Quasar Spectral Energy Distributions in the Rest-frame Extreme Ultraviolet: Hubble-COS Spectra of Two Ultraluminous Quasars

Abstract Using the Cosmic Origins Spectrograph (COS) on board 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 (LLSs) that fall in the gap between the FUV and optical. Each active galactic nucleus (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 Å), these AGN have flux distributions, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>F</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msup> </mml:math> , with spectral indices α ν = 1.11 ± 0.22 (SBS 1010+535, z AGN = 1.5086) and α ν = 0.98 ± 0.22 (HS 0747+4259, z AGN = 1.9006), both considerably harder than the mean index, α ν = 1.41 ± 0.15, 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 (0.4-0.9) × 10 48 erg s −1 ), estimated black-hole masses (0.5–1) × 10 10 M ⊙ , and effects on the inner accretion disk and Comptonized winds.