2018ApJ...863..198P

We identified a peculiar proximate subdamped Lyα absorption system (sub-DLA) at z = 3.234 with a neutral hydrogen column density of N_HI=10^19.70±0.15cm^–2 toward the quasar SDSS J125903.26+621211.5 in two epoch optical spectra of the Sloan Digital Sky Survey. We detected Lyα residue in the proximate sub-DLA trough at a >8σ level. To our surprise, significant (>4σ) extreme ultraviolet (EUV) continuum emission was detected in the corresponding Lyman 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 Lyα 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 Lyα emission are excluded in turn, and we speculate that the residual Lyα and EUV emissions are due to photon scattering (broad Lyα and the continuum emission) of electrons residing at a spatial scale larger than that of the proximate sub-DLA. Electron scattering is wavelength-independent, and the scattered light is therefore a copy of the incident spectrum that might originate in the accretion disk. With assistance 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 α = 0.65 ± 0.25 (F_ν∝ν^α), 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.