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

TL;DR

This paper reports the detection of unexpected EUV continuum emission in a quasar spectrum, attributed to electron scattering of photons from the accretion disk, revealing insights into the EUV emission mechanisms.

Contribution

It introduces the first evidence of scattered EUV continuum in a quasar, using a proximate sub-DLA as a natural coronagraph and mirror to study the accretion disk emission.

Findings

Detection of significant EUV continuum emission at the Lyman limit

Spectral index of the EUV emission consistent with accretion disk models

Electron scattering explains the residual Lya and EUV emission

Abstract

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).