Fig. 3: Jet collimation profile.

Red filled circles mark the measured jet transverse width for the observations reported here. The error bars (1σ) are within the symbols (see Supplementary Information section 8 for more details on measuring the jet width). Grey filled squares, dots and triangles denote previous measurements of the width on larger scales^16,17,28, for which a power-law fit with a fixed power-law index of 0.58 is shown by the dashed line. The vertical dashed line marks the position at which the intrinsic half-opening angle θ of the fitted parabolic jet equals the jet viewing angle of θ_v = 17° (that is, boundary condition for a down-the-pipe jet²⁹). The horizontal blue solid line marks the measured diameter of the ring at 3.5 mm, whereas the horizontal black dashed line marks the ring diameter measured with the EHT at 1.3 mm. In each case, the shaded area denotes the corresponding measurement uncertainty. The light-grey-shaded area denotes the outermost streamlines of the envelope of the parabolic jet from theoretical simulations (projected for θ_v = 17°; ref. ³⁰) that are anchored at the event horizon¹⁹ for a range of black hole spins (dimensionless spin parameters, a = 0.0–0.9). The lower and upper boundaries of this shaded area correspond to the highest (a = 0.9) and lowest (a = 0.0) spin, respectively. As the jet footpoint is anchored at the event horizon, some flattening of the jet width profile is expected near the black hole. This is further enhanced by geometrical projection effects in the region where the intrinsic jet half-opening angle (θ) is larger than the jet viewing angle (θ_v). The quasi-cylindrical shape in region I requires some change in the physical conditions to connect the innermost Blandford–Znajek jet from the event horizon to the upstream jet (region II).