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 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.