3 Conclusions

The obtained upper limits show that INCA could observe GRBs with fluence $F \sim 10^{-5}\div 10 ^{-4}~{\rm erg}~{\rm cm}^{-2}$ in the $1~{\rm GeV}\div 1~{\rm TeV}$ energy range, occurring at small zenith angles. About 1/10 of the bursts observed by BATSE have a fluence larger than $10^{-5}~{\rm erg}~{\rm cm}^{-2}$ in the $20\div 300$ KeV region. Moreover fluences of few $10^{-5}~{\rm erg}~{\rm cm}^{-2}$ have been measured by EGRET in the $30~{\rm MeV}\div 5~{\rm GeV}$ energy range during GRB 940217 and in the $1~{\rm MeV}\div 1~{\rm GeV}$ energy range during GRB 930131. In both cases the spectrum slope was $\alpha\sim -2$ and no energy cutoff was visible up to some GeV, suggesting that a considerable fraction of GRBs could have a high energy tail. A slope $\alpha\sim -2$ implies equal amount of power per decade of energy, hence a fluence F>10^-5 erg cm^-2 is expected in the INCA energy region from the most intense bursts, provided the spectrum does not change its slope up to 1 TeV and the distance of the burst is $z\leq 0.1$ to avoid gamma ray absorption.