GRB 170511A: MAXI/GSC detection

At 2017-05-11T06:00 UT, the MAXI/GSC detected an uncatalogued X-ray transient source. The position reported to the MAXI new-transient mailing list was not accurate due to an error of attitude determination. With the corrected attitude information, we update the source position assuming a constant source flux over the scan transit to be (R.A., Dec) = (300.633 deg, 33.315 deg) = (20 02 31, +33 18 53) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.29 deg and 0.25 deg, respectively. The roll angle of long axis from the north direction is 170.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The position and the time of the burst are consistent with a burst detected by Fermi/GBM (trigger #516175156). Thus, we consider this event to be a GRB. According to a quick-look light curve of Fermi/GBM, MAXI/GSC observed a tail part of the GRB. The averaged X-ray flux was 228 +- 35 mCrab (4-10 keV). There were no significant excess fluxes at the previous and followed scan transits at 2017/05/11 04:27 UT and 07:33 UT, respectively, with an upper limit of 20 mCrab for each.