[vsnet-grb-info 20375] Swift detection of MAXI J1820+070

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22471
SUBJECT: Swift detection of MAXI J1820+070
DATE:    18/03/12 00:59:40 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

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

At 00:11:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located MAXI J1820+070 (Kawamuro et al, 2018; ATel #11399) 
(trigger=813771, retriggering as trigger=813772).  
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 275.025, +7.192 which is 
   RA(J2000) = 18h 20m 06s
   Dec(J2000) = +07d 11' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Because of the of duration of this
image trigger (17 minutes) no BAT lightcurve is immediately available. 

The XRT began observing the field at 00:31:42.2 UT, 1202.8 seconds
after the BAT trigger. XRT found a bright X-ray source located at RA,
Dec 275.0895, 7.1832 which is equivalent to:
   RA(J2000)  = +18h 20m 21.48s
   Dec(J2000) = +07d 10' 59.5"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 232 arcseconds from the BAT onboard position. This position
is 33.1 arcseconds from that of a known X-ray source: 1RXS
J182022.9+071125 in the ROSAT RASSFSC catalogue. 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 1.37e-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 150 seconds with the White
filter starting 1212 seconds after the BAT trigger. There is a 
16.8 mag source at 
  RA(J2000) = 18:20:21.25 = 275.08856
  DEC(J2000) = 07:10:54.7 = 7.18186
with a 90%-confidence error radius of about 0.61 arc seconds. 
This position is 5.9 arc seconds from the center of the XRT position. 
No correction has been made for the expected extinction
corresponding to E(B-V) of 0.23. 

The XRT and UVOT locations of the counterpart are near the location 
of ASASSN-18ey (distances=10.5 arcsec, 16 arcsec respectively), 
which was suggested as the counterpart to the MAXI source by 
Denisenko (ATel #11400).