Soft Gamma‐Ray Repeaters in Nearby Galaxies: Rate, Luminosity Function, and Fraction among Short Gamma‐Ray Bursts

Abstract

It was suggested that some of the short-duration Gamma-Ray Bursts (GRB) are giant flares of Soft Gammaray 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-σ error region coincides with the apparent disk of one of 316 bright, star-forming galaxies found within 20 Mpc. I find a single match of GRB 000420B 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 with fluence above ∼ 10 −5 erg cm −2 is < 16% (< 27%) at the 95% (99.73%) confidence level. 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 (Cheng et al.), to derive the rate of giant flares with energy above 4 × 10 46 erg. I find that the rate of such giant flares is about (0.4 − 5) × 10 −4 yr −1 per SGR. This rate is marginally consistent with the observed Galactic rate, of a single giant flare with energy above 4 × 10 46 erg in 30 years. Comparison of the Galactic rate with the inferred extragalactic rate implies a gradual cut-off (or steepening) of the flare energy distribution at 3 × 10 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.

GRB 070610: A Curious Galactic Transient

“…Recently, hypergiant flares from magnetars in our own Galaxy and nearby galaxies have been found to contaminate the SHB sample. The Galactic rate of the hypergiant flares is likely 10 À3 yr À1 (Ofek 2007), much larger than the estimated Galactic SHB rate of 10 À6 yr À1 (Nakar et al 2006;Guetta & Piran 2006).…”

Long-Duration Radio Transients Lacking Optical Counterparts Are Possibly Galactic Neutron Stars

“…Assuming a fraction of 10 −4 of the energy released in hard X-rays/γ -rays is emitted in radio over 1 hr, a giant X-ray flare with luminosity L X ∼ 10 46 erg can be observed to a distance of a about 100 Mpc. However, the rate of SGR giant flares with such energy is about 10 −4 -10 −5 Mpc −3 yr −1 (Ofek 2007). Therefore, the expected observed rate of Galactic and extragalactic SGRs is at least 3 orders of magnitude smaller than the rate of long-duration radio transients.…”

Soft gamma repeaters activity in time

“…However, any rescaling of the galactic rate of HFs is extremely uncertain as only one event has been observed. Results presented in Popov & Stern (2006), Lazzati et al (2005), and Ofek (2007) show that the rate of extragalactic HFs is much lower than that expected from a naive scaling (based on starformation rate or supernova rate, etc.) given one observed event from SGR 1806-20 in our Galaxy.…”

“…Still, below I shall use a conservative estimate 0.02 GFs per year per source. Data on possible extragalactic SGR flares (Ofek 2007) is only marginally compatible with this rate, and I briefly discuss this problem below.…”

“…Previously several authors Popov & Stern 2006;Lazzati, Ghirlanda & Ghisellini 2005;Ofek 2007) addressed the problem of extragalactic SGR flares. They used different archive data (BATSE, IPN) to search for examples of extragalactic GFs and HFs, and to put limits on the fraction of SGR flares among short GRBs (see Nakar 2007 for a review on short GRBs and for some discussion of the fraction of SGR flares among them).…”