Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Variability in GRBs - A Clue   We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.

27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
15
The categories of this paper are: nucl-ex