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

TL;DR

This study measures the EUV spectra of two ultra-luminous quasars using Hubble-COS, revealing their unusually hard ionizing spectra and providing insights into their extreme luminosity and black hole properties.

Contribution

First to obtain rest-frame EUV spectra of ultra-luminous quasars with COS, revealing their harder-than-average spectral indices and implications for accretion physics.

Findings

EUV spectral indices of ~1.1 and ~1.0 for the two quasars.

Both quasars are outliers with harder spectra than the average AGN.

High UV luminosity and black hole mass may influence their spectral hardness.

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.