  We present the results of the spectral analysis of a sample of short bright
$\gamma$--ray bursts (GRB) detected by BATSE and compare them with the average
and time resolved spectral properties of long bright bursts. While the spectral
parameters of short GRBs confirm, as expected from previous works based on the
hardness ratio, that they are harder than long events, we find that this
difference is mainly due to a harder low energy spectral component present in
short bursts, rather than to a (marginally) different peak energy. Intriguingly
our analysis also reveals that the emission properties of short GRBs are
similar to the first 2 s of long events. This might suggest that the central
engine of long and short GRBs is the same, just working for a longer time for
long GRBs. We find that short bursts do not obey the correlation between peak
frequency and isotropic emitted energy for any assumed redshift, while they can
obey the similar correlation between the peak frequency and isotropic emitted
luminosity. This is consistent with (although not a proof of) the idea that
short GRBs emit a $\gamma$--ray luminosity similar to long GRBs. If they indeed
obey the peak frequency -- isotropic luminosity relation, we can estimate the
redshift distribution of short bursts, which turns out to be consistent with
that of long bursts just with a slightly smaller average redshift.

Astrophysics