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Swift detection of MAXI J1820+070

ATel #11403; 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:
on 12 Mar 2018; 01:02 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

This is reposted from GCN #22471 as it is of interest to the transient community.

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).