Possible Class of Nearby Gamma-Ray Burst / Gravitational Wave Sources

Abstract

A possible subclass of gamma‐ray bursts — those with few, wide pulses, spectral lags of order one to several seconds, and soft spectra — has been identified. Their Log[N]‐Log[Fp] distribution approximates a −3/2 power‐law, suggesting homogeneity and relatively nearby sources. These mostly dim bursts account for ∼ 50% of the BATSE sample of long bursts near that instrument’s trigger threshold, suggesting that this subluminous class constitutes a more common variety than the more familiar burst sources which lie at truly cosmological distances. Theoretical scenarios predicted such a class, motivated by their exemplar GRB 980425 (SN 1998bw) lying at a distance of ∼ 38 Mpc. The observations are explained by invoking off‐axis viewing of the GRB jet and/or bulk Lorentz factors of order a few. Long‐lag bursts show a tendency to concentrate near the Supergalactic Plane with a quadrupole moment of −0.10 ± 0.04, similar to that for SNe type Ib/c within the same volume. The rate of the observed subluminous bursts is of order 14 that of SNe Ib/c. Evidence for a sequential relationship between SNe Ib/c and GRBs is critiqued for two cases, as simultaneity of the SN and GRB events may be important for detection of the expected gravitational wave signal; at most, SN to GRB delays appear to be a few days. SN asymmetries and ultrarelativistic GRB jets suggest the possibility of rapid rotation in the pre‐collapse objects, a primary condition required for highly non‐axisymmetric SN collapse to produce strong gravitational waves.