[vsnet-grb-info 8238] GRB 090621A: MAGIC telescope GeV observation

[GCN Circulars]

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  <garcz at mppmu.mpg.de>

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.