The Halo Beaming Model for Gamma‐Ray Bursts

We consider a model for gamma-ray bursts (GRBs) from high-velocity neutronstars in the galactic halo. In this model, bursters are born in the galacticdisk with large recoil velocities V_r, and GRBs are beamed to within emissioncones of half-angle \phi centered on V_r. We describe scenarios formagnetically -channeled GRBs that have such beaming characteristics. We thenmake detailed comparisons of this halo beaming model (HBM) to BATSE and PVOdata for GRB intensity & angular position distributions. Acceptable fits toobservations of over 1000 bursts are obtained for \phi = 15 - 30 degrees andfor a BATSE sampling depth ~ 180 kpc. Present data favor a truly isotropic(cosmological) model over the HBM, but not by a statistically compellingmargin. Bursters born in nearby external galaxies, such as M31, are almostentirely undetectable in the HBM because of misdirected beaming. We analyzeseveral refinements of the basic HBM: gamma-ray intensities that vary withangle from the beam axis; non-standard-candle GRB luminosity functions; andmodels including a subset of bursters that do not escape from the galaxy. Wealso discuss the energy budgets for the bursters, the origins of their recoils,and the physics of burst beaming and alignment. One possible physical model isbased on the magnetar model of soft gamma repeaters (SGRs). Empirical bounds onthe rate of formation and peculiar velocities of SGRs imply that there exist ~10^4 to ~ 10^7 aged SGRs in the galactic halo within a distance of 100 kpc. TheHBM gives an acceptable fit to observations only if it satisfies certainconditions (e.g. \phi ~ 20 deg) which are possible, but for which there existno clear & compelling theoretical justifications. The cosmological bursterhypothesis is more generic and thus more attractive in this sense. (AbbreviatedAbstract).Comment: ApJ accepted, 9 figures, AASTE