Portrayal of the disc geometry based on the water maser components. The disc (in cyan), assumed to be in Keplerian rotation, is edge-on and oriented north–south, with the black hole at the centre. The positions of the detected maser components M1 and M2 are marked with a green cross ‘×’ and a red plus symbol ‘+’, respectively. The position of the supposed blueshifted maser component M3 is indicated with a blue plus symbol ‘+’ (see Sect. 5.1).