------------------------------------------------------------------- THE ASTRONOMER Electronic Circular No 3119 2015 Oct 27 11.31UT Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise, Basingstoke, Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074 INTERNET: GUY@TAHQ.DEMON.CO.UK Backup: gmh@wdcc1.bnsc.rl.ac.uk WORLD WIDE WEB http://www.theastronomer.org ------------------------------------------------------------------- GRB 151027A AND BRIGHT OPTICAL COUNTERPART A. Maselli (INAF-IASFPA) et. al. report on GCN 18478 on behalf of the SWIFT team: At 03:58:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151027A (trigger=661775). Swift slewed immediately to the burst. The BAT on-board calculated location a RA 18h 09m 58s DEC +61d 22' 51" (2000) with an uncertainty of 3' The BAT light curve showed a complex peaked structure lasting at least 150 seconds. The XRT began observing the field at 03:59:51.1 UT, 87.0 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at: RA 18h 09m 57.26s DEC +61d 21' 05.4" (2000) with an uncertainty of 4.8". UVOT took a finding chart exposure of 150 seconds with the White filter starting 95 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA 18:09:56.68 DEC +61:21:13.4 with a 90%-confidence error radius of about 0.61 arc sec. This position is 9.06 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.46 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. ON GCN 18480, Alexei Pozanenko at IKI, Moscow relays that L. Elenin et.al. observed the field with 0.4-m telescope of ISON-NM observatory in robotic mode starting on Oct., 27 (UT) 03:59:43 i.e. 79 seconds after burst trigger. They obtained 50 unfiltered images of 30 s exposure and clearly detected the optical counterpart. In the first image they estimated brightness of the optical counterpart as ~14m. On GCN 18481 Daniel Perley at Caltech advised that the Palomar 60-inch robotic telescope responded automatically to the alert and began a sequence of i, r, and z-band imaging at 04:00:49 UT, 145 seconds after the BAT trigger. They identify a very bright afterglow consistent with the UVOT position. The preliminary afterglow light curve shows a slow rise to a peak at about ~5 minutes after the GRB trigger, then slowly fades by about 1 magnitude between then and approximately 30 minutes after the trigger. Over the next 20 minutes it then rapidly re-brightens by about 0.7 magnitude. Little or no fading is seen after that point (through to at least 90 minutes after the trigger.) Continued monitoring is encouraged, and should remain possible even from small telescopes for quite some time. Editor: Readers are encouraged to follow up on this object and also consider use of remote robotic telescopes which may currently have a dark period. Guy M Hurst