A Search for H I Lyman $\alpha$ Counterparts to Ultra-Fast X-ray Outflows

Prompted by the H I Ly$\alpha$ absorption associated with the X-ray ultra-fast outflow at -17,300 $\rm km~s^{-1}$ in the quasar PG~1211+143, we have searched archival UV spectra at the expected locations of H I Ly$\alpha$ absorption for a large sample of ultra-fast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outflows have predicted H I Ly$\alpha$ wavelengths falling within the bandpass of spectra from either the Far Ultraviolet Spectroscopic Explorer or the Hubble Space Telescope, although none of the archival observations were simultaneous with the X-ray observations in which UFOs were detected. In our spectra broad features with full-width at half-maximum of 1000 $\rm km~s^{-1}$ have 2-$\sigma$ upper limits on the H I column density of generally <$2\times10^{13}~\rm cm^{-2}$. Using grids of photoionization models covering a broad range of spectral energy distributions, we find that producing Fe XXVI Ly$\alpha$ X-ray absorption with equivalent widths $>30$ eV and associated H I Ly$\alpha$ absorption with $\rm N_{HI}<2\times10^{13}~cm^{-2}$ requires total absorbing column densities $\rm N_{H}>5\times10^{22}~cm^{-2}$ and ionization parameters log $\xi$ > 3.7. Nevertheless, a wide range of SEDs would predict observable H I Ly$\alpha$ absorption if ionization parameters are only slightly below peak ionization fractions for Fe XXV and Fe XXVI. The lack of Ly$\alpha$ features in the archival UV spectra indicates that either the UFOs have very high ionization parameters, very hard UV-ionizing spectra, or that they were not present at the time of the UV spectral observations due to variability.
Comment: 18 pages, 11 figures. Accepted for publication in ApJ, 4/16/18