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

TL;DR

This paper reports the first deep NuSTAR X-ray observations of a highly luminous quasar at redshift 6.19, revealing significant variability over weeks, and discusses potential mechanisms behind this rapid change during the Epoch of Reionization.

Contribution

It presents the first deep NuSTAR observations of a z>6 quasar, detecting extreme X-ray variability and analyzing its implications for early universe black hole activity.

Findings

Detected a factor of 2.6 X-ray flux variation over two weeks in the quasar.

Identified two nearby faint X-ray sources affecting the observations.

Concluded the variability is among the most extreme at this epoch.

Abstract

With enough X-ray flux to be detected in a 160s 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 $\sim180$ 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 $\sim110$ 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 $\sim2.6$ 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.