TITLE:   GCN CIRCULAR
NUMBER:  23946
SUBJECT: GRB 190311A: Swift detection of a burst with optical counterpart
DATE:    19/03/11 14:43:26 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <siegel@swift.psu.edu>

E. Troja (NASA/GSFC/UMCP), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
M. J. Moss (George Washington University), K. L. Page (U Leicester)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 14:23:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 190311A (trigger=892607).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 212.102, +53.519 which is 
   RA(J2000) = 14h 08m 24s
   Dec(J2000) = +53d 31' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). From the available data, the BAT light curve showed 
a single-peaked structure with duration of at least 10 sec. However, due to 
a data gap from ~T+10 s to ~ T+ 110 s, there might be additional emission
in that time interval. The peak count rate was ~1700 counts/sec (15-350 keV), 
at ~ 0 sec after the trigger. 

The XRT began observing the field at 14:24:54.0 UT, 80.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
212.0649, 53.5007 which is equivalent to:
   RA(J2000)  = 14h 08m 15.58s
   Dec(J2000) = +53d 30' 02.7"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 103 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 (1.17 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.1
(+3.27/-2.92) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.45e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	14:08:15.65 = 212.06522
  DEC(J2000) = +53:30:03.0  =  53.50084
with a 90%-confidence error radius of about 0.60 arc sec. This position is 0.85
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.47 with a 1-sigma error of about  0.12. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 
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/)