Panel a: best-fit photocentres for the 13 brightest spectral channels covering the Brγ line to the differential phase data. The colour indicates wavelength and the ellipses outline the uncertainty on the photocentres. The red cross plots the (0,0) position. Panel b: best fit ‘2-pole’ photocentre model where all blueshifted (blue point) and redshifted (red point) channels are assumed to have the same photocentre with the addition of the wavelength-independent offset. The black point indicates the best-fit position of the ‘null’ model where all channels are assumed to have the same photocentre. Panel c: photocentres produced from our best-fitting BLR model (see Sect. 3.2) which we calculated by first producing mock differential phase spectra before fitting them in the same way as panel a. Colours are the same as in panel a.