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

Abstract

Prompted by the H I Ly$\alpha$ absorption associated with the X-ray ultra-fast outflow at -17,300 $\mathrm{km}{\mathrm{s}}^{-1}$ 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 $\mathrm{km}{\mathrm{s}}^{-1}$ have 2-$\sigma$ upper limits on the H I column density of generally <$2\times 10^{13}\mathrm{c}{\mathrm{m}}^{-2}$. 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 ${\mathrm{N}}_{\mathrm{HI}}<2\times 10^{13}\mathrm{c}{\mathrm{m}}^{-2}$ requires total absorbing column densities ${\mathrm{N}}_{\mathrm{H}}>5\times 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$ 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.

18 pages, 11 figures. Accepted for publication in ApJ, 4/16/18

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• Mapping the "invisible" circumgalactic medium around a z $\sim$ 4.5 radio galaxy with MUSE
• Resolving the X-ray obscuration in a low flux observation of the quasar PDS 456
• Supermassive Black Hole Winds in X-rays: SUBWAYS. II. HST UV spectroscopy of winds at intermediate redshifts
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• UV counterpart of an X-ray ultra-fast outflow in IRAS 17020+4544