TITLE: GCN CIRCULAR NUMBER: 24553 SUBJECT: GRB 190515B: Swift detection of a burst DATE: 19/05/15 16:17:18 GMT FROM: Michael Moss at George Washington U M. J. Moss (George Washington University), C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 16:04:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190515B (trigger=903845). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 98.840, +52.295 which is RA(J2000) = 06h 35m 22s Dec(J2000) = +52d 17' 41" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 20 sec. The peak count rate was ~800 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 16:05:43.7 UT, 82.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 98.84892, 52.31725 which is equivalent to: RA(J2000) = 06h 35m 23.74s Dec(J2000) = +52d 19' 02.1" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 82 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.27 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 7.78e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 92 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 84% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.09. Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) [GCNOPS NOTE(15may19): Per author's request, the GRB name "190515A" was changed to "190515B" in the Subject lines in the first paragraph.]