Radio and X-ray observations of an exceptional radio flare in the extreme z= 4.72 blazar GB B1428+4217

We report on the extreme behaviour of the high-redshift blazar GB B1428+4217 at z= 4.72. A continued programme of radio measurements has revealed an exceptional flare in the light curve, with the 15.2-GHz flux density rising by a factor of ∼3 from ∼140 to ∼430 mJy in a rest-frame time-scale of only ∼4 months – much larger than any previous flares observed in this source. In addition to new measurements of the 1.4–43 GHz radio spectrum, we also present the analysis and results of a target-of-opportunity X-ray observation using XMM–Newton, made close to the peak in radio flux. Although the X-ray data do not show a flare in the high-energy light curve, we are able to confirm the X-ray spectral variability hinted at in previous observations. GB B1428+4217 is one of several high-redshift radio-loud quasars that display a low-energy break in the X-ray spectrum, probably due to the presence of excess absorption in the source. X-ray spectral analysis of the latest XMM–Newton data is shown to be consistent with the warm-absorption scenario which we have hypothesized previously. Warm absorption is also consistent with the observed X-ray spectral variability of the source, in which the spectral changes can be successfully accounted-for with a fixed column density of material in which the ionization state is correlated with hardness of the underlying power-law emission.