TITLE:   GCN CIRCULAR
NUMBER:  9764
SUBJECT: GRB 090621B: MAGIC telescope GeV observation
DATE:    09/08/11 07:10:46 GMT
FROM:    Markus Garczarczyk at MPI/MAGIC  <garcz@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 short burst GRB090621B (Curran et al., GCN Circ. 
9545). The burst was also triggered by Fermi-GBM (Goldstein et al., GCN 
Circ. 9562). We received the GCN alert at 22:07:38 UT (T0+13s). Data
taking with MAGIC started at 22:43:33 UT (T0+2169s), when the GRB zenith
angle went below 60 degrees. The observation continued for 12570 s.

No evidence for VHE gamma-ray emission above the analysis threshold 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, including a 30%
systematic uncertainty on the telescope efficiency.

Because of improving sensitivities and analysis threshold due to 
decreasing zenith angle during the observation, the data sample
was split into 4 parts:

E ( 300-1000 GeV): 0.42 * 10^-10 erg cm^-2 s^-1
for a time window from 22:43:33 UT to 23:38:37 UT
Analysis threshold: 407 GeV

E ( 175- 300 GeV): 5.61 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV): 0.19 * 10^-10 erg cm^-2 s^-1
for a time window from 23:38:37 UT to 00:32:31 UT
Analysis threshold: 291 GeV

E ( 175- 300 GeV): 4.97 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV): 0.24 * 10^-10 erg cm^-2 s^-1
for a time window from 00:32:31 UT to 01:23:06 UT
Analysis threshold: 224 GeV

E ( 125- 175 GeV): 15.27 * 10^-10 erg cm^-2 s^-1
E ( 175- 300 GeV):  1.25 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV):  0.22 * 10^-10 erg cm^-2 s^-1
for a time window from 01:23:06 UT to 02:13:04 UT
Analysis threshold: 186 GeV

We can also exclude emission of a constant flux in any 100s time bin
smaller than:

E ( 300-1000 GeV): 4.02 * 10^-10 erg cm^-2 s^-1
for a time window from 22:43:33 UT to 23:38:37 UT
Analysis threshold: 407 GeV

E ( 175- 300 GeV): 23.60 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV):  1.52 * 10^-10 erg cm^-2 s^-1
for a time window from 23:38:37 UT to 00:32:31 UT
Analysis threshold: 291 GeV

E ( 175- 300 GeV): 15.33 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV):  1.41 * 10^-10 erg cm^-2 s^-1
for a time window from 00:32:31 UT to 01:23:06 UT
Analysis threshold: 224 GeV

E ( 125- 175 GeV): 22.23 * 10^-10 erg cm^-2 s^-1
E ( 175- 300 GeV):  7.87 * 10^-10 erg cm^-2 s^-1
E ( 300-1000 GeV):  1.15 * 10^-10 erg cm^-2 s^-1
for a time window from 01:23:06 UT to 02:13:04 UT
Analysis threshold: 186 GeV

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