Context : GRB 050509b , detected by the Swift satellite , is the first case where an X-ray afterglow has been observed associated with a short gamma-ray burst ( GRB ) .
Within the fireshell model , the canonical GRB light curve presents two different components : the proper-GRB ( P-GRB ) and the extended afterglow .
Their relative intensity is a function of the fireshell baryon loading parameter B and of the CircumBurst Medium ( CBM ) density ( n _ { CBM } ) .
In particular , the traditionally called short GRBs can be either “ genuine ” short GRBs ( with B \lesssim 10 ^ { -5 } , where the P-GRB is energetically predominant ) or “ disguised ” short GRBs ( with B \gtrsim 3.0 \times 10 ^ { -4 } and n _ { CBM } \ll 1 , where the extended afterglow is energetically predominant ) .

Aims : We verify whether GRB 050509b can be classified as a “ genuine ” short or a “ disguised ” short GRB , in the fireshell model .

Methods : We investigate two alternative scenarios .
In the first , we start from the assumption that this GRB is a “ genuine ” short burst .
In the second attempt , we assume that this GRB is a “ disguised ” burst .

Results : If GRB 050509b were a genuine short GRB , there should initially be very hard emission which is ruled out by the observations .
The analysis that assumes that this is a disguised short GRB is compatible with the observations .
The theoretical model predicts a value of the extended afterglow energy peak that is consistent with the Amati relation .

Conclusions : GRB 050509b can not be classified as a “ genuine ” short GRB .
The observational data are consistent with a “ disguised ” short GRB classification , i.e. , a long burst with a weak extended afterglow “ deflated ” by the low density of the CBM .
We expect that all short GRBs with measured redshifts are disguised short GRBs because of a selection effect : if there is enough energy in the afterglow to measure the redshift , then the proper GRB must be less energetic than the afterglow .
The Amati relation is found to be fulfilled only by the extended afterglow excluding the P-GRB .