TITLE: GCN CIRCULAR NUMBER: 11890 SUBJECT: GRB 110406A: An exceptionally bright burst detected by INTEGRAL SPI-ACS DATE: 11/04/06 20:24:50 GMT FROM: Volodymyr Savchenko at ISDC,U of Geneve 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. Götz (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.