GCN Circular 26678

A. Melandri (INAF-OAB), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), N. J. Klingler (PSU),
S Laha (GSFC/UMBC/CRESST), D. M. Palmer (LANL), M. H. Siegel (PSU),
K. K. Simpson (PSU) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 01:46:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200109A (trigger=948361).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 307.171, +53.003 which is 
   RA(J2000) = 20h 28m 41s
   Dec(J2000) = +53d 00' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of 70 sec.  The peak count rate
was ~1600 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 01:47:59.1 UT, 102.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
307.1258, 52.9932 which is equivalent to:
   RA(J2000)  = 20h 28m 30.19s
   Dec(J2000) = +52d 59' 35.5"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 104 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
https://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 3.98
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.12e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 109 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. The list of sources is typically complete to
about 18 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.29. 

Burst Advocate for this burst is A. Melandri (andrea.melandri 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/)