despite a veneer of consensus, impartiality is in fact highly contested. As the collection of principles it refers to has expanded to include human rights and civilian protection, deep disagreements have arisen over what keeping peace impartially actually means. Beyond the semantics, the book shows how this contestation, together with the varying expectations and incentives created by the norm, has resulted in perverse and unintended consequences that have politicized peacekeeping and, in some cases, effectively converted UN forces into one warring party among many. The author assesses the implications of this radical transformation for the future of peacekeeping and for the UN\u27s role as guarantor of international peace and security

Afterglows as Diagnostics of Gamma Ray Burst Beaming

If gamma ray bursts are highly collimated, radiating into only a small fraction of the sky, the energy requirements of each event may be reduced by several (up to 4 - 6) orders of magnitude, and the event rate increased correspondingly. The large Lorentz factors (Gamma > 100) inferred from GRB spectra imply relativistic beaming of the gamma rays into an angle 1/Gamma. We are at present ignorant of whether there are ejecta outside this narrow cone. Afterglows allow empirical tests of whether GRBs are well-collimated jets or spherical fireballs. The bulk Lorentz factor decreases and radiation is beamed into an ever increasing solid angle as the burst remnant expands. It follows that if gamma ray bursts are highly collimated, many more optical and radio transients should be observed without associated gamma rays than with them. In addition, a burst whose ejecta are beamed into angle zeta undergoes a qualitative change in evolution when Gamma < 1/zeta: Before this, Gamma ~ r^{-3/2}, while afterwards, Gamma decays exponentially with r. This change results in a potentially observable break in the afterglow light curve. Successful application of either test would eliminate the largest remaining uncertainty in the energy requirements and space density of gamma ray bursters.Comment: 5 pages, LaTex, uses aipproc and psfig style files. To appear in the proceedings of the Fourth Huntsville Gamma Ray Burst Symposiu

A reduced star catalog containing 537 named stars

Positional and color magnitude data for the 537 named stars, are given. A brief translation of the star names and their source language is also presented

Supersonic transport lubrication system investigation, phase 2 Periodical report

Evaluation tests of ball bearings, face seals, and lubricants for use on Mach 3 aircraft gas turbine engine mainshaft

A technique for computation of star magnitudes relative to an optical sensor

The theory and techniques used to compute star magnitudes relative to any optical detector (such as the Mariner Mars 1971 Canopus star tracker) are described. Results are given relative to various star detectors

Cosmic Ray Rejection by Linear Filtering of Single Images

We present a convolution-based algorithm for finding cosmic rays in single well-sampled astronomical images. The spatial filter used is the point spread function (approximated by a Gaussian) minus a scaled delta function, and cosmic rays are identified by thresholding the filtered image. This filter searches for features with significant power at spatial frequencies too high for legitimate objects. Noise properties of the filtered image are readily calculated, which allows us to compute the probability of rejecting a pixel not contaminated by a cosmic ray (the false alarm probability). We demonstrate that the false alarm probability for a pixel containing object flux will never exceed the corresponding probability for a blank sky pixel, provided we choose the convolution kernel appropriately. This allows confident rejection of cosmic rays superposed on real objects. Identification of multiple-pixel cosmic ray hits can be enhanced by running the algorithm iteratively, replacing flagged pixels with the background level at each iteration.Comment: Accepted for publication in PASP (May 2000 issue). An iraf script implementing the algorithm is available from the author, or from http://sol.stsci.edu/~rhoads/ . 16 pages including 3 figures. Uses AASTeX aaspp4 styl