The Soft X-ray Imager ( SXI ) is one of the three instruments on board EXIST , a multi-wavelength observatory in charge of performing a global survey of the sky in hard X-rays searching for Sup er-massive Black Holes ( Grindlay & Natalucci , these Proceedings ) .
One of the primary objectives of EXIST is also to study with unprecedented sensitivity the most unknown high energy sources in the Universe , like high redshift GRBs , which will be pointed promptly by the Spacecraft by autonomous trigger based on hard X-ray localization on board .
The presence of a soft X-ray telescope with an effective area of \sim 950 cm ^ { 2 } in the energy band 0.2-3 keV and extended response up to 10 keV will allow to make broadband studies from 0.1 to 600 keV .
In particular , investigations of the spectra components and states of AGNs and monitoring of variability of sources , study of the prompt and afterglow emission of GRBs since the early phases , which will help to constrain the emission models and finally , help the identification of sources in the EXIST hard X-ray survey and the characterization of the transient events detected .
SXI will also perform surveys : a scanning survey with sky coverage \sim 2 \pi and a limiting flux of \sim 5 \times 10 ^ { -14 } cgs plus other serendipitous .