GRB 221028B: MAXI/GSC detection

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 2022-10-28T20:57:52 UT. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (215.098 deg, -36.880 deg) = (14 20 23, -36 52 48) (J2000) with a 90% C.L. statistical error of 0.23 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 159 +- 26 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (214.489, -35.809) deg = (14 17 57, -35 48 32) (J2000) (214.045, -36.075) deg = (14 16 10, -36 04 30) (J2000) (215.717, -37.852) deg = (14 22 52, -37 51 07) (J2000) (216.165, -37.579) deg = (14 24 39, -37 34 44) (J2000) There was no significant excess flux in the previous transit at 19:24 UT and in the next transit at 22:09 UT with an upper limit of 20 mCrab for each.