9.4. New Puzzles from Afterglow observations
Afterglow observations fit well the fireball picture that was
developed for explaining the GRB phenomena. The available data is not
good enough to distinguish between different specific models. But in
the future we expect to be able to distinguish between those models
and even to be able to determine the parameters of the burst E and
0
(if the data is taken early enough), the surrounding ISM
density and the intrinsic parameters of the relativistic shock
e,
B and
p. Still the current data is
sufficient to raise new puzzles and present us with new questions.
Why afterglow accompany some GRBs and not others?
X-ray, Optical and radio afterglows have been observed in some bursts but not in others. According to the current model afterglow is produces when the ejecta that produced the GRB is shocked by the surrounding matter. Possible explanations to this puzzle invoke environmental effects. A detectable afterglow might be generated efficiently in some range of ISM densities and inefficiently in another. High ISM densities would slow down of the ejecta more rapidly. This could make some afterglows detectable and others undetectable. ISM absorption is another alternative. While most interstellar environments are optically thin to gamma-rays high density ISM regions can absorb and attenuate efficiently x-rays and optical radiation.
How can we explain the 1053 ergs required for isotropic emission
in GRB971214? As we discuss in the next section this
amount is
marginal for most models that are based on the formation of a compact
source. This problem can be resolved if we invoke beaming, with
~ 0.1. However, such
beaming would results in a break at
the light curve when the local Lorentz factor would reach a value of
1/. Such a break was not
seen in other afterglows for which
there are good data. Note that recently Perna & Loeb
[263]
inferred from the lack of radio transients that GRB beamns cannot be
very narrow. If typical GRBs are beamed, the beam width
should be larger than
6°.
SN1998bw (and the associated GRB980425) is a factor of a hundred nearer than a typical GRB (which are expected to be at z ~ 1). The corresponding (isotropic) gamma-ray energy, ~ 5 × 1047 ergs, is four order of magnitude lower than a regular burst. This can be in agreement with the peak flux distribution only if the bursts with such a low luminosity compose a very small fraction of GRBs. This leads naturally to the question is there an observational coincidence between GRBs and SNs? To which there are conflicting answers [264, 265, 266].