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Left, our best surface brightness model of the 1.7-GHz global VLBI observation used here, which has been reconvolved with the main lobe of the interferometer’s point spread function and added to the residuals (34 μJy per beam r.m.s.). For reference, red contours show the surface brightness at 2.1 μm observed by the W. M. Keck Observatory adaptive optics system30. The positions of two low-mass perturbers are each marked with a black X. The 2 × 108 M⊙ object first detected by ref. 24 is labelled , and the 1.13 × 106 M⊙ detection reported here is labelled . The zoomed-in region shown in the right-hand two panels is indicated by the black square, which has a side length of 60 mas. Top right, detail of the bright arc around , with the colour scale modified to emphasize the gap in the arc produced by the gravitational perturbation of . Bottom right, GI corrections to the lensing convergence (expressed in units of lens-plane surface mass density), showing a compact, positive feature whose position and mass are consistent with the independent parametric modelling results for . The dashed black circle has a radius of 80 pc and the lensed emission is indicated by the black contours.