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

Lea Marcotulli, Thomas Connor, Eduardo Bañados, Peter G. Boorman, Giulia Migliori, Brian W. Grefenstette, Emmanuel Momjian, Aneta Siemiginowska, Daniel Stern, Silvia Belladitta, C. C. Cheung, Andrew Fabian, Yana Khusanova, Chiara Mazzucchelli, Sofía Rojas-Ruiz, C. Megan Urry

Published 2025-01-13Version 1

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.