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GCN Circular 247

GRB990123, is the "radio flare" due to an inhomogeneous medium?
1999-02-03T18:17:17Z (25 years ago)
Xiangdong Shi at UCSD <>
Xiangdong Shi and Geza Gyuk (UC, San Diego):

In GCN#211,239 S. R. Kulkarni and D. A. Frail reported radio observations
of GRB990123, which show a "radio flare" on Jan. 24. GRB990123 appears
otherwise to be radio-quiet as of yet (GCN#200,212,239).  We suggest the
possibility that this "flare" resulted from the GRB external shock running
into a cloud off line-of-sight. This section of the shock was decelerated
efficiently and its sychrotron frequency quickly fell into the range of
radio waves. This inhomogeneity of the medium may explain the abruptness
and shortness of the radio emission.  The "flares" in other wavelengths
were brief and might also have been buried in the main afterglow.

We speculate that the cloud may have been composed of prior ejecta from
the gamma-ray burst progenitor. If there are still other nearby clouds, it
will be interesting to see whether the "flares" recur.

One implication of this hypothesis, if it is true, is that the GRB990123
is not likely to be strongly beamed (>10).  Its intrinsic energy output
would in turn be indeed huge.

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