GCN Circular 22021
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and B. Sbarufatti (PSU) report on behalf of the Swift Team: At 17:38:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located Vela X-1 (trigger=779762). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 135.515, -40.554 which is RA(J2000) = 09h 02m 04s Dec(J2000) = -40d 33' 15" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 40 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 17:40:35.2 UT, 145.4 seconds after the BAT trigger. XRT found a bright X-ray source located at RA, Dec 135.5273, -40.5544 which is equivalent to: RA(J2000) = +09h 02m 6.55s Dec(J2000) = -40d 33' 15.8" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This position is 3.4 arcseconds from a known X-ray source: 1SXPS J090206.7-403318 (= Vela X-1). This source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate of 7.20 +/- 0.21 ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ090206.7-403318 for details of these previous observations. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 6.17e-09 erg cm^-2 s^-1 (0.2-10 keV). The BAT Transient Monitor shows flaring in the light-curve of Vela X-1 over the past few days (https://swift.gsfc.nasa.gov/results/transients/VelaX-1/).