Session 28 - Diffuse Gamma-Ray Emission & Gamma-Ray Bursts.
Oral session, Monday, January 13
Harbour B,

## [28.03] Time Dilation and Homogeneous, Soft-Spectrum GRBs

J. P. Norris (NASA/GSFC), J. T. Bonnell (USRA), R. J. Nemiroff (MTU), J. D. Scargle (NASA/ARC), G. N. Pendleton (UAH), C. Kouveliotou (USRA), G. Pizzichini (TESRE/CNR)

Recently, BATSE gamma-ray bursts selected for soft average spectra have been shown to follow more nearly a -3/2 power law in their number-intensity relation, indicative of a spatially homogeneous population, unlike the whole BATSE burst sample which deviates significantly from a -3/2 signature. The softer bursts might therefore be closer, and the reported time dilation as a function of peak flux in the whole burst sample (Bonnell et al., ApJ submitted) might be expected to be different for soft bursts. We have investigated this possibility with a sample of 500 long bursts (T_90 > 2 s) from the BATSE 3B catalog, defining soft bursts (\sim 20% of total) using the three hardness ratios derived from fluences in BATSE's four energy channels (25--55, 55--110, 110--320, > 320 keV). The relative time-dilation factors (TDFs) were calculated using a brightness-independent algorithm for duration. The expected effect is observed: The average log[duration] of soft bursts is significantly lower (factor of \sim 2) than that for harder bursts, or for the whole set, to much dimmer peak fluxes -- consistent with unity TDF (compared to bright bursts in the whole sample) down to peak flux of \sim 1.0 photon cm^-2 s^-1. Using a Kolmogorov-Smirnov test, we find that T_90 and T_50 duration distributions of soft and hard bursts above this peak flux value are different, with a confidence level > 99%. This result is qualitatively consistent with a GRB luminosity function implied by the apparent homogeneity of the bright-to-intermediate peak-flux soft bursts. However, dimmer soft bursts are time-dilated relative to bright bursts in the whole sample, suggesting that spectral redshift compounds the definition of the soft burst class.