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
