SGRs in nearby galaxies: rate, luminosity function and fraction among short GRBs

Abstract

It was suggested that some of the short-duration Gamma-Ray Bursts (GRB) are giant flares of Soft Gamma-ray Repeaters (SGR) in nearby galaxies. To test this hypothesis, I have constructed a sample of 47 short GRBs, detected by the Inter-Planetary Network (IPN), for which the position is constrained by at least one annulus on the celestial sphere. For each burst, I have checked whether its IPN 3-sigma error region coincides with the apparent disk of one of 316 galaxies found within 20 Mpc. I find a single match of a GRB with M74, which could, however, be due to a chance coincidence. I estimate the IPN efficiency as a function of fluence and derive the galaxy sample completeness. I find that assuming there is a cut-off in the observed energy distribution of SGR flares at <10^47 erg, the fraction of SGRs among short GRBs is <16% (95% confidence). I estimate the number of active SGRs in each one of the galaxies in the sample, and combine it with the distances to these galaxies, the IPN efficiency, and the SGR flare energy distribution, to derive the rate of giant flares with energy above 4x10^46 erg. I find that the rate of such giant flares is about (0.4-5)x10^-4 yr^-1 per SGR. This rate is marginally consistent with the observed Galactic rate. Comparison of the Galactic rate with the inferred extragalactic rate implies a steepening of the flare energy distribution at <3x10^46 erg (95% confidence). Using the Galactic SGR flare rate, I set a lower limit of one percent on the fraction of SGR flares among short GRBs.