8 Discussion

Our simultaneous Chandra and HST observations are the first definitive confirmation of an ultra-fast outflow detected simultaneously in both X-ray and UV spectra. Highly ionized gas at an outflow velocity of $-17\,300~\rm km~s^{-1}$ ($-0.0577c$) in our Chandra spectrum is an excellent match to the broad Ly$\alpha$ absorption at $-16\,980~\rm km~s^{-1}$ ($-0.0551c$) in our HST-COS spectrum (Kriss et al., 2018). Previous X-ray observations of PG1211+143 found evidence for outflows clustered near several different velocities. In the original observations using XMM-Newton, Pounds et al. (2003) identified an ultra-high velocity outflow at $\sim -0.09c$ ( $-27\,000~\rm km\,s^{-1}$), detected only in very highly ionized gas producing the FeXXVI K$\alpha$ transition. Re-analysis of this same data set reaffirmed the detection of ultra-high velocity gas, but at $-0.14 \pm 0.01~c$, with additional high-ionization lines at $\sim-0.07 c$ (Pounds, 2014). Most recently, the deepest observations to date of PG1211+143 using XMM-Newton ($\sim$450 ks) in 2014 again detected ultra-high velocity gas at $-0.129c$ and high-velocity gas at $-0.066 \pm 0.003 c$ (Reeves et al., 2018; Pounds et al., 2016b). The UFO at $-0.129~c$ is not seen in contemporaneous NuSTAR data (Zoghbi et al., 2015), but a combined analysis of the XMM-Newton and NuSTAR spectra show that the spectral structure around 7 keV is quite complex. Pounds et al. (2016b) show that the $-0.129~c$ feature is quite variable, both in column density and in ionization parameter. Given the complexity of the spectrum around 7 keV and the lower column density of the $-0.129c$ feature in 2014, from a joint analysis of the NuSTAR and XMM-Newton spectra, Lobban et al. (2016) argue that it is not surprising that it is not detectable in the NuSTAR data.

Subsequent analysis of the 2014 XMM-Newton RGS data by Reeves et al. (2018) shows that two lower-ionization, lower velocity absorbers are also present with velocities of $-0.062 \pm 0.001 c$ ( $v_{\rm out} = -18\,600 \pm 300$ kms$^{-1}$) and $-0.059 \pm 0.002 c$ ( $v_{\rm out} = -17\,700 \pm 600$ kms$^{-1}$), the latter of which is a good match to the $-17\,300$ kms$^{-1}$ ($-0.0577c$) warm absorber we detect in our Chandra spectrum. However, the absorber we detect is slightly lower in ionization (log $\xi = 2.87^{+0.32}_{-0.10}$ vs. log $\xi = 3.4 \pm 0.1$), and nearly an order of magnitude lower in column density ( $N_{\rm H} = 3 \times 10^{21}$ cm$^{-2}$ vs. $1 \times 10^{22}$ cm$^{-2}$). Both the velocity and the lower total column density are compatible with the Ly$\alpha$ absorption detected in the simultaneous HST-COS spectrum (Kriss et al., 2018).

Simultaneously detecting the same kinematic outflow with both Chandra and HST provide the first opportunity to assess the physical characteristics of an ultra-fast outflow using both X-ray and UV spectra. However, we note that this absorber is fairly high ionization, both in the X-ray and the UV. This is consistent with our detection of only broad Ly$\alpha$ in our UV spectrum-the ionization is too high to produce significant populations of the usually seen UV ions. Kriss et al. (2018) also do not detect UV absorption lines that might be associated with a lower-ionization warm absorber, either in the COS spectrum or in archival spectra from earlier epochs. This is consistent with no evidence for a lower-ionization X-ray WA in PG1211+143. Despite the curvature in the Chandra spectrum that might suggest a lower-ionization absorber, there are no detected absorption lines, either in the X-ray nor in the UV. The UV is especially sensitive in this regard. All X-ray WAs also show UV absorption in CIV (Crenshaw et al., 2003) or OVI (Dunn et al., 2008). Although Tombesi et al. (2013) has suggested that WAs may be a lower-ionization manifestation of the same wind structure represented by UFOs, but at larger distances from the black hole, this has been disputed by Laha et al. (2016). The lack of a low-ionization absorber in PG1211+143, unfortunately, does not have much bearing on this dispute since $\sim 30$% of sources containing UFOs do not have associated WAs.

The lower ionization state of the gas in our Chandra observation is expected, given the $\sim2 \times$ lower X-ray flux in 2015 compared to 2014. Such an ionization response has been seen in longer, more extensive observations of other UFOs. The long XMM-Newton observation of IRAS 13224$-$3809 shows variability of its high-ionization UFO in concert with variations in the X-ray flux (Parker et al., 2017b; Parker et al., 2017a), consistent with the response of photoionized gas. However, we also see a significant decrease in total column density, $\log N_{\rm H} = 21.5$ cm$^{-2}$ compared to $\log N_{\rm H} = 22.0$ cm$^{-2}$ Reeves et al. (2018) and $23.3$ cm$^{-2}$ Pounds et al. (2016a) in 2014. Column density variations are also seen in other UFOs such as PDS 456, where Reeves et al. (2016) suggest that the broad, variable soft X-ray absorption lines they see are due to lower-velocity clumps in the overall outflow. In PG1211+143 itself, Pounds et al. (2016a) find that both ionization and column density vary in the 2014 XMM-Newton observation.

Although in §7 we have presented a notional model for driving the observed outflow in PG1211+143 via shocks from a jet associated with the radio source, the most popular mechanism for explaining ultra-fast outflows is via a wind driven from the accretion disk. In both radiative and MHD models of accretion disk winds, the velocity of the outflow is expected to reflect the orbital velocity at which the wind was launched (e.g., Fukumura et al., 2010a; Proga et al., 2000; Proga, 2003; Kazanas et al., 2012). Based on reverberation mapping, the black-hole mass of PG1211+143 is $1.46 \times 10^8~M_{\odot}$ (Peterson et al., 2004) (although this is poorly constrained). For an orbital velocity of $17\,300$ kms$^{-1}$, the wind would have originated at $r = 6.5 \times 10^{15}$ cm, or $\sim 300$ gravitational radii ($r_g$). Interestingly, the half-light radius (at 2500 Å) for an accretion disk surrounding a black hole of this mass is approximately $5 \times 10^{15}~\rm cm$ (this estimate is based on the compilation of reverberation-mapping and gravitational micro-lensing results in Edelson et al., 2015). At the rest UV wavelength of the observed Ly$\alpha$ absorption feature $\rm (1240 \AA/1.0809 = 1147 \AA$), scaling by the $\lambda^{4/3}$ temperature profile of typical accretion disks (Edelson et al., 2015), the half-light radius of the UV continuum is then $1.8 \times 10^{15}~\rm cm$. Given that the X-ray absorber fully covers the continuum source, and that the UV absorber seems to cover only $\sim$40 percent, then the projected size of the outflow is also roughly $1.8 \times 10^{15}~\rm cm$. This suggests that the outflow originates only from a portion of the disk, perhaps from selective active regions, or that it is restricted to a conical volume with opening angle smaller than the inclination to our line of sight. In the latter case, the outflow could obscure the far side of the disk (and the full X-ray emitting region), but leave our line of sight to the near, outer side of the disk unobstructed.

As shown by several authors (e.g., Reeves & Pounds, 2012; King, 2010; King & Pounds, 2015; Nardini et al., 2015), the high outflow velocity in UFOs and their often substantial column density can lead to a large injection of energy into the interstellar medium of the AGN host galaxy. Even though our arguments above establish a plausible origin for the UFO in the outer portion of the accretion disk (at a few hundred gravitational radii), assessing its mass flux and kinetic power depend on its overall extent and covering fraction. Kriss et al. (2018) discuss several alternatives for determining the energy in the outflow observed in our joint campaign. For the case in which the outflow is restricted to a thin shell near its origin at the accretion disk, the impact is minimal. They find a minimum mass flow of $> 0.013~M_{\odot}$ yr$^{-1}$, and a minimum power in the outflow of $>1.2 \times 10^{43}$ ergs$^{-1}$. Our SED for PG1211+143 (see §5.1) gives a total bolometric luminosity of $5.3 \times 10^{45}$ ergs$^{-1}$, so such an outflow would comprise only 0.02% of the total energy output of the AGN. In contrast, feedback from AGN at levels of 0.5-5% of their radiated luminosity are required to have an evolutionary impact on the host galaxy in most models (Di Matteo et al., 2005; Hopkins & Elvis, 2010). For the more likely case where the power in the outflow is comparable to the minimum kinetic luminosity in the jet (§7: $3 \times 10^{44}~$ergs$^{-1}$), the outflow and the jet would be injecting mechanical energy at 0.6% of the AGN radiated luminosity, which is sufficient to have an impact. A definitive answer to whether the feedback from this UFO affects the host galaxy, however, requires a conclusive determination of its total size and extent.