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
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
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.