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

TL;DR

This study searches for H I Ly$ extalpha$ counterparts to ultra-fast X-ray outflows in quasars, finding no such features in archival UV spectra, which suggests high ionization, spectral hardness, or variability of the outflows.

Contribution

It provides the first systematic search for UV Ly$ extalpha$ counterparts to X-ray ultra-fast outflows using archival data, constraining their ionization states and physical conditions.

Findings

No H I Ly$ extalpha$ absorption detected in archival spectra.

High ionization parameters or spectral hardness likely explain the absence.

Outflows may be variable or have very high ionization states.

Abstract

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.