There is a strong evidence that gamma-ray bursters are at the
cosmological distances, but there is no rigorous proof yet. The most
important question we can ask at this time is:
how can one distance scale or another be proven?
Note, that all types of objects known to exist in our galaxy are always
found in other galaxies as soon as the detectors are sensitive enough to
uncover those objects at extragalactic distances. Therefore, we may have
full confidence that if the known gamma-ray bursts are associated with our
galaxy there should also be gamma-ray bursts associated with M31, which
is the nearest giant spiral, pretty much like our own
(Atteia & Hurley 1986). This issue cannot be resolved by the BATSE, at
least not yet (Hakkila et al. 1994), but
a new experiment with 
times BATSE's sensitivity should
resolve the issue (Harrison et al. 1994). If a concentration of very weak
bursts towards M31 is detected we shall have to accept the
fact that the BATSE bursters are in the corona of our Galaxy, establishing
a new type of a distribution of astronomical objects.
If no signature of M31 is found we shall have to accept the
cosmological origin of the bursts. I hope that all participants of
this debate agree with my assessment of the decisive role of the
experiment aimed at M31. Note, that extending the observed luminosity
function of gamma-ray bursts to the level 10 below the current BATSE
limit is almost certain to be interesting in its own right:
if the bursters are in the galactic corona we shall find where
the corona is truncated; if they are cosmological we shall find them
at larger redshifts and they may turn out to be useful for cosmology.
The current situation may be summarized as follows:
times more sensitive than BATSE will
definitely determine the distance scale by comparing the number of the
weakest bursts towards M31 with the number in any other direction.
arc second positions would
greatly improve the likelihood that counterparts of gamma-ray bursters
are finally detected. This may be a critical step towards the determination
of the nature of the bursters. A new interplanetary network would offer
the best opportunity to achieve this goal.
It is a great pleasure for me to acknowledge many helpful comments and suggestions by W. H. G. Lewin, R. J. Nemiroff, R. E. Rutledge, K. Z. Stanek and R. A. M. J. Wijers. This work was supported with the NASA grant NAG5-1901.
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