The PdBI observations of GRB 970508 gave a significant detection at 86.2 GHz and a non-detection at 232 GHz. Studying the frequency dependencies of interstellar scintillation, we find that the 86.2 GHz fluxes are very likely intrinsic to the source. With high significance, the source has passed from detection to non-detection in the millimetre range between days 14 to 19 after the burst, without showing correlated flares in the optical or cm range.
The light curve at 86.2 GHz is not well fitted by the power law decline seen at higher frequencies, although it is not excluded. A better match would be the form "fast rise - nearly constant level - fast decline".
We conclude that mm observations provide an new aspect of the spectral development of GRB's, allowing to probe the interior of the relativistic shock fronts with a low risk of distortion by interstellar scintillation.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998