Exploratory X-Ray monitoring of luminous radio quiet quasars at high redshift: no evidence for evolution in X- Ray variability

Abstract

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 similar to 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 approximate to 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 approximate to 1 day timescales, although flux variations of up to similar to 25% are observed on approximate to 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.