TITLE: GCN CIRCULAR NUMBER: 20053 SUBJECT: GRB161004A: Further analysis of the burst nature DATE: 16/10/16 00:06:51 GMT FROM: Amy Lien at GSFC S. D. Barthelmy (GSFC), P.A. Evans (U Leicester), N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), J. P. Norris (BSU), P. O'Brien (U Leicester), K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC), and H. Ziaeepour report on behalf of the Swift Team: We performed further analysis of GRB161004A (trigger #715084; Evans et al., GCN Circ. 19979), to explore the possibility to clarify its short/long nature. Using the full BAT event data set from T-240 to T+962 sec, the updated T90 is 3 +/- 1.4 s (using the 1-s binned light curve). This T90 is longer than the 1.08 +- 0.21 s originally reported (Palmer et al, GCN Circ. 20001). However, we note that due to the weakness of the burst, the standard pipeline finds a T90 that varies from ~ 1.3 s to ~ 3 s when using light curves with different bin sizes. The image analysis with the BAT data in 15-150 keV finds no significant detections for extended emission within several hundred seconds after the trigger. However, the BAT image from T+250 s to T+350 s, which is the time period around the peak of the XRT flare, does show a slight increase of significance of 3.8 sigma (comparing to ~ 1 to 2 sigma detections for other periods). Therefore, it is possible that the XRT flare might be the second peak of the prompt emission, but was close to the detection limit of the BAT. The time averaged spectrum is best fit by a simple power-law model, with an index of 1.3 +/- 0.33. This value is consistent with the average value of the short GRB distribution, and slightly on the harder side of the long burst distribution, but still within the majority (Lien & Sakamoto et al. 2016). The lag analysis is still unavailable due to the weakness of the burst (Palmer et al, GCN Circ. 20001). The bright flare in the XRT light curve around T+300 s is unusual for short GRBs. However, Swift have detected short GRBs with flares before (e.g., GRB 070724A, GRB 100816A; Margutti et al., 2011, MNRAS, 417, 2144), and thus we cannot rule out the short-burst possibility base on this. Therefore, we conclude that the burst nature remains ambiguous with current information.