  We study the spectral evolution of 13 short duration Gamma Ray Bursts (GRBs)
detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra
resolved in time at the level of 2-512 ms in the 8 keV-35 MeV energy range. We
find a strong correlation between the observed peak energy Ep and the flux P
within individual short GRBs. The slope of the Ep P^s correlation for
individual bursts ranges between ~0.4 and ~1. There is no correlation between
the low energy spectral index and the peak energy or the flux. Our results show
that in our 13 short GRBs Ep evolves in time tracking the flux. This behavior
is similar to what found in the population of long GRBs and it is in agreement
with the evidence that long GRBs and (the still few) short GRBs with measured
redshifts follow the same rest frame Ep-Liso correlation. Its origin is most
likely to be found in the radiative mechanism that has to be the same in both
classes of GRBs.

High Energy Astrophysical Phenomena