The extreme ultraviolet variability of quasars

We study the extreme ultraviolet (EUV) variability (rest frame wavelengths 500–920 Å) of high-luminosity quasars using Hubble Space Telescope (HST) (low to intermediate redshift sample) and Sloan Digital sky Survey (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 $>2\times {10}^{7}s$ compared to $<1.5\times {10}^7$ s. Based on an excess variance analysis, for time intervals $<2\times {10}^{7}s$ in the quasar rest frame, 10% of the quasars (4/40) show evidence of EUV variability. Similarly, for time intervals $>2\times {10}^{7}s$ 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 $2.5\times {10}^{7}s$ and $3.16\times {10}^{7}s$ (1 year). The temporal behavior is one of a threshold time interval for significant variability as opposed to a gradual increase on these timescales. A threshold timescale 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 timescale.