Temporal and Spectral Characteristics of Short Bursts from the Soft Gamma Repeaters 1806-20 and 1900+14

We study the temporal and coarse spectral properties of 268 bursts from SGR 1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray Timing Explorer/Proportional Counter Array. Hardness ratios and temporal parameters, such as T_90 durations and tau_90 emission times are determined for these bursts. We find a lognormal distribution of burst durations, ranging over more than two orders of magnitude: T_90 ~ 10^-2 to >~ 1 s, with a peak at ~ 0.1 s. The burst light curves tend to be asymmetrical, with more than half of all events showing rise times t_r < 0.3 T_90. We find that there exists a correlation between the duration and fluence of bursts from both sources. We also find a significant anti-correlation between hardness ratio and fluence for SGR 1806-20 bursts and a marginal anti-correlation for SGR 1900+14 events. Finally, we discuss possible physical implications of these results within the framework of the magnetar model.Comment: 24 pages, 11 figures, accepted for publication in Ap