TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1162 SUBJECT: GRB010921 - Milagro GeV/TeV Observations DATE: 01/11/28 19:03:40 GMT FROM: Julie McEnery at U. Wisconsin Julie McEnery on behalf of the Milagro collaboration reports: Milagro observed GRB010921 at GeV/TeV energies during the burst duration (12 s) reported by the HETE WXM (GCN 1096). No evidence for GeV/TeV emission was found. A preliminary analysis (assuming a differential photon spectral index of -2.4) gives an upper limit on the fluence at the 99% confidence level of: J(E > 2 TeV) < 2 * 10^(-7) erg cm^(-2) The spectrum of the host galaxy of the proposed afterglow of GRB010921 implies a redhift of 0.45 (GCN 1108). We expect that TeV photons will be attenuated by pair production with infrared photons in intergalactic space so we also calculate an upper limit on the fluence, assuming a spectrum truncated at 150 GeV. In this case we find an upper limit on the fluence at the 99% confidence level of: J(100-150 GeV) < 4 * 10^(-5) erg cm^(-2) These upper limits are preliminary and will be refined with further analysis. These limits do not incorporate systematic uncertainties which may be of order 50%.