The prompt emission of Gamma Ray Bursts ( GRBs ) is usually well described by the Band function : two power-laws joined smoothly at a given break energy .
In addition to the Band component , a few bursts ( GRB941017 , GRB090510 , GRB090902B and GRB090926A ) show clear evidence for a distinct high-energy spectral component , which in some cases evolves independently from the prompt keV component and is well described by a power-law ( PL ) , sometimes with a cut-off energy ; this component is found to have long duration , even longer than the burst itself for all the four bursts .
Here we report the observation of an anomalous short duration high energy component in GRB980923 .
GRB980923 is one of the brightest Gamma-Ray Bursts ( GRBs ) observed by BATSE .
Its light curve is characterized by a rapid variability phase lasting \sim 40 s , followed by a smooth emission tail lasting \sim 400 s. A detailed joint analysis of BATSE ( LAD and SD ) and EGRET TASC data of GRB980923 reveles the presence of an anomalous keV to MeV component in the spectrum that evolves independently from the prompt keV one .
This component is well described by a PL with a spectral index of -1.44 and lasts only \sim 2 s ; it represents one of the three clearly separated spectral components identified in GRB980923 , the other two being the keV prompt emission , well described by the Band function and the tail , well fit by a Smoothly Broken Power Law ( SBPL ) .