GRB 241218A: Swift-BAT refined analysis

Using the data set from T-520 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 241218A (trigger #1275005) (M. A. Williams, et al., GCN Circ. 38601). The BAT ground-calculated position is RA, Dec = 185.687, 29.984 deg which is RA(J2000) = 12h 22m 44.8s Dec(J2000) = +29d 59' 04.2" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 52%. The mask-weighted light curve shows a single pulse of emission lasting 40 seconds with a possible second pulse at T+70 seconds. The T90 (15-350 keV) is 23.31 +- 4.19 sec (estimated error including systematics). The time-averaged spectrum from T-1.82 to T+26.16 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.31 +- 0.26. The fluence in the 15-150 keV band is 5.5 +- 1.0 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T+16.64 sec in the 15-150 keV band is 0.6 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1275005