[vsnet-grb-info 17870] GRB 160417A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19316
SUBJECT: GRB 160417A: Swift detection of a burst with an optical counterpart
DATE:    16/04/17 04:34:46 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 04:23:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160417A (trigger=683076).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 120.240, +7.671 which is 
   RA(J2000) = 08h 00m 58s
   Dec(J2000) = +07d 40' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex peak
structure with a duration of about 30 sec.  The peak count rate
was ~1100 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 04:25:53.1 UT, 132.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 120.25774, 7.66265 which
is equivalent to:
   RA(J2000)  = 08h 01m 01.86s
   Dec(J2000) = +07d 39' 45.5"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 70 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.92
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 135 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	08:01:01.64 = 120.25682
  DEC(J2000) = +07:39:46.4  =	7.66290
with a 90%-confidence error radius of about 0.64 arc sec. This position is 4.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.15 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.inaf.it). 
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/too.html.)