Minute-timescale variability in the X-ray emission of the highest redshift blazar

TL;DR

This study reports the first observation of minute-timescale spectral variability in the X-ray emission of the most distant known blazar, PSO J0309+27, revealing rapid spectral softening likely caused by inverse Compton scattering in the jet.

Contribution

It provides the first evidence of ultra-rapid spectral variability in a high-redshift blazar, supporting the presence of bulk comptonization in its jet.

Findings

Confirmed high flux level in the X-ray emission.

Detected spectral softening on a 250s timescale.

First evidence of bulk comptonization in a blazar.

Abstract

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.