NuSTAR observations of a varying-flux quasar in the Epoch of Reionization

With enough X-ray flux to be detected in a 160 s scan by SRG/eROSITA, the z = 6.19 quasar CFHQS J142952+544717 is, by far, the most luminous X-ray source known at z > 6. We present deep (245 ks) NuSTAR observations of this source; with ∼180 net counts in the combined observations, CFHQS J142952+544717 is the most distant object ever observed by the observatory. Fortuitously, this source was independently observed by Chandra ∼110 days earlier, enabling the identification of two nearby (30′′ and 45′′ away), fainter X-ray sources. We jointly fit both Chandra and NuSTAR observations—self-consistently including interloper sources—and find that, to greater than 90% confidence, the observed 3 − 7 keV flux varied by a factor of ∼ 2.6 during that period, corresponding to approximately two weeks in the quasar rest-frame. This brightening is one the most extreme instances of statistically significant X-ray variability seen in the Epoch of Reionization. We discuss possible scenarios that could produce such rapid change, including X-ray emission from jets too faint at radio frequencies to be observed.