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 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." This message may be cited.