The Extreme Ultraviolet Variability of Quasars
  We study the extreme ultraviolet (EUV) variability (rest frame wavelengths
500 - 920 $\AA$) of high luminosity quasars using HST (low to intermediate
redshift sample) and SDSS (high redshift sample) archives. The combined HST and
SDSS data indicates a much more pronounced variability when the sampling time
between observations in the quasar rest frame is $> 2\times 10^{7}$ sec
compared to $< 1.5\times 10^{7}$ sec. Based on an excess variance analysis, for
time intervals $< 2\times 10^{7}$ sec in the quasar rest frame, $10\%$ of the
quasars (4/40) show evidence of EUV variability. Similarly, for time intervals
$>2\times 10^{7}$ sec in the quasar rest frame, $55\%$ of the quasars (21/38)
show evidence of EUV variability. The propensity for variability does not show
any statistically significant change between $2.5\times 10^{7}$ sec and
$3.16\times 10^{7}$ sec (1 yr). The temporal behavior is one of a threshold
time interval for significant variability as opposed to a gradual increase on
these time scales. A threshold time scale can indicate a characteristic spatial
dimension of the EUV region. We explore this concept in the context of the slim
disk models of accretion. We find that for rapidly spinning black holes, the
radial infall time to the plunge region of the optically thin surface layer of
the slim disk that is responsible for the preponderance of the EUV flux
emission (primarily within 0 - 7 black hole radii from the inner edge of the
disk) is consistent with the empirically determined variability time scale.
