Long‐Lag, Wide‐Pulse Gamma‐Ray Bursts

Currently, the best available probe of the early phase of gamma-ray burst(GRB) jet attributes is the prompt gamma-ray emission, in which severalintrinsic and extrinsic variables determine GRB pulse evolution. Bright,usually complex bursts have many narrow pulses that are difficult to model dueto overlap. However, the relatively simple, long spectral lag, wide-pulsebursts may have simpler physics and are easier to model. In this work weanalyze the temporal and spectral behavior of wide pulses in 24 long-lagbursts, using a pulse model with two shape parameters -- width and asymmetry --and the Band spectral model with three shape parameters. We find that pulses inlong-lag bursts are distinguished both temporally and spectrally from those inbright bursts: the pulses in long spectral lag bursts are few in number, and ~100 times wider (10s of seconds), have systematically lower peaks in nu*F(nu),harder low-energy spectra and softer high-energy spectra. We find that thesefive pulse descriptors are essentially uncorrelated for our long-lag sample,suggesting that at least five parameters are needed to model burst temporal andspectral behavior. However, pulse width is strongly correlated with spectrallag; hence these two parameters may be viewed as mutual surrogates. We inferthat accurate formulations for estimating GRB luminosity and total energy willdepend on several gamma-ray attributes, at least for long-lag bursts. Theprevalence of long-lag bursts near the BATSE trigger threshold, theirpredominantly low nF(n) spectral peaks, and relatively steep upper power-lawspectral indices indicate that Swift will detect many such bursts.Comment: 57 pages, 26 figures, 6 tables. Accepted for publication in the Astrophysical Journa