GCN Circular 10976
T. N. Ukwatta (GWU/GSFC), S. D. Barthelmy (GSFC), N. Gehrels (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), J. Norris (U. Denver), and T. Sakamoto (GSFC/UMBC) (for the Swift-BAT team): For GRB 100724A (Markwardt et al GCN 10968), the spectral lag analysis of the mask-weighted data from T-0.5 sec to T+2 sec yields a lag of 316 +/- 82 msec for the 25-50 to 50-100 keV bands using light curve binning of 64 msec. This relatively large observed lag value puts this burst in the long burst category. The observed T90 (1.4 +/- 0.2 sec; Swift/BAT 15-350 keV, Markwardt et al., GCN 10972) value is in the middle of the the soft/long bimodal distribution. The power-law fit to the BAT spectrum gives index (PLI) of 1.92 +/- 0.21 (Markwardt et al., GCN 10972), which is in the middle of the Long GRB PLI distribution and in the soft side tail of the Short GRB PLI distribution. In summary, the observed T90 value for GRB 100724A is inconclusive. The relatively large spectral lag value and soft spectrum points towards long soft burst. Our conclusion is that this burst is likely to be in the long burst category. Further observations are encouraged to characterize the host galaxy.