Univ Calif Los Alamos Natl Lab, Los Alamos, NM 87545 USA
; Chinese Acad Sci, Inst High Energy Phys, Cosm Ray & High Energy Astrophys Lab, Beijing 100039, Peoples R China
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 Letter, 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 (similar to 2 keV). We found that lags between energy bands are small. The lag between the peak of similar to 50 keV photons and that of similar to 200 keV photons is similar to 0.08 s. The upper limit in the lag between similar to 9 and similar to 90 keV photons is similar to 0.5 s. 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, we found that 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 that angular effects are independent of energy. Thus, only hydrodynamics can produce these lags. Perhaps the radiation process varies as the reverse shock moves through the shell.