We identified a peculiar proximate sub-damped Ly \alpha  absorption system ( sub-DLA ) at z=3.234 with a neutral hydrogen column density of N _ { H { \footnotesize I } } = 10 ^ { 19.70 \pm 0.15 } ~ { } cm ^ { -2 } toward the quasar SDSS J125903.26+621211.5 in two epoch optical spectra of the Sloan Digital Sky Survey .
We detected Ly \alpha  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 Ly \alpha  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 Ly \alpha  emission are excluded in turn , and we speculate that the residual Ly \alpha  and EUV emission is due to photons scattering ( broad Ly \alpha  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 ) .