Annu. Rev. Astron. Astrophys. 2015. 53:115-154 Copyright © 2015 by Annual Reviews. All rights reserved

2. THE OBSERVATIONAL EVIDENCE FOR ULTRA FAST OUTFLOWS

As noted above, the requirement of X-ray observations with high sensitivity and good spectral resolution over a wide energy band delayed the discovery of powerful, highly ionized winds from non-BAL AGN until the launch of Chandra and XMM-Newton. A decade after the first reports (Pounds et al. 2003, Reeves et al. 2003), high-velocity (v ∼ 0.1c), highly-ionized winds are now established to be common in low redshift AGN.

2.1. The fast outflow in PG1211+143

Exploring the nature of the ’soft excess’ in a sample of luminous Palomar Green AGN was a primary target in the Guaranteed Time programme awarded to Martin Turner, Project Scientist for the EPIC Camera on XMM-Newton (Turner et al. 2001). At that time the X-ray spectrum in AGN above ∼ 1 keV was expected to be a rather featureless power law apart from a fluorescent emission line at ∼ 6.4 keV from near-neutral Fe. One source, PG1211+143, showed a surprisingly ’noisy’ X-ray spectrum which one of us (KP) volunteered to explore.

PG1211+143, at a redshift of 0.0809 (Marziani et al. 1996), is one of the brightest AGN at soft X-ray energies. It was classified (Kaspi et al. 2000) as a Narrow Line Seyfert 1 galaxy (FWHM Hβ 1800 km s⁻¹), with black hole mass ∼ 4 × 10⁷M_⊙ and bolometric luminosity 4 × 10⁴⁵erg s⁻¹, indicating a mean accretion rate close to Eddington.

Analysis of the unusual spectral structure in the 2001 XMM-Newton observation of PG1211+143 showed it to be dominated by blue-shifted absorption lines of highly ionized metals, providing the first evidence for a high velocity ionized outflow in a non-BAL AGN, with the initial identication of a deep blue-shifted Fe Lyman-α absorption line indicating an outflow velocity of ∼ 0.09c (Pounds et al. 2003). That observation, closely followed by the detection of a still higher outflow velocity from the luminous QSO PDS 456 (Reeves et al. 2003), attracted wide attention, potentially involving the ejection of a significant fraction of the bolometric luminosity, and perhaps characteristic of AGN accreting near the Eddington rate (King & Pounds 2003).

Appropriately for such an unexpected discovery, the validity of the high velocity in PG1211+143 was not unchallenged. The near–coincidence of the observed absorption line blueshift and the redshift of the host galaxy was a concern, notwithstanding the uncomfortably high column density of heavy metals implied by a local origin. Then, in a detailed modelling of the soft X-ray RGS data, Kaspi & Behar (2006) found only a much lower velocity. Any doubts relating to the absorption being local were removed, however, by a revised velocity of 0.13−0.15c based on the inclusion of additional absorption lines from intermediate–mass ions (Pounds & Page 2006), and when repeated observations of PG1211+143 demonstrated that the strong Fe K absorption line was variable over several years (Reeves et al. 2008).

Here we use the 2001 XMM-Newton observation of PG1211+143 with the pn camera (Strueder et al. 2001) to illustrate the two methods used then – and since – to parameterise the ionized outflow. Figure 1 shows the ratio of EPIC pn data to a simple power law continuum, with a deep absorption line seen near 7 keV and additional spectral structure at ∼ 1 − 4 keV.

Figure 1. Ratio of EPIC pn data to a simple power law continuum for the 2001 XMM-Newton observation of PG1211+143 showing a deep absorption line near 7 keV and additional structure between ∼ 1 and 4 keV. Deriving an outflow velocity requires the correct identification of the individual absorption lines, which ideally requires spectral modelling with a photoionised absorber.

Fitting a negative Gaussian to the deep ∼ 7 keV absorption line (figure 2, top panel), finds an observed line energy of 7.06 ± 0.02 keV, or 7.63 ± 0.02 keV at the AGN redshift of 0.0809. The line is clearly resolved, with 1σ width ∼ 100 ± 30 eV. Assuming identification with the Fe XXV resonance (6.70 keV rest energy), the blueshifted line corresponds to an AGN outflow velocity v∼ 0.122 ± 0.005c. The most likely a priori alternative identification, with the Fe XXVI Lyman–α line (6.97 keV rest energy), conservatively adopted in the initial analysis (Pounds et al. 2003), yields a lower outflow velocity v ∼ 0.095 ± 0.005c.

An alternative procedure, which also provides additional parameters of the gas flow, requires full spectral modelling, as in Pounds & Page (2006), and more widely in recent outflow studies (Section 2.3). For the 2001 XMM-Newton pn spectrum of PG1211+143, modelling the absorption from 1–10 keV with a photoionized gas derived from the XSTAR code of Kallman et al. (1996) gives an excellent fit, for a column density N_H ∼ 3.2 ± 0.7 × 10²³ cm⁻², ionization parameter logξ = 2.7 ± 0.1 erg cm s⁻¹, and an outflow velocity (in the AGN rest frame) v ∼ 0.149 ± 0.003c. The model profile (figure 2, lower panel) shows significant inner shell absorption components to the low energy wing of the 1s-2p resonance line which explains why simply identifying the absorption near 7 keV with the 6.7 keV rest energy of Fe XXV gives too low a velocity.

Figure 2. (top) A Gaussian fit to the ∼ 7 keV absorption feature finds a line energy of 7.06 ± 0.02 keV with (1σ) width 100 ± 30 eV. Identification with the Fe XXV 1s-2p resonance line (6.70 keV rest energy) gives an outflow velocity v ∼ 0.12 ± 0.01c. (lower) Alternative modelling with a photoionised gas over the wider 1–10 keV spectral band yields a good fit with a relatively high column density NH ∼ 3.2 ± 0.7 × 1023 cm−2, moderate ionisation parameter logξ = 2.7 ± 0.1 erg cm s−1, and outflow velocity of v ∼ 0.15 ± 0.01c. The Fe XXV absorption line profile is seen to include lower energy components due to the addition of one or more L-shell electrons, showing why the simple Gaussian fit gives too low a velocity.

Although individually weaker than the Fe absorption, the combination of resonance lines of He– and H–like Mg, Si, S and Ar in the broadband spectral fit is evidently driving the spectral fit. That conclusion is confirmed with Gaussian fits to corresponding absorption features in Figure 1, which find a weighted observed blueshift of 0.055 ± 0.05 and outflow velocity (at the AGN redshift) of v ∼ 0.14 ± 0.01c, a value consistent with that found from spectral modelling, but significantly higher than from simply identifying the ∼ 7 keV absorption line with the resonance 1s-2p transitions of either FeXXV or FeXXVI.

An interesting by-product of the XSTAR modelling in the above case is that the observed broadening of the ∼ 7 keV absorption line does not require high turbulence (we used grid 25 with v_turb of 200 km s⁻¹) or an accelerating/decelerating flow. Instead, intrinsically narrow absorption components remain consistent with a radial outflow, coasting post–launch.

2.2. Mass rate and mechanical energy in the PG1211+143 outflow

Although the detection of high-speed winds in a substantial fraction of bright AGN suggests most such flows have a large covering factor, PG1211+143 is one of very few where a wide angle flow has been demonstrated directly.

Using stacked data from 4 XMM-Newton observations between 2001 and 2007, Pounds & Reeves (2007, 2009) examined the relative strength of ionized emission and absorption spectra modelled by XSTAR to estimate the covering factor and collimation of the outflowing ionized gas. The summed pn data of PG1211+143 also shows a well defined P Cygni profile in the Fe K band (Figure 3), the classical signature of an outflow, with emission and absorption components of comparable equivalent width. Both methods indicated a covering factor b (= Ω / 2π) of 0.75 ± 0.25. Analysis of a Suzaku observation of PG1211+143 gives a similar result (Reeves et al. 2008), with an intrinsic emission component of ∼ 6.5 keV and width of σ ∼ 250 eV, corresponding to a flow cone of half angle ∼ 50 deg, assuming velocity broadening in a radial flow.

Figure 3. The PCygni profile of Fe XXV from stacked XMM-Newton pn observations of PG1211+143 is characteristic of a wide angle outflow. The comparable equivalent width of emission and blue-shifted absorption components indicates the highly ionized outflow has a large covering factor. From Pounds and Reeves 2009.

The outflow mass rate and mechanical energy can then be estimated, since for a uniform radial outflow of velocity v the mass rate is:

(9)

where n is the gas density at a radial distance r, and m_p is the proton mass.

The observed values for PG1211+143 find a mass loss rate of _out ∼ 7 × 10²⁵ gm s⁻¹ (∼ 2.5M_⊙ yr⁻¹), and mechanical energy ∼ 4.5 × 10⁴⁴ erg s⁻¹ (Pounds & Reeves 2009).

The mass loss rate is comparable to the Eddington accretion rate _Edd = 1.3 M_⊙ yr⁻¹ for a supermassive black hole of mass ∼ 4 × 10⁷ M_⊙ accreting at an efficiency of 10%, while the outflow mechanical energy is only ∼ 6% of the Eddington luminosity, close to that predicted by continuum driving (equation 5 in Section 3 below). As noted elsewhere that energy flow rate would be more than sufficient to unbind the gas of the host galaxy bulge if all its energy were efficiently communicated.

2.3. High speed winds are common

The evidence for high velocity winds as an important property of AGN remained dependent on the prototype case of PG1211+143 for several years, with fast outflows in two BAL AGN (Chartas et al. 2002) and in the most luminous low redshift QSO PDS 456 (Reeves et al. 2003, O'Brien et al. 2005) seen as rare objects. That began to change with the detection of a highly significant outflow of velocity ∼ 0.1c in the Seyfert 1 galaxy IC4329A (Markowitz et al. 2006), and several outflow detections in the range ∼ 0.14−0.2c in multiple observations of Mrk 509 (Dadina et al. 2005). A review in 2006 (Cappi et al. 2006) listed 7 non-BAL objects with outflows of v ∼ 0.1c and several with red-shifted absorption lines.

A major step forward came with the results of an XMM-Newton archival search of bright AGN by Tombesi et al. (2010), finding strong statistical evidence in 15 of 42 radio-quiet objects of blue-shifted iron K absorption lines, identification with FeXXV or XXVI resonance absorption lines implying ultrafast outflow (UFO) velocities up to ∼ 0.3c, and clustering near v ∼ 0.1c. A later analysis based on broad-band modelling with XSTAR photoionized grids (Tombesi et al. 2011) led to several revised velocities and confirmed that the outflows were typically highly ionized, with logξ ∼ 3 − 6 erg cm s⁻¹, with column densities in the range N_H ∼ 10²² − 10²⁴ cm⁻². A similar search of the Suzaku data archive (Gofford et al. 2013) yielded a further group of UFO detections, finding significant absorption in the Fe K band in 20 (of 51) AGN with velocities up to ∼ 0.3c and a flatter distribution than the XMM-Newton sample, but a median value again v ∼ 0.1c.

Figure 4 brings together the results from the spectral modelling analyses of the XMM-Newton and Suzaku surveys. We follow Tombesi et al. (2011) in defining UFOs as having outflow velocities greater than 10⁴ km s⁻¹, to discriminate against WAs or post-shock flows (Sections 2.4 and 4). The velocity plot shows a peak at ∼ 0.1c, with a tail extending to ∼ 0.3c. In terms of the continuum-driving Black Hole Winds model (King & Pounds 2003) the higher velocities would imply a higher value of the accretion efficiency η, with the future potential for such observations to provide a measure of black hole spin. Equation (22) also suggests the low velocity tail in both the Tombesi et al. and Gofford et al. distributions could relate to primary outflows formed at a higher accretion ratio (but see Section 3.1).

Figure 4. Distribution of outflow velocities, ionization parameter (erg cm s−1) and column density (cm−2) obtained from modelling the individual spectra from extended observations of type 1 AGN in the XMM-Newton and Suzaku data archives (Tombesi et al. 2011, Gofford et al. 2013). The red lined histogram refers to lower limits in column density.

Figure 4 also shows the distribution of ionization parameter and absorption column density from the surveys of Tombesi et al. (2011) and Gofford et al. (2013). The high ionization parameter, peaking near logξ ∼ 4, explains why the detection of UFOs has been almost exclusively from X–ray observations in the Fe K band, leaving open the possibility that fully ionized outflows (also consistent with continuum driving) will become detectable when the AGN luminosity (and hence ionization) falls. In assessing observational data it is important to note that for a radial outflow the observed column density is a line–of–sight integration over the flow time, dominated by the higher density at small radii, while the ionization parameter is governed by the current AGN luminosity. The column density, which generally lies below N_H ∼ 10²⁴ cm⁻², can vary rapidly and turns out to be a powerful diagnostic of the flow history and dynamics. We return to the observability of UFOs in Section 3.3.

2.4. Evidence for a shocked flow

The mechanical energy in a fast wind, such as that in PG1211+143, was noted in Section 2.2 to be incompatible with the continued growth of the black hole and stellar bulge of the host galaxy, unless the flow is short-lived or the coupling of wind energy to bulge gas is highly inefficient. A recent XMM-Newton observation of the narrow-line Seyfert galaxy NGC 4051 has provided the first evidence of a fast ionized wind being shocked, with subsequent strong cooling leading to most of the initial flow energy being lost before it can be communicated to the bulge gas. We outline a possible scenario for that event below.

NGC 4051 was found in the XMM-Newton archival search to have a high velocity wind during an observation in 2002 when the source was in an unusually low state, the initial identification with Fe XXVI Lyman-α in Tombesi et al. (2010) indicating a velocity of ∼ 0.15c. In a full spectral fit (Tombesi et al. 2011) identification with Fe XXV was preferred, with an increased velocity ∼ 0.20c. Significantly, in a 2001 observation of NGC 4051, when the X-ray flux was much higher, a strong outflow was detected at ∼ 6000 km s⁻¹, but no ultra-fast wind was seen.

It seems that the detection of a UFO in NGC 4051 is unusually dependent on the source flux, with evidence for a high velocity wind (v ∼ 0.13c) again found only during periods when the ionizing continuum was low during a further XMM-Newton observation in 2009 (Pounds & Vaughan 2012). An additional factor may be the low redshift (z = 0.00234) of NGC 4051, which makes a high velocity wind more difficult to detect with current observing facilities.

The 600 ks XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009, extending over 6 weeks and 15 spacecraft orbits, broke new ground by finding an unusually rich absorption spectrum with multiple outflow velocities, in both RGS (den Herder et al. 2001) and EPIC spectra, up to ∼ 9000 km s⁻¹ (Pounds & Vaughan 2011a). Inter-orbit variability is seen in both absorption and emission lines, with strong recombination continua (RRC) and velocity-broadened resonance lines providing constraints on the dynamics and geometry of the putative post-shock flow (Pounds and Vaughan 2011b, 2012).

2.5. A self-consistent model for the shocked wind in NGC 4051

More complete modelling of both RGS and EPIC pn absorption spectra of NGC 4051 found a highly significant correlation of outflow velocity and ionization state (figure 5), as expected from mass conservation in a post–shock flow (King 2010, Pounds & King 2013). The additional analysis also found a range of column densities to be required by the individual XSTAR absorption components, suggesting an inhomogeneous shocked flow, perhaps with lower ionization gas clumps or filaments embedded in a more extended, lower density and more highly ionized flow.

Figure 5. The outflow velocity and ionization parameters for 6 XSTAR photoionised absorbers used to fit the RGS and EPIC spectra of NGC 4051, together with a high point representative of the pre-shock wind, show the linear correlation expected for a mass-conserved cooling flow (see Pounds and King 2013).

Theoretical considerations suggested a key factor in determining the structure of the post-shock flow was likely to be the cooling time, as discussed in more detail in Section 4. In particular, the fate of a fast wind depends on the distance it travels before colliding with the ISM or slower-moving ejecta, with Compton cooling dominating for a shock occurring sufficiently close to the AGN continuum source.

Importantly, flux-linked variations in the ratio of FeXXV to Fe XXVI absorption in the 2009 XMM-Newton observation (figure 6) provided a measure of the Compton cooling time, the mean flow speed then determining the shell thickness of the hotter, more highly ionized flow component. The detection of strong recombination continua (RRC) in the soft X-ray spectra furthermore suggested an increasing density in the decelerating post-shock flow, with two-body cooling becoming increasingly important.

Figure 6. Fe K profiles from observations of NGC 4051 several days apart show an increased level of ionization coinciding with a hard X-ray flare (data from Pounds and Vaughan 2012). The ratio of resonance absorption lines of Fe XXV and Fe XXVI is a sensitive measure of the ionization state of the absorbing gas.

To pursue that idea we note that at the (adiabatic) shock the free–free (thermal bremsstrahlung) and Compton cooling times are

(10)

and

(11)

respectively (see King et al. 2011: here T, N are the postshock temperature and number density, R₁₆ is the shock radius in units of 10¹⁶ cm, M₇ is the black hole mass in units of 10⁷ M_⊙, and ∼ 1 is the Eddington factor of the mass outflow rate).

After the adiabatic shock, the gas cools rapidly via inverse Compton cooling, while its density rises as N ∝ T⁻¹ (pressure is almost constant in an isothermal shock), and

(12)

which means that the free–free cooling time decreases sharply while the Compton time does not change. Eventually free–free (and other atomic two–body processes) become faster than Compton when T has decreased sufficiently below the original shock temperature T_s ∼ 1.6 × 10¹⁰ K. From (10, 11) above this requires

(13)

or

(14)

The temperature of ionization species forming around a few keV is therefore likely to be determined by atomic cooling processes rather than Compton cooling. The strong recombination continua in NGC 4051 (Pounds & Vaughan 2011b, Pounds & King 2013) are direct evidence for that additional cooling, with the RRC flux yielding an emission measure for the related flow component. In particular, the onset of strong two-body cooling results in the lower-ionization, lower-velocity gas being confined in a relatively narrow region in the later stages of the post-shock flow. The structure and scale of both high and low ionization flow regions can be deduced from the observations and modelling parameters.

For the highly ionized post-shock flow, the iron Ly–α to He–α ratio will be governed by the ionizing continuum and recombination time. Significant variations in this ratio are found on inter-orbit timescales (Pounds & Vaughan 2012), with an example shown in figure 6. For a mean temperature of ∼ 1 keV, and recombination coefficient of 4.6 × 10⁻¹² cm³ s⁻¹ (Verner & Ferland 1996), the observed recombination timescale of ∼ 2 × 10⁵ s corresponds to an average particle density of ∼ 4 × 10⁶ cm⁻³. Comparison with a relevant absorption column N_H ∼ 4 × 10²² cm⁻² from the XSTAR modelling indicates a column length scale of ∼ 10¹⁶ cm. Assuming a mean velocity of the highly ionized post-shock flow of 6000 km s⁻¹, the observed absorption length corresponds to a flow time ∼ 1.7 × 10⁷ s (0.6 yr). Equation (11) finds a comparable cooling time for NGC 4051 at a shock radius R ∼ 10¹⁷ cm.

For the low-ionization flow component, decay of strong RRC of NVII, CVI and CV (Pounds & Vaughan 2011b, Pounds & King 2013), occurs over ∼ 2−6 days. With an electron temperature from the mean RRC profile of ∼ 5 eV, and recombination coefficient for CVI of ∼ 10⁻¹¹ cm³ s⁻¹ (Verner & Ferland 1996), the observed RRC decay timescale corresponds to a (minimum) electron density of ∼ 2 × 10⁶ cm⁻³. A column density of 1.5 × 10²¹ cm⁻² from modelling absorption in the main low-ionization flow component then corresponds to an absorbing path length of 7 × 10¹⁴ cm.

The RRC emission flux provides a consistency check on the above scaling. Assuming solar abundances, and 30 percent of recombinations direct to the ground state, a CVI RRC flux of ∼ 10⁻⁵ photons cm⁻² s⁻¹ corresponds to an emission measure of ∼ 2 × 10⁶² cm⁻³, assuming a Tully–Fisher distance to NGC 4051 of 15.2 Mpc. With a mean particle density of ∼ 2 × 10⁶ cm⁻³ the emission volume (4π R²Δ R) is ∼ 5 × 10⁴⁹ cm³. Assuming a spherical shell geometry of the flow, with fractional solid angle b, shell thickness Δ R ∼ 7 × 10¹⁴ cm, and shell radius R ∼ 10¹⁷ cm, the measured RRC flux is reproduced for b ∼ 0.5.

Although this excellent agreement may be fortuitous given the approximate nature and averaging of several observed and modelled parameters, the mutual consistency of absorption and emission of the photoionized flow is encouraging. Given that only blueshifted RRC emission is seen, b ∼ 0.5 is consistent with a wide-angle flow, visible only on the near side of the accretion disc.

Figure 7 illustrates the main features of the overall NGC 4051 outflow, a fast primary wind being shocked at a radial distance of order 0.1pc, within the zone of influence of an SMBH of 1.7 × 10⁶ M_⊙. The initially hot gas then cools in the strong radiation field of the AGN, with a Compton cooling length determining the absorption columns of Fe and the other heavy metal ions. Two–body recombination provides additional cooling as the density rises downstream, eventually becoming dominant. Absorption (and emission) in the soft X–ray band is located primarily in this thinner outer layer of the post–shock flow.

Figure 7. Schematic view of the shock pattern resulting from the impact of a black hole wind (blue) on the interstellar gas (red) of the host galaxy. The accreting supermassive black hole drives a fast wind (velocity v ∼ ηc / ∼ 0.1c), whose ionization state makes it observable in X–ray absorption lines. It collides with the ambient gas in the host galaxy and is slowed in a strong shock. The inverse Compton effect from the quasar's radiation field rapidly cools the shocked gas, removing its thermal energy and strongly compressing and slowing it over a very narrow radial extent. In the most compressed gas, two–body cooling becomes important, and the flow rapidly cools and slows over an even narrower region. In NGC 4051 this region is detected in the soft X–ray spectrum, where absorption (and emission) are dominated by the lighter metals. The cooled gas exerts the preshock ram pressure on the galaxy's interstellar gas and sweeps it up into a dense shell (‘snowplow’). The shell's motion then drives a milder outward shock into the ambient interstellar medium. This shock ultimately stalls unless the SMBH mass has reached the value Mσ satisfying the M − σ relation (from Pounds and King 2013).

It is interesting to note that similar shocking of fast outflows provides a natural link between UFOs and the equally common ‘warm absorbers’ in AGN (Tombesi et al. 2013). While the onset of strong two–body cooling, resulting in the intermediate column densities being small, might explain why evidence for intermediate-flow velocities has awaited an unusually long observation of a low mass AGN, the accumulated ‘debris’ of shocked wind and ISM could be a major component of the ‘warm absorber’. See Section 7.4.1 for a discussion.

2.6. Variability of UFOs

While it is likely that powerful winds blow continuously in AGN in rapid growth phases, it is important to note that the existing observations of UFOs are restricted to bright, low-redshift AGN, z ≤ 0.1, where the X-ray fluxes are sufficient to yield high quality spectra. Repeated observation of several bright AGN frequently show changes in the equivalent width in the primary Fe K absorption line.

Variability in the strength of blueshifted Fe-K absorption over several years in PG1211+143 was first noted in a comparison of the initial XMM-Newton and Chandra observations (Reeves et al. 2008), and confirmed by repeated XMM-Newton observations (Pounds and Reeves 2009). Multiple observations of the luminous Seyfert 1 galaxy Mrk 509 (Cappi et al. 2009) found variations in both intensity and blueshift of Fe K absorption lines. The archival searches provide the most comprehensive variability data, with repeated observations of several AGN demonstrating that variability of absorption line equivalent width (EW) over several years is common. More rapid variability in EW, over a few months, is reported in the XMM-Newton archive for Mrk 509, Mrk 79 and Mrk 841, with both velocity and EW change in ≤2 days for Mrk 766.

In addition, the ‘hit rate’ of UFOs for multiply-observed AGN in the archival XMM-Newton data search (Tombesi et al. 2010) was relatively low, being 1 of 6 observations for NGC 4151, MCG-6-30-15 (0/5), Mrk 509 (3/5), NGC 4051(1/2), Mrk 79 (1/3), Mrk 205 (1/3), and Mrk 290 (1/4). Overall, though 101 suitably extended observations yielded 36 narrow absorption line detections in the Fe K band, only 22 were observed at > 7 keV. While the UFO ‘hit rate’ of ∼ 22% is a lower limit set by the sensitivity of available exposures it seems clear that the fast outflows currently being detected in low-redshift AGN are far from continuous.