TITLE: GCN CIRCULAR NUMBER: 13368 SUBJECT: Trigger 524584: Swift detection of Algol DATE: 12/06/18 12:00:18 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), V. D'Elia (ASDC), S. R. Oates (UCL-MSSL), G. Stratta (ASDC) and V. N. Yershov (UCL-MSSL) report on behalf of the Swift Team: At 11:33:11 UT, the Swift Burst Alert Telescope (BAT) triggered on the flare star Algol (trigger=524584). Swift did not slew because of the Sun observing constraint. The BAT on-board calculated location is RA, Dec 47.017, +40.966, which is RA(J2000) = 03h 08m 04s Dec(J2000) = +40d 57' 57" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers (7 min), the TDRSS lightcurve does not show anything significant. This trigger was in the new category of subthreshold matchup with known sources in the on-board catalog (imager signif=5.83 sgima). However, the BAT position is 1.2 arcmin from Algol, so the chance coincidence proability is low, but we can not rule that out.