Exploratory X-ray Monitoring of Luminous Radio-Quiet Quasars at High Redshift

We present new results from an ongoing X-ray monitoring project of two groups of comparably luminous radio-quiet quasars (RQQs). The first consists of four sources at 4.10<= z<= 4.35, monitored by Chandra, and the second is a comparison sample of three sources at 1.33<= z<= 2.74, monitored by Swift. Together with archival X-ray data, the total rest-frame temporal baseline spans ˜2-4 yr and ˜5-13 yr for the first and second group, respectively. The main finding is that luminous RQQs at the highest accessible redshifts exhibit pronounced X-ray variability, above that expected from their luminosities, which is quite similar to many of their nearby and far less luminous counterparts. We show that this surprising result may be attributed to luminous sources at the highest redshifts having relatively high accretion rates. Complementary UV-optical monitoring of the two groups of sources shows that variations in their optical-X-ray spectral energy distribution are dominated by the X-ray variations. This project is designed to provide a basic assessment of the X-ray variability properties of RQQs at the highest accessible redshifts that will serve as a benchmark for more ambitious and systematic monitoring of such sources with future X-ray missions.