We measure the local rates of “ low-luminosity ” ( LL-GRBs , i.e . L < 10 ^ { 48 \div 49 } erg/sec ) and “ high-luminosity ” Gamma-ray Bursts ( HL-GRBs ) .
The values are in the range \dot { n } _ { \circ } = 100 \div 1800 Gpc ^ { -3 } yr ^ { -1 } and \dot { n } _ { \circ } = 100 \div 550 Gpc ^ { -3 } yr ^ { -1 } , respectively , and the ratios to SNe-Ibc \sim 1 \% - 9 \% and 0.4 \% - 3 \% .
These data may suggest the existence of two physically distinct classes of GRBs in which LL-GRBs are ( intrinsically ) more frequent events than HL-GRBs .
However , with the present data we can not exclude the possibility of a single population of GRBs which give rise to both an isotropic low-luminous emission ( LL-GRBs : detectable only in nearby GRBs ) and to a highly collimated high-luminous emission ( HL-GRBs : detectable preferentially at high-z ) .
We compute also the rate of SNe-Ibc characterized by broad-lined spectra ( Hypernovae ) and found it to be about 1.5 \times 10 ^ { -4 } HNe yr ^ { -1 } 10 ^ { 10 } L _ { B \odot } ( i.e less than 10 \% of SNe-Ibc occurring in Spirals ) .
This result implies that the ratio HL-GRBs/HNe is smaller than 1 , possibly in the range 0.04–0.3 .
We have used the ratio between Hypernovae and LL-GRBs to constrain their beaming factor to f _ { b } ^ { -1 } \sim 10 or less .