Most theorists expect GRBs to be significantly beamed - many energetic astrophysical phenomena are (examples include young protostars; the so-called "microquasars'', which are black hole binaries in the Galaxy; radio galaxies; and AGN). And theorists desire beaming because it saves their models. Several speakers at this workshop have emphasized these points (see, e.g., Dar 1999; Fargion 1999, and Rees 1999). Strong beaming probably requires strong magnetic fields, but no detailed physical model of how this might happen has been put forward as yet.
One can ask: Where is the observational evidence for beaming...?
Fenimore (1999)
told us that there is none in the time histories of the
bursts themselves. Worse yet,
Greiner (1999)
reported that 
from an analysis of
the ROSAT all sky survey. This constraint may not be as strong as it
appears, because the duration of the temporal exposure in the ROSAT
all-sky survey is only a few hundred seconds, and thus the sensitivity
of the survey is relatively poor. Consequently, soft X-ray afterglows
would be detectable by the ROSAT all-sky survey only within a day or so
after the burst, when (in the relativistic external shock model of
afterglows - see below) the soft X-ray emission is still highly
beamed.
Constraints on so-called "orphan'' optical afterglows, and therefore
on the beaming of GRBs, will be strengthened by new low-z SN Ia
searches that will soon be underway. These searches will monitor an
area of the sky that is roughly ten times larger than that monitored by
current high-z SN Ia searches down to the same limiting magnitude
(
)
(Perlmutter 1999).
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