TITLE: GCN CIRCULAR NUMBER: 23155 SUBJECT: GRB 180818B: Swift-BAT refined analysis DATE: 18/08/19 01:00:33 GMT FROM: Amy Lien at GSFC J. R. Cummings (CPI), S. D. Barthelmy (GSFC), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 180818B (trigger #853882) (Marshall et al., GCN Circ. 23149). The BAT ground-calculated position is RA, Dec = 104.227, 39.310 deg which is RA(J2000) = 06h 56m 54.5s Dec(J2000) = +39d 18' 35.1" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 89%. The mask-weighted light curve shows two pulses. The first pulse starts at at ~T-100 s, peaks at ~T-80 s, and ends at ~T-50 s. The second pulse starts at ~T+10 s, peaks at ~T+30 s, and ends at ~T+60 s. T90 (15-350 keV) is 134.4 +- 9.8 sec (estimated error including systematics). However, note that the burst came into the BAT FOV at ~T-128 s, and thus there might be additional burst emission beforehand. The time-averaged spectrum from T-93.58 to T+62.74 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 +- 0.12. The fluence in the 15-150 keV band is 2.3 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T-82.11 sec in the 15-150 keV band is 1.2 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/853882/BA/