TITLE: GCN CIRCULAR NUMBER: 9763 SUBJECT: GRB 090621A: MAGIC telescope GeV observation DATE: 09/08/11 07:00:09 GMT FROM: Markus Garczarczyk at MPI/MAGIC Carosi A. (INAF Rome), Gaug M. (IAC Tenerife), Antonelli L.A. (INAF Rome), Bastieri D. (Univ. Padova), Becerra Gonzalez J. (IAC Tenerife), Covino S. (INAF Rome), Galante N. (MPI Munich), Garczarczyk M. (IFAE Barcelona), La Barbera A. (INAF Palermo), Longo F. (INFN Trieste), Scapin V. (Univ. Udine) and Spiro S. (INAF Rome) for the MAGIC collaboration The MAGIC Imaging Atmospheric Cherenkov Telescope performed a follow-up observation of the BAT burst GRB090621A (Curran et al., GCN Circ. 9540). The burst was also triggered by Fermi-GBM (Arne Rau, GCN Circ. 9556) We received the GCN alert at 04:23:38 UT (T0+55s). Data taking with MAGIC started at 04:36:18 UT (T0+815s), when the GRB zenith angle reached 46 degrees. The observation continued for 1833 s. No evidence for VHE gamma-ray emission above the analysis threshold of 196 GeV was found. The observation was carried out in twilight condition. A preliminary analysis, for the hypothesis of steady emission and assumption of a differential photon spectral index of -2.5, yields the following 95% CL differential flux upper limits, including a 30% systematic uncertainty on the telescope efficiency: E(175 - 300 GeV): 0.25 * 10^-10 erg cm^-2 s^-1 E(300 - 1000 GeV): 0.10 * 10^-10 erg cm^-2 s^-1 for the whole time of observation (from 04:36:18 UT to 05:06:51 UT). We can also exclude emission of a constant flux in any 100s time bin smaller than: E(175 - 300 GeV): 2.74 * 10^-10 erg cm^-2 s^-1 E(300 - 1000 GeV): 1.01 * 10^-10 erg cm^-2 s^-1 for a time window from 04:36:18 UT to 05:06:51 UT This message can be cited.