[vsnet-grb-info 469] GRB 021004 - Unusual Radio-to-Submm Spectrum

[GCN Circulars <gcncirc@lheawww.gsfc.nasa.gov>]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1612
SUBJECT: GRB 021004 - Unusual Radio-to-Submm Spectrum
DATE:    02/10/10 03:24:04 GMT
FROM:    Edo Berger at Caltech  <ejb@astro.caltech.edu>

E. Berger, S. R. Kulkarni D. A. Frail report on behalf of the larger
Caltech-NRAO-CARA GRB collaboration:

"We have continued to monitor the radio afterglow of GRB 021004 (GCN 1574)
with the VLA at 8.5 and 22.5 GHz.  Combining our observations with the
Ryle Telescope 15 GHz measurements (GCN 1588) and the IRAM Plateau de Bure
Interferometer 86 GHz measurement (GCN 1590) we find a spectral slope
nu^0.9 between 8.5 and 86 GHz.  There is no sign of curvature in the range
of 8.5 to 86 GHz as would be expected for a transition from
optically-thick (nu^2) to optically-thin (nu^1/3) emission.

Millimetric emission from an underlying host galaxy could steepen the
spectrum (from the reasonably expected nu^1/3).  However, the upper limit
at 232 GHz from IRAM PdBI (GCN 1590) indicates that there is no
significant millimeteric emission from a host galaxy.

It therefore appears that the radio spectrum is unusual compared to the
radio afterglows of most other well-studied afterglows.  However, we note
that the early spectrum of the radio afterglow of SN 1998bw/GRB 980425
(Kulkarni et al. Nature 395, 663, 1998) also did not conform to the
synchrotron spectrum expected from a single power law electron energy
spectrum with uniform magnetic fields.  Multiple components or some other
emission mechanism may be needed to explain this unusual spectrum."

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