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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 intwo epoch optical spectra of the Sloan Digital Sky Survey. We detected Lyaresidue 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 FUVand NUV bands by the Galaxy Evolution Explorer. The high neutral hydrogencolumn density should allow a negligible transmission of both the Lya linephotons and EUV continuum photons due to the high optical depth of the gas. Thepossible scenarios of foreground galaxy contamination, partial coverage,emission from the quasar host galaxy, and extended Lya emission are excluded inturn, and we speculate that the residual Lya and EUV emission is due to photonsscattering (broad Lya and the continuum emission) of electrons residing at aspatial scale larger than that of the proximate sub-DLA. Electron scattering iswavelength-independent, the scattered light is therefore a copy of the incidentspectrum that might originate in the accretion disk. With the assistances fromthe neutral hydrogen absorbers as the "natural coronagraph" and the scatterersas 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 standardpicture 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 etal. (2008).

A Strange EUV Emission: Scattered Continuum in the Lymann Limit Absorption Edge toward the Quasar SDSS J125903.26+621211.5?