2025/08/13 by Shull, J. Michael, Bordoloi, Rongmon, Danforth, Charles W.
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2508.10097
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, Fν ∝ ν-αν, with spectral indices αν = 1.11±0.22 (SBS 1010+535, z\rm AGN = 1.5086) and αν = 0.98±0.22 (HS 0747+4259, z\rm AGN = 1.9006), both considerably harder than the mean index, αν = 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 (1048~\rm erg~s-1), estimated black-hole masses (0.5-1)×1010 M\odot, and effects on the inner accretion disk and Comptonized winds.