Spectral Lags of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs

Abstract

The γ‐ray instruments on board INTEGRAL detected and localised 47 GRBs from its launch in October 2002 up to July 2007. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all‐sky rate of GRBs above ∼0.15 ph cm−2 s−1 is ∼1400yr−1 in the fully coded field of view of IBIS. Spectral lags i.e. the time delay in the arrival of low‐energy γ‐rays with respect to high‐energy γ‐rays, are measured for 31 of the GRBs. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags 0.75 s. Most of the long‐lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X‐ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q = −0.225±0.090 and hence reflect the local...