GRB 221008A: MAXI/GSC detection

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 2022-10-08T00:53:50 UT. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (29.687 deg, -18.876 deg) = (01 58 44, -18 52 33) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.12 deg and 0.09 deg, respectively. The roll angle of long axis from the north direction is 25.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (29.400, -18.665) deg = (01 57 35, -18 39 53) (J2000) (29.338, -18.887) deg = (01 57 21, -18 53 13) (J2000) (30.076, -19.070) deg = (02 00 18, -19 04 12) (J2000) (30.137, -18.847) deg = (02 00 32, -18 50 49) (J2000) The X-ray flux averaged over the scan was 803 +- 56 mCrab (4.0-10.0keV, 1 sigma error). There was no significant excess flux in the previous transit at 10/07 23:21 UT with an upper limit of 20 mCrab.