TITLE: GCN CIRCULAR NUMBER: 23900 SUBJECT: Trigger 889483: Swift detection of Swift J1858.6-0814 DATE: 19/02/18 09:40:22 GMT FROM: Kim Page at U.of Leicester A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 08:54:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located Swift J1858.6-0814 (trigger=889483). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 284.640, -8.235 which is RA(J2000) = 18h 58m 34s Dec(J2000) = -08d 14' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual for an image trigger, there is no obvious variation in the immediately-available BAT lightcurve. The XRT began observing the field at 09:03:09.4 UT, 539.9 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source located at RA, Dec 284.64412, -8.23735 which is equivalent to: RA(J2000) = 18h 58m 34.59s Dec(J2000) = -08d 14' 14.5" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 16 arcseconds from the BAT onboard position, within the BAT error circle. This localisation is consistent with the previously reported position (ATel 12160). A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 3.21 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 543 seconds after the BAT trigger. There is a source in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 18:58:34.91 = 284.64544 DEC(J2000) = -08:14:14.8 = -8.23745 with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.34 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.26. This source was originally discovered by Swift-BAT in the transient monitor (ATel 12151), and Swift triggered on this source on 2018 Nov 23 (GCN 23431). The source has just emerged from behind the Sun, and NuSTAR have shown it still to be active in X-rays (ATel 12512). The BAT transient monitor page for this source is available at https://swift.gsfc.nasa.gov/results/transients/weak/SWIFTJ1858.6-0814/