A Strange EUV Emission: Scattered Continuum in the Lymann Limit
  Absorption Edge toward the Quasar SDSS J125903.26+621211.5?
  We identified a peculiar proximate sub-damped Lya absorption system (sub-DLA)
at z=3.234 with a neutral hydrogen column density of $N_{H}
=10^{19.70\pm0.15}~\rm cm^{-2}$ toward the quasar SDSS J125903.26+621211.5 in
two epoch optical spectra of the Sloan Digital Sky Survey. We detected Lya
residue in the proximate sub-DLA trough at $> 8\sigma$ level. To our surprise,
extreme ultraviolet (EUV) continuum emission is significantly ($> 4\sigma$)
detected in the corresponding Lymann limit absorption edge at both of the FUV
and NUV bands by the Galaxy Evolution Explorer. The high neutral hydrogen
column density should allow a negligible transmission of both the Lya line
photons and EUV continuum photons due to the high optical depth of the gas. The
possible scenarios of foreground galaxy contamination, partial coverage,
emission from the quasar host galaxy, and extended Lya emission are excluded in
turn, and we speculate that the residual Lya and EUV emission is due to photons
scattering (broad Lya and the continuum emission) of electrons residing at a
spatial scale larger than that of the proximate sub-DLA. Electron scattering is
wavelength-independent, the scattered light is therefore a copy of the incident
spectrum that might originate in the accretion disk. With the assistances from
the neutral hydrogen absorbers as the "natural coronagraph" and the scatterers
as the "natural mirror", we yielded a very hard EUV spectral index of $\alpha =
0.65\pm 0.25$ ($F_{\nu}\propto \nu^{\alpha}$), consistent with the standard
picture of the locally heated accretion disk in the inner EUV-emitting radii,
as well as in the outer near-infrared-emitting radii suggested by Kishimoto et
al. (2008).
