  We report on two Chandra observations of the quasar PSO J0309+27, the most
distant blazar observed so far (z=6.1), performed eight months apart, in March
and November 2020. Previous Swift-XRT observation showed that this object is
one of the brightest X-ray sources beyond redshift 6.0 ever observed so far.
This new data-set confirmed the high flux level and unveiled a spectral change
occurred on a very short timescale (250s rest-frame), caused by a significant
softening of the emission spectrum. This kind of spectral variability, on a
such short interval, has never been reported in the X-ray emission of a flat
spectrum radio quasar. A possible explanation is given by the emission produced
by the inverse Compton scatter of the quasar UV photons by the cold electrons
present in a fast shell moving along the jet. Although this bulk comptonization
emission should be an unavoidable consequence of the standard leptonic jet
model, this would be the first time that it is observed.
