[vsnet-grb-info 21801] Trigger 889483: Swift detection of Swift J1858.6-0814

[GCN Circulars]

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  <klp5 at leicester.ac.uk>

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/