GCN Circular 22489

S. D. Barthelmy (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), B. Sbarufatti (PSU), M. H. Siegel (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:11:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located MAXI J1820+070 (trigger=814259).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 275.089, +7.181 which is 
   RA(J2000) = 18h 20m 21s
   Dec(J2000) = +07d 10' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure that lasts beyond currently available data (T0+400 s).  
The peak count rate was ~800 counts/sec (15-350 keV), at ~-2 sec 
after the trigger. 

The XRT began observing the field at 19:13:54.3 UT, 143.0 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
275.0898, 7.1831 which is equivalent to:
   RA(J2000)  = +18h 20m 21.55s
   Dec(J2000) = +07d 10' 59.2"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
position is 10 arcseconds from MAXI J1820+070, although we believe this
is the same source. Assuming a typical GRB power-law spectrum 
(NH=3e21 cm^-2, gamma=2), the catalogue entry for this object 
is equivalent to a 0.2-10 keV flux of 9.3e-13 erg cm^-2 s^-1; 
the flux in the 2.5 s XRT image was 3.32e-09 erg cm^-2 s^-1. No
event data are yet available to determine the column density using
X-ray spectroscopy. 

UVOT took a finding chart exposure of 250 seconds with the U filter 
starting  308 seconds after the BAT trigger. The source is  detected
at a magnitude of 12.28 +/- 0.04.  No correction has been  made for
the expected extinction corresponding to E(B-V) of  0.23.