Surface brightness profile of comet Borisov (yellow circles) compared with models in which the nucleus radius (for assumed geometric albedo p_V = 0.04) is r_n = 0 km (thick black line), 0.3 km (dashed red line), 0.7 km (dashed–dotted blue line), and 1.0 km (thin black line). Lines in the lower left indicate logarithmic surface brightness gradients m = 1 and m = −1.5, for comparison.