Exploratory X-ray Monitoring of Luminous Radio-Quiet Quasars at High
  Redshift: Initial Results
  We present initial results from an exploratory 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. Six of these sources show significant X-ray variability over
rest-frame timescales of ~10^2 - 10^3 d; three of these also show significant
X-ray variability on rest-frame timescales of ~1-10 d. The X-ray variability
properties of our variable sources are similar to those exhibited by nearby and
far less luminous active galactic nuclei (AGNs). While we do not directly
detect a trend of increasing X-ray variability with redshift, we do confirm
previous reports of luminous AGNs exhibiting X-ray variability above that
expected from their luminosities, based on simplistic extrapolation from lower
luminosity sources. This result may be attributed to luminous sources at the
highest redshifts having relatively high accretion rates. Complementary
UV-optical monitoring of our sources shows that variations in their
optical-X-ray spectral energy distribution are dominated by the X-ray
variations. We confirm previous reports of X-ray spectral variations in one of
our sources, HS 1700+6416, but do not detect such variations in any of our
other sources in spite of X-ray flux variations of up to a factor of ~4. 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 systematic monitoring of such sources with future X-ray
missions.
