Exploratory X-Ray Monitoring of Luminous Radio-quiet Quasars at High Redshift: No Evidence for Evolution in X-Ray Variability

We report on the second installment of an X-ray monitoring project of seven luminous radio-quiet quasars (RQQs). New Chandra observations of four of these, at 4.10 ≤ z ≤ 4.35, yield a total of six X-ray epochs per source, with temporal baselines of ∼850-1600 days in the rest frame. These data provide the best X-ray light curves for RQQs at z > 4 to date, enabling qualitative investigations of the X-ray variability behavior of such sources for the first time. On average, these sources follow the trend of decreasing variability amplitude with increasing luminosity, and there is no evidence for X-ray variability increasing toward higher redshifts, in contrast with earlier predictions of potential evolutionary scenarios. An ensemble variability structure function reveals that their variability level remains relatively flat across ≈20-1000 days in the rest frame and it is generally lower than that of three similarly luminous RQQs at 1.33 ≤ z ≤ 2.74 over the same temporal range. We discuss possible explanations for the increased variability of the lower-redshift subsample and, in particular, whether higher accretion rates play a leading role. Near-simultaneous optical monitoring of the sources at 4.10 ≤ z ≤ 4.35 indicates that none is variable on ≈1 day timescales, although flux variations of up to ∼25% are observed on ≈100 day timescales, typical of RQQs at similar redshifts. Significant optical-X-ray spectral slope variations observed in two of these sources are consistent with the levels observed in luminous RQQs and are dominated by X-ray variations.