Spectral index maps of PSO J0309+27 (S_ν ∝ ν^α) for the frequency pairs 1.5−5.0 GHz (left) and 5.0−8.4 GHz (right). The black contours are the 1.5 GHz emission on the left, and the 5 GHz emission on the right, with the same increasing scheme as Fig. 1. The circular restoring beam is 13 mas FWHM (1.5−5.0 GHz image) and 4 mas FWHM (5.0−8.4 GHz image), and it is shown in the bottom left corner of each map.