Panel a: polarization posterior distributions for GRB 170207A with the 1 and 2σ credibility intervals as well as that corresponding to 99% credibility. The polarization angle shown here is in the POLAR coordinate system; a rotation in the positive direction of 176 degrees transforms this to the coordinate system as defined by the IAU. Panel b: measured scattering angle distribution (gray data points with a 5 degree bin size) with the posterior model predictions superimposed in red. The errors on the data points are the Poisson errors corrected for the background.