How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts

TL;DR

This paper proposes an empirical test to determine if gamma ray bursts are highly collimated jets or spherical fireballs by analyzing the evolution of their afterglow emissions over time.

Contribution

It introduces a novel observational test based on the changing beaming angle of afterglow emissions to distinguish between collimated jets and spherical models.

Findings

More optical and radio transients should be observed without gamma rays if bursts are collimated.

Existing supernova search data can be used to test gamma ray beaming models.

The evolution of afterglow emissions can reveal the geometry of gamma ray burst ejecta.

Abstract

If gamma ray bursts are highly collimated, the energy requirements of each event may be reduced by several (~ 4-6) orders of magnitude, and the event rate increased correspondingly. Extreme conditions in gamma ray bursters lead to highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in strong forward beaming of the emitted radiation in the observer's rest frame. Thus, all information on gamma ray bursts comes from those ejecta emitted in a narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at present ignorant of whether there are ejecta outside that cone or not. The recent detection of longer wavelength transients following gamma ray bursts allows an empirical test of whether gamma ray bursts are collimated jets or spherical fireballs. The bulk Lorentz factor of the burst ejecta will decrease with time after the event, as the ejecta sweep up the surrounding medium. Thus, radiation from the ejecta is beamed into an ever increasing solid angle as the burst remnant evolves. 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. Published supernova searches may contain enough data to test the most extreme models of gamma ray beaming. We close with a brief discussion of other possible consequences of beaming, including its effect on the evolution of burst remnants.