Spectral Lags of Gamma-Ray Bursts From [ITAL]Ginga[/ITAL] and BATSE

The analysis of spectral lag between energy bands, which combines temporal and spectral analyses, can add strict constraints to gamma-ray burst (GRB) models. In previous studies, the lag analysis focused on the lags between channel 1 (25-57 keV) and channel 3 (115-320 keV) from the Burst and Transient Source Experiment (BATSE). In this paper, we analyzed the cross-correlation average lags (including approximate uncertainties) between energy bands for two GRB samples: 19 events detected by Ginga, and 109 events detected by BATSE. We paid special attention to the BATSE GRBs with known redshifts, because there has been a reported connection between lag and luminosity. This extends our knowledge of spectral lags to lower energy ($sim 2$ keV). We found that lags between energy bands are small. The lag between the peak of $sim 50$ keV photons and $sim 200$ keV photons is $sim 0.08$ sec. The upper limit in the lag between $sim 9$ keV photons and $sim 90$ keV photons is $sim 0.5$ sec. Thus, there are not large shifts at low energy. We found that about 20% of GRBs have detectable lags between energy bands in the Ginga and BATSE samples. From the internal shock model there are three sources of time structure in GRB pulses: cooling, hydrodynamics, and angular effects. We argue that cooling is much too fast to account for our observed lags and angular effects are energy independent. Thus, only hydrodynamics can produce these lags. Perhaps the radiation process varies as the reverse shock moves through the shell.Comment: 12 pages, 1 embeddedfigure, Latex, Accepted by ApJ Letter