TITLE: GCN CIRCULAR NUMBER: 23989 SUBJECT: Swift detection of LS I +61 303 DATE: 19/03/23 21:16:23 GMT FROM: Boris Sbarufatti at PSU S. D. Barthelmy (GSFC), J.D. Gropp (PSU), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:30:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located LS I +61 303 (trigger=894557). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 40.153, +61.246 which is RA(J2000) = 02h 40m 37s Dec(J2000) = +61d 14' 45" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed multiple short spikes with a total duration of about 20 sec. The peak count rate was ~2400 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 20:32:12.5 UT, 85.3 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 40.1330, 61.2290 which is equivalent to: RA(J2000) = 02h 40m 31.91s Dec(J2000) = +61d 13' 44.2" with an uncertainty of 2.0 arcseconds (radius, 90% containment). This location is 70 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. This position is 2.0 arcseconds from LS I +61 303. This source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate of 0.19600 +/- 0.00093 ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ024031.5%2B611344 for details of these previous observations. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 9.01 x 10^21 cm^-2 (Willingale et al. 2013). UVOT data cannot properly be analyzed at this time. A full analysis will be possible once the full dataset comes to the ground. The light curve from the BAT transient monitor is available at https://swift.gsfc.nasa.gov/results/transients/weak/LSIp61303/