LOCALIZATIONS AND SPECTRA OF GAMMA-RAY BURSTS OBSERVED WITHIN THE SIDELOBES OF THE SIGMA TELESCOPE

Two cylindrical interruptions of the SIGMA telescope passive shield ac t as collimators, projecting arc-shaped images on the detector. The fi eld of view (FOV) of these collimators consists of significant sidelob es in the SIGMA sensitivity diagram. GRB localizations, provided by th is imaging system, can be combined with the annulus derived from trian gulation with other spacecraft, in order to provide a narrow window fo r the search for counterparts at other wavelengths. 4 cosmic gamma-ray bursts (GRBs) have been detected so far by the lower collimator. Loca lizations with an accuracy of a fraction of degree, as well as time st ructures and broad band spectroscopy in the 30-400 keV energy range, o f these 4 GRBs are presented.