[vsnet-grb-info 3972] GRB061028: MAGIC telescope GeV observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  5789
SUBJECT: GRB061028: MAGIC telescope GeV observation
DATE:    06/11/08 11:45:41 GMT
FROM:    Markus Garczarczyk at MPI/MAGIC  <garcz at mppmu.mpg.de>

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).

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