[vsnet-grb-info 5154] GRB070920A: MAGIC telescope GeV observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6854
SUBJECT: GRB070920A: MAGIC telescope GeV observation
DATE:    07/10/06 08:21:21 GMT
FROM:    Nicola Galante at MPI,Muenchen  <galante at mppmu.mpg.de>

Galante N. (MPI/Muenchen), Garczarczyk M. (MPI/Muenchen),
Gaug M. (IAC/Tenerife), Bastieri D. (U. Padova), Longo F. (INFN/Trieste)
and Scapin V. (U. Udine), for the MAGIC Collaboration

The MAGIC Imaging Atmospheric Cherenkov Telescope performed a follow-up
observation of the SWIFT/BAT burst GRB070920A (GCN circular 6805,
Grupe D., et al.). We received the GCN alert at T0+539s. The data taking
by MAGIC started at T0+690s, 2.5 minutes after the alert income.
We observed the sky positon for 36min spanning a zenith angle
in the range 53-50 degrees.

No evidence for VHE gamma-ray emission above the analysis threshold of
about 250 GeV was found. A preliminary analysis, taking advantage
for the first time of the fast 2GHz digitalization from the
new data acquisition system, and 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 efficiency):

E(225-300 GeV)  : 7.1 x 10^(-8) erg/cm^2
E(300-400 GeV)  : 4.6 x 10^(-8) erg/cm^2
E(400-1000 GeV) : 2.2 x 10^(-8) erg/cm^2
E(1-10 TeV)     : 0.53 x 10^(-8) erg/cm^2

The upper limits correspond to the time period between T0+690s and
T0+2490s. We can also exclude emission of a constant flux during our observation
in any 100s time bin higher than:

E(225-300 GeV)  : 1.9 x 10^(-7) erg/cm^2
E(300-400 GeV)  : 1.0 x 10^(-7) erg/cm^2
E(400-1000 GeV) : 2.4 x 10^(-8) erg/cm^2

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