GCN Circular 11890

V. Savchenko, C. Ferrigno, E. Bozzo, M. Nikolajuk, M. Beck (ISDC), S.
Mereghetti (INAF/IASF-Milano). V. Beckmann (APC), Arne Rau (MPE), J.
Borkowski (CAMK/Torun), D. G�tz (CEA/Saclay), and A. von Kienlin (MPE)
report on behalf of the INTEGRAL SPI-ACS GRB team:

An exceptionally bright burst has been detected by the SPI
Anti-Coincidence System (ACS) on-board INTEGRAL at 2011-04-06T03:44:10.
The SPI-ACS light curve shows a single smooth peak with a peak
count rate of ~19000 counts/50msec of a duration of about 3 seconds.
The roughly estimated energy peak flux is of the order of 6e-5
erg/cm2/s. This was one of the brightest bursts ever detected with SPI-ACS.

The SPI-ACS light curves are available (both as images and data files)
at http://isdc.unige.ch/Soft/ibas/ibas_acs_web.cgi

The light curves, binned at 50 ms, are derived from 91 independent
detectors with different lower energy thresholds (mainly between 80 keV
and 150 keV) and an upper threshold at about 100 MeV. The ACS response
varies as a function of the GRB incident angle. For these reasons we
caution that the count rates cannot be easily translated into physical
flux units. It is not possible to localize a burst based on the SPI-ACS
data.