On the universal GeV emission in short GRBs

Within the classification of gamma-ray bursts (GRBs) in different subclasses we give further evidence that short bursts, originating from binary neutron star (B-NS) progenitors, exist in two subclasses: the short gamma-ray flashes (S-GRFs) and the short gamma-ray bursts (S-GRBs). It has already been shown that S-GRFs occur when the B-NS mergers lead to a massive neutron star (M-NS), having the isotropic energy $\lesssim$ $10^{52}$ erg and a soft spectrum with a peak at a value of $E_{\rm p,i}\sim 0.2$--$2$ MeV. Similarly, S-GRBs occur when B-NS merging leads to the formation of a black hole (BH), with isotropic energy $\gtrsim 10^{52}$ erg and a hard spectrum with a peak at a value of $E_{\rm p,i}\sim 2$--$8$ MeV. We here focus on 18 S-GRFs and 6 S-GRBs, all with known or derived cosmological redshifts following \textit{Fermi}-LAT observations. We evidence that \textit{all} S-GRFs have no GeV emission. The S-GRBs \textit{all} have GeV emission and their $0.1$--$100$ GeV luminosity light-curves as a function of time in the rest-frame follow a universal power-law, $L(t)= (0.88\pm 0.13) \times 10^{52}~t^{-(1.29 \pm 0.06)}$~erg~s$^{-1}$. From the mass formula of a Kerr BH we can correspondingly infer for S-GRBs a minimum BH mass in the range of $2.24$--$2.89 M_\odot$ and a corresponding maximum dimensionless spin in the range of $0.18$--$0.33$.