A Population of Long Spectral Lag Gamma-Ray Bursts

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Scientific paper

Spectral lag measurements for the long (T_90 > 2 s) BATSE GRB sample reveal a subpopulation of ~ 350 bursts with few, wide pulses, lags of a few tenths to several seconds, and soft spectra. Their Log[N] - Log[PF] approximates a -3/2 power-law, suggesting homogeneity and relatively nearby sources. The proportion of these long-lag bursts increases from negligible among bright BATSE bursts to 50 % at trigger threshold. Bursts with very long lags (> 1 s) show a 2-2.5 σ tendency to concentrate near the Supergalactic Plane. GRB 980425 (SN 1998bw) is a member of this subsample of ~ 100 bursts with estimated distances less than 100 Mpc. Assuming beaming fractions near unity, the frequency of these ultra-low luminosity bursts would be ~ 1/4 that of SNe Ib/c. If these sources are truly nearby, the associated core-collapse events might produce gravitational radiation detectable by LIGO-II, and may help explain the flattening of the cosmic ray spectrum at ultra-high energies as recently observed by AGASA. In a sensitivity-limited regime, 10-5.6 < L_53 < 10-2, a power-law scaling relation is indicated for the ultra-low luminosity sources, dN/dL ~ L-1, flatter than expected (dN/dL ~ L^1/6) if viewing angle alone governed observed luminosity.

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