A long Gamma-Ray Burst ( GRB ) was detected with the instruments on board the INTEGRAL satellite on January 31 2003 .
Although most of the GRB , which lasted \sim 150 seconds , occurred during a satellite slew , the automatic software of the INTEGRAL Burst Alert System was able to detect it in near-real time .
Here we report the results obtained with the IBIS instrument , which detected GRB 030131 in the 15 keV - 200 keV energy range , and ESO/VLT observations of its optical transient .
The burst displays a complex time profile with numerous peaks .
The peak spectrum can be described by a single power law with photon index \Gamma \simeq 1.7 and has a flux of \sim 2 photons cm ^ { -2 } s ^ { -1 } in the 20-200 keV energy band .
The high sensitivity of IBIS has made it possible for the first time to perform detailed time-resolved spectroscopy of a GRB with a fluence of 7 \times 10 ^ { -6 } erg cm ^ { -2 } ( 20-200 keV ) .