GRB 220618A: Swift-BAT refined analysis

Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 220618A (trigger #1110821) (Troja, et al., GCN Circ. 1110821). The BAT ground-calculated position is RA, Dec = 38.986, 55.218 deg which is RA(J2000) = 02h 35m 56.6s Dec(J2000) = +55d 13' 05.0" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%. The BAT light curve showed a single peak structure with a duration of about 40 sec. T90 (15-350 keV) is 96.00 +- 65.97 sec (estimated error including systematics). The time-averaged spectrum from -15.264 to 128.736 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.26 +- 0.85, and Epeak of 65.0 +- 16.9 keV (chi squared 53.40 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.2 +- 0.3 x 10^-06 erg/cm2 and the 1-sec peak flux measured from T+8.24 sec in the 15-150 keV band is 0.8 +- 0.4 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.61 +- 0.16 (chi squared 63.41 for 57 d.o.f.). 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/1110821/BA/