Quasar Spectral Energy Distributions in the Rest-Frame EUV: Hubble-COS Spectra of Two Ultra-luminous Quasars

Abstract

Using the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope with both far-UV (FUV) and near-UV (NUV) gratings, we measure the ionizing spectra of two bright, intermediate-redshift quasars in their rest-frame extreme ultraviolet (EUV). The availability of both NUV and FUV spectra allows us to define the quasar continuum and correct for strong Lyman-limit systems (LLS) that fall in the gap between the FUV and optical. Each AGN has a prominent LLS, but the flux recovery shortward of their Lyman edges allows us to fit and restore the true AGN continuum. In the EUV (450-912 A) these AGN have flux distributions, $F_{\nu} \propto \nu^{-\alpha_{\nu}}$, with spectral indices $\alpha_{\nu} = 1.11\pm0.22$ (SBS 1010+535, $z_{\rm AGN} = 1.5086$) and $\alpha_{\nu} = 0.98\pm0.22$ (HS 0747+4259, $z_{\rm AGN} = 1.9006$), both considerably harder than the mean index, $\alpha_{\nu} = 1.41\pm0.15$, in a COS composite spectrum of 159 UV-bright AGN. These two AGN are outliers in the index distribution, perhaps resulting from their extremely high UV luminosity ($10^{48}~{\rm erg~s}^{-1}$), estimated black-hole masses (0.5-1)$\times10^{10} M_{\odot}$, and effects on the inner accretion disk and Comptonized winds.