GCN Circular 5789
Gaug M. and Garczarczyk M., Bastieri D., Fagiolini M., Galante N., Longo F., Mizobuchi S., Piccioli A., Scapin V. and Stamerra A. report for the MAGIC collaboration: The MAGIC Imaging Atmospheric Cherenkov Telescope performed a follow-up observation of the SWIFT-BAT burst GRB061028 (GCN circular 5762, Sakamoto T. et al.). We received the GCN alert at T0+140s. The telescope slewed immediately to the GRB sky coordinates and started data taking 29s after the alert, at T0+169s. Due to the long time delay of the alert and the burst duration of T90=106s, only the early afterglow phase could be observed by MAGIC. Our observation window covered the weak tail emission measured by BAT and the X-ray afterglow measured by XRT (Sakamoto T. et al.). The observation continued for 100min. No evidence for VHE gamma-ray emission above the analysis threshold of 110GeV was found. 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 (inc. 30% systematic error on the absolute flux level): E(100-125 GeV) : 1.6 x 10^(-10) erg/cm^2/s E(125-175 GeV) : 0.8 x 10^(-10) erg/cm^2/s E(175-300 GeV) : 0.7 x 10^(-10) erg/cm^2/s E(300-1000 GeV) : 0.3 x 10^(-10) erg/cm^2/s The upper limits apply of the time window between T0+169s and T0+1969s (the first 30min of the MAGIC data sample). This message can be cited.