TITLE:   GCN CIRCULAR
NUMBER:  23908
SUBJECT: GRB 190220A: Swift detection of a burst with an optical counterpart
DATE:    19/02/20 03:14:43 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

K. L. Page (U Leicester), N. J. Klingler (PSU),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU)
and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 02:59:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 190220A (trigger=889785).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 330.142, -76.691 which is 
   RA(J2000) = 22h 00m 34s
   Dec(J2000) = -76d 41' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 10 sec.  The peak count rate
was ~1700 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 03:00:55.5 UT, 80.8 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find an uncatalogued, fading X-ray source located at RA, Dec 330.16325,
-76.68047 which is equivalent to:
   RA(J2000)  = 22h 00m 39.18s
   Dec(J2000) = -76d 40' 49.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 41 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 in excess of the Galactic value (1.10 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.9
(+3.65/-3.07) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 86 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:00:39.14 = 330.16309
  DEC(J2000) = -76:40:50.4  = -76.68067
with a 90%-confidence error radius of about 0.66 arc sec. This position is 3.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.29 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.13. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
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/)