A search for giant flares from soft gamma-repeaters in nearby galaxies in the Konus-Wind short burst sample

The knowledge of the rate of soft gamma-ray repeater (SGR) giant flares is important for understanding the giant flare mechanism and the SGR energy budget in the framework of the magnetar model. We estimate the upper limit to the rate using the results of an extensive search for extragalactic soft gamma-repeater giant flares (GFs) among 140 short gamma-ray bursts detected between 1994 and 2010 by Konus-Wind using InterPlanetary Network (IPN) localizations and temporal parameters. We show that Konus-Wind and the IPN are capable of detecting GFs with energies of 2.3x10^46 erg (which is the energy of the GF from SGR 1806-20 assuming a distance of 15 kpc) at distances of up to about 30 Mpc and GFs with energies of <10^45 erg (which is the energy of the GF from SGR 0526-66) at distances of up to about 6 Mpc. Using a sample of 1896 nearby galaxies we found that only two bursts, GRB 051103 and GRB 070201, have a low chance coincidence probability between an IPN localization and a nearby galaxy. We found the upper limit to the fraction of GFs among short GRBs with fluence above ~5x10^-7 erg cm^-2 to be <8% (95% confidence level). Assuming that the number of active SGRs in nearby galaxies is proportional to their core-collapse supernova rate, we derived the one-sided 95% upper limit to the rate of GFs with energy output similar to the GF from SGR 1806-20 to be (0.6--1.2)x10^-4 Q_46^-1.5 yr^-1 per SGR, where Q_46 is the GF energy output in 10^46 erg.
Comment: 6 pages, 3 figures. Accepted for publication in MNRAS