A new relativistic component of the accretion disk wind in PDS 456
James Reeves Valentina Braito Emanuele Nardini Andrew Lobban Gabriele Matzeu Michele Costa
astro-ph.HE astro-ph.GA


A relativistic wind in PDS 456.
Reeves et al.

1Center for Space Science and Technology, 
University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA; email jreeves@umbc.edu
2INAF, Osservatorio Astronomico di Brera, Via Bianchi 46 I-23807 Merate (LC), Italy
3INAF - - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
4Astrophysics Group, School of Physical and Geographical Sciences, Keele 
University, Keele, Staffordshire, ST5 5BG, UK
5European Space Astronomy Centre (ESA/ESAC), E-28691 Villanueva de la Canada, Madrid, Spain




Past X-ray observations of the nearby luminous quasar PDS 456 (at z=0.184) have revealed a wide angle accretion disk wind <cit.>, with an outflow velocity of ∼-0.25c. Here we unveil a new, relativistic component of the wind through hard X-ray observations with NuSTAR and XMM-Newton, obtained in March 2017 when the quasar was in a low flux state.
This very fast wind component, with an outflow velocity of -0.46±0.02c, is detected in the 
iron K band, in addition to the -0.25c wind zone. 
The relativistic component may arise from the innermost disk wind, launched from close to the black hole at radius of ∼10 gravitational radii.
The opacity of the fast wind also increases during a possible obscuration event lasting for 50 ks. We suggest that the very fast wind may only be apparent during the lowest X-ray flux states 
of PDS 456, becoming overly ionized as the luminosity increases. Overall, the total wind power may even 
approach the Eddington value.












































§ INTRODUCTION


Outflows are an important phenomenon in Active Galactic Nuclei (AGN) and can play a key role in the
co-evolution of the massive black hole and the host galaxy <cit.>.
Black holes
grow by accretion and strong nuclear outflows can 
quench this process by shutting off their supply of matter, thereby setting the 
M-σ relation that is seen today <cit.>.



A number of high column density (N_ H∼ 10^23 cm^-2), fast (∼0.1c)
outflows have now been found in luminous AGN <cit.>, 
through detections of blue-shifted Fe K absorption, at rest-frame energies greater than 7 keV. These ultra fast outflows may be the missing link in the 
galactic feedback process, by driving massive molecular outflows out to large (∼ kpc) 
scales in galaxies <cit.>.

A prototype ultra fast outflow occurs in the nearby (z=0.184) quasar, PDS 456.
With a bolometric luminosity of ∼10^47 erg s^-1, PDS 456 is the most luminous QSO in the local Universe <cit.> and the radio-quiet analogue of 3C 273. However, PDS 456 is most notable for its powerful and fast (∼0.25c) X-ray wind. 
Indeed, since its initial detection in 2001 with  
<cit.>, the presence of the ultra fast outflow in PDS 456 has now been established through over a decade's worth of X-ray observations <cit.>. 
Intriguingly, <cit.> recently claimed a fast UV counterpart to the X-ray wind.

In a series of five XMM-Newton 
and NuSTAR observations of PDS 456 in 2013–2014, <cit.>
detected a persistent P-Cygni-like profile from highly ionized (He or H-like) iron, blueshifted to 9 keV in the quasar rest frame. The broad P-Cygni profile established the wide-angle character of the outflow, while the wind variability 
provided a robust estimate of the wind radial distance, on the scale of the AGN accretion disk.
From this, the large mass outflow rate inferred, of ∼10 M_⊙ yr^-1, implied that the wind power is at least 15% of the Eddington luminosity. This is more than sufficient to provide the feedback required by models of black hole and host galaxy co-evolution <cit.>, which likely plays a critical role in black hole growth and feedback in the early Universe. 


 









§ OBSERVATIONS AND DATA REDUCTION


PDS 456 was subsequently observed with NuSTAR from March 23–26, 2017, with a total duration of 305 ks. This coincided with two simultaneous XMM-Newton observations, hereafter OBS 1 and OBS 2, 
taken over two consecutive satellite orbits (see Table 1) and both in Large Window mode for EPIC-pn.


All data were processed using the nustardas v1.7.1, 
XMM-Newton sas v16.0 and heasoft v6.20 software.
NuSTAR source spectra were extracted using a 50 circular region centered on the source and background from a 65 circular region clear from stray light. 
XMM-Newton EPIC-pn spectra were extracted from single and double events, using a 30 source region and 2×34 background regions on the same chip. 
All spectra are binned to at least 50 counts per bin, while the background rates 
are <10% of the net source rates. 


The 3-40 keV NuSTAR lightcurve is shown in Figure <ref>, where OBS 1 coincided with a pronounced dip in the source count rate ∼50 ks into the NuSTAR observation.

Note that outflow velocities are given in the rest-frame of PDS 456 at z=0.184, 
after correcting for relativistic Doppler shifts. Errors are quoted at 90% confidence for one interesting parameter (or Δχ^2=2.7).







§ SPECTRAL ANALYSIS


Initially we analyzed the time-averaged NuSTAR spectrum.
The count rate spectra from the FPMA+FPMB modules are shown 
in Figure <ref>, where the cross normalizations of FPMA and FPMB agree to within ±5% of each other. This is compared to a 
power-law of photon index of Γ=2.3 and
corrected for Galactic absorption, where N_ H=2.4×10^21 cm^-2, <cit.>.
PDS 456 has a low flux throughout the 2017 observations, where the
mean 2-10 keV flux of 2.6×10^-12 erg cm^-2 s^-1 is at the low end of the range 
measured in previous observations by <cit.> and <cit.>.

While the hard X-ray continuum is well described by a power-law, the overall fit is very poor, with χ_ν^2=327.1/169, rejected with a null hypothesis probability of P_ N=3.6×10^-12. Two strong absorption profiles are present in the residuals of both detectors between 8-12 keV. Modeling these with Gaussian profiles gave rest frame centroid energies of E=8.93±0.15 keV 
and E=11.4±0.3 keV, with equivalent widths of EW=-430±80 eV and EW=-380±100 eV 
respectively.
The addition of both lines significantly improved the fit by 
Δχ^2=-61.7 and Δχ^2=-39.3. 
The lower energy line is consistent with the energy of the Fe K absorption profile measured previously
in PDS 456 <cit.>, however 
the second line appears at a substantially higher energy.

The lines are broadened and were fitted with a common velocity width, of σ_v=20 000^+8 000_-4 000 km s^-1, corresponding to σ=600^+250_-130 eV at 8.93 keV,  consistent with the width measured by <cit.> previously.
















If we associate the two lines with the Lyα and Lyβ transitions from H-like Fe at 6.97 keV and 8.27 keV respectively, then the inferred outflow velocities are inconsistent, with v_ out=-0.24±0.02c and 
v_ out=-0.31±0.02c.  Note it would also be unusual for a higher order line to have such a high 
equivalent width as is observed here. 


Alternatively, the higher energy feature may be associated with an Fe K absorption edge. This gave a threshold energy of E=10.7±0.2 keV, resulting in an acceptable fit. However, the velocity inferred from the edge is also inconsistent; e.g. for H-like Fe, a K-shell edge (at 9.27 keV) blue-shifted to 10.7 keV gives v_ out=-0.14±0.02c, while the Fe xxvi Lyα line (at 6.97 keV) blue-shifted to 8.9 keV gives v_ out=-0.24±0.02c. The velocities are also inconsistent if the two features instead arise from He-like iron. 

Given the lack of a plausible identification at a self consistent velocity, the two absorption lines are likely to arise from two outflowing systems with 
different velocities. If they are both associated with Fe xxvi Lyα, then 
the outflow velocities are v_ out=-0.24±0.02c and v_ out=-0.46±0.03c (with 
slightly higher velocities inferred for He-like Fe). Thus while the lower energy line is consistent with the 
outflow velocities usually measured at iron K in PDS 456 <cit.>, 
which are typically 0.25-0.3c, the higher energy feature may originate from a new, 
very fast component of the wind.






 §.§ Photoionization Modelling
 

To test this, we fitted the NuSTAR spectrum with a self consistent xstar 
photoionization model, 
which accounts for any weak higher order lines and edges in addition to the 
strong 1s→ 2p resonance absorption. 
We adopt the same absorption model grids used in <cit.>, 
where the optical to X-ray SED of PDS 456 was used as the input continuum, which has 
an ionizing (1-1 000 Ryd) luminosity of L_ ion=5×10^46 erg s^-1. A turbulence velocity width of 15 000 km s^-1 was used, consistent with the Gaussian line width.
Solar abundances of <cit.> were used throughout.
As the FPMA and FPMB spectra were consistent within errors, these were combined using mathpha into a single mean NuSTAR spectrum to maximize S/N. The response files were  combined using equal weightings for both modules, while the source to background area (backscal) scaling factors were propagated through to the combined spectrum.



The best-fit xstar model fitted to the mean PDS 456 spectrum is shown in Figure <ref> (panel a).
Two outflowing absorption zones are required, one slower zone with v_ out=-0.25±0.02c and the 
faster zone with v_ out=-0.46±0.02c; see Table 1 for details. 
These velocities are consistent with the Gaussian analysis and the high ionization 
parameter of the absorbers (logξ=5.5) is consistent with the absorption 
lines arising from H-like iron (Fe xxvi Lyα). 
Figure <ref> (panel b) shows the residuals against a model including only the 
slower -0.25c zone, which leaves significant residuals around 11 keV and can then only be modeled by the 
fast 0.46c zone (see panel c).
Indeed, both absorption zones are required at >99.99% 
confidence (with Δχ^2=-80.3, slow zone and Δχ^2=-29.1, fast zone), resulting in an acceptable fit statistic of χ_ν^2=177.9/161. 
As a final consistency check, we attempted to model the high energy feature with a lower ionization partial coverer, having the same velocity as the -0.25c zone, but where the K-shell edge is blueshifted to higher energies. 
This single velocity absorber is rejected as the fit statistic is worse by Δχ^2=28.2 (for Δν=2) compared to the two velocity model.

The spectrum shows significant excess emission, observed redwards 
of the absorption lines between 6-8 keV (see Figure <ref>). 
This was first measured by <cit.>, who resolved the broad P-Cygni profile at Fe K from 
PDS 456. As per <cit.>, the emission was modeled 
with an additive xstar emission grid and convolved with a Gaussian profile of 
width σ=0.6^+0.3_-0.2 keV (or σ_ v∼24 000 km s^-1).
The high equivalent width of the emission, with EW∼350 eV, is consistent with the wide angle wind characterized by <cit.>.



 §.§ The XMM-Newton Observations 
 

The spectra obtained from the two XMM-Newton observations, OBS 1 and OBS 2, were also analyzed. 
Simultaneous NuSTAR spectra were extracted using identical good time intervals and then fitted jointly with 
their corresponding XMM-Newton spectra. The combined FPMA and FPMB spectra were used after first checking that the two modules were consistent. During the OBS 1 spectrum, PDS 456 was caught at an even lower flux (F_2-10 keV=1.8×10^-12 erg cm^-2 s^-1) coincident with the dip in the lightcurve. Note this may correspond to a short-lived absorption event, which will be discussed in a forthcoming paper on the broad band spectra.



The best-fit spectrum from OBS 1 is shown in Figure <ref> and is well fitted by the dual velocity absorber.
Both absorption lines are detected at >99.99% confidence, while the equivalent width of the high energy line is slightly stronger than the mean value, with EW=-530±150 eV. Note that the high energy feature is also independently detected in both the 
NuSTAR and XMM-Newton spectra, as the bandpass of the latter extends to 12 keV in the QSO rest frame. 
For ease of comparison, we assumed a constant ionization 
across the mean, OBS 1 and OBS 2 spectra; see Table 1 for parameters.
The column density of the fast zone is slightly higher in OBS 1 (N_ H=5.0^+1.4_-1.7×10^23 cm^-2), when compared to the mean (N_ H=2.7^+1.0_-1.2×10^23 cm^-2), 
to account for the increased depth of the high energy feature. 
The OBS 2 spectrum is consistent with the mean and the faster zone appears somewhat weaker compared to the lowest flux OBS 1 spectrum.



§ DISCUSSION
 

Low flux observations of PDS 456, obtained with NuSTAR and XMM-Newton in March 2017, 
have revealed a new relativistic component of the fast wind in this quasar, with v=-0.46±0.02c. 
Compared to studies of fast winds in other local, predominantly radio-quiet AGN <cit.>, this is the fastest wind detected to date in a nearby AGN. Perhaps the only other AGN with such an extreme fast wind is the high redshift (z=3.9) BAL QSO, APM 08279+5255, where the outflow velocity may reach ∼-0.7c <cit.>. Note, <cit.> subsequently claimed the outflow velocity in APM 08279+5255 may be somewhat lower, with v∼0.1-0.2c. 

Here, the high velocity absorber appears considerably faster than all of the previous Fe K wind measurements in PDS 456. <cit.> studied all twelve previous X-ray observations of PDS 456, observed with either XMM-Newton, NuSTAR or Suzaku. They showed the wind velocity in PDS 456 varies within the range 0.24c-0.34c, while the strong positive correlation observed between the outflow velocity and X-ray luminosity was interpreted as possible evidence for a radiatively driven wind. 
The mean X-ray luminosity in 2017, of L_ 2-10 keV=2.6×10^44 erg s^-1, lies at the low luminosity end of the range observed by <cit.>, with L_ 2-10 keV=2.8-10.5×10^44 erg s^-1 and the velocity of the slower zone, with v∼-0.25c 
is consistent with the trend between velocity and luminosity. 

lccc

The 2017 PDS 456 Observations and Outflow Parameters.
0pt

    Mean     OBS 1     OBS 2
Observations:-

Telescope     NuSTAR     XMM-Newton     XMM-Newton

Detector     FPMA+FPMB     EPIC-pn     EPIC-pn

OBSID     60201020002     0780690201     0780690301

Start date     2017/03/23     2017/03/23     2017/03/25

Start time     05:31:09     19:25:01     06:27:09 

Exposure^a     157.0     39.6     64.9 
  
Net rate (s^-1)^b     0.162±0.002     0.774±0.004     1.189±0.004

Slower Zone:-

N_ H^c     4.2^+1.3_-1.1     3.7^+1.3_-1.1     3.9^+1.0_-0.9

logξ^d     5.5^t     5.5^+0.3_-0.2     5.5^t 

v/c      -0.25±0.02     -0.21±0.02     -0.27±0.02 

Δχ^2     80.3     24.0     42.2

Faster Zone:-

N_ H^c     2.7^+1.0_-1.2     5.0^+1.4_-1.7     2.9^+1.4_-1.1

logξ^d     5.5^t     5.5^t     5.5^t 

v/c      -0.46±0.02     -0.43±0.02     -0.43^t 

Δχ^2     29.1     23.6     10.4

Continuum:-

Γ     2.18^+0.08_-0.05     2.41±0.09     2.41^t
 
F_ 2-10 keV^e     2.58     1.84     2.54

χ_ν^2     177.9/161     133.5/148     246.5/255
aNet exposure after background screening and deadtime correction, in ks.
bNet count rates, over 3–40 keV for NuSTAR and 0.4–10 keV for XMM-Newton.
cUnits of column density ×10^23 cm^-2.
dIonization parameter (where ξ=L/nR^2) in units of erg cm s^-1.
eObserved 2–10 keV flux in units of ×10^-12 erg cm^-2 s^-1.
tDenotes parameter is tied.



The fast, ∼-0.45c component of the wind is not apparent 
in the past observations of PDS456, e.g. <cit.> and see 
<cit.>, Figure 1. 
To confirm this, we checked all the previous NuSTAR observations of PDS 456 for any significant higher energy absorption above 10 keV, in addition to the persistent, but slower, -0.25c wind component. However, aside from the 2017 observation, none was found. The most stringent constraint comes from the first NuSTAR observation of PDS 456 in 2013 (hereafter, OBS A, ), where an upper-limit of EW<80 eV is placed on any Gaussian absorption line profile above 10 keV. Here the 2-10 keV X-ray luminosity is 8×10^44 erg s^-1, more than ×3 higher than in 2017. 

The upper limit on the column density of the fast zone during this high luminosity observation  
is N_ H<1.1×10^23 cm^-2 for a given ionization of logξ=5.5, significantly lower than for the low flux 2017 observations (see Table 1).

The fast component may arise from an inner stream-line of an accretion disk wind <cit.>, launched from very close to the black hole. Magneto-hydrodynamical mechanisms are also capable of accelerating winds up to these velocities, especially if the illuminating X-ray continuum is steep <cit.>, as is the case here with Γ>2. 
For a radiatively accelerated wind, the launch radius is:-

    R_ w≈ 2 (αL/L_ Edd - 1)(v_∞/c)^-2

where R_ w is the wind launch radius in gravitational units (R_ G), v_∞ is the terminal velocity and α is the force multiplier factor. 
In PDS 456, with L/L_ Edd=1, v_∞=-0.45c and 
for a modest multiplier of α=2, then R_ w∼10 R_ G 
(or ∼10^15 cm for a black hole mass of 10^9  M_⊙). 
The maximum likely radial distance of the absorber can be derived under the assumption that Δ R / R <1, for a given wind streamline. For an ionizing 
luminosity of L_ ion=5×10^46 erg s^-1, N_ H=5×10^23 cm^-2 and 
logξ=5.5, then R<L_ ion/N_ Hξ<10^17 cm
This is consistent with the radial distance of R∼10^16 cm estimated by <cit.>, from the wind variability timescale in PDS 456.

If the fast wind component is launched from close to the black hole and is fully exposed to the central X-ray source, then its ionization state will likely be very high. At these distances and for iron not to become fully ionized (where logξ<6), this requires densities of n∼10^7 - 10^11 cm^-3 and which could be associated to matter lifted off the surface of the inner accretion disk. Given that this very fast wind component is detected during a low flux observation of PDS 456, while it appears absent at higher fluxes (such as during OBS A in 2013), this may suggest that an inner wind zone would only be detected when its ionization state is low enough for the gas to not be fully ionized. 




Alternatively the innermost wind may be shielded by denser gas near the launch point, which could arise from the high column partial covering gas often seen in the lower flux X-ray spectra of PDS 456 <cit.>.

Nonetheless, the fast zone may have important implications for the overall wind energetics.  For the -0.25c wind in PDS 456, <cit.> estimated the kinetic power and mass outflow rate to be ∼15% and ∼50% of Eddington respectively.
As the power goes as Ė_ kin∝ṁ v^2∝ v^3, then the -0.45c zone could require the wind power to be a factor of ×6 higher. Then the total kinetic power could reach Eddington, if the overall mass outflow rate of the fastest zone is similar to the slower one. In reality, it may be likely that we are observing a structured wind, with multiple velocity components, which are launched at different disk radii.  
Future high resolution X-ray calorimeter observations, with 
XARM and Athena, will be able to further reveal the velocity structure in high velocity winds such as PDS 456.



§ ACKNOWLEDGEMENTS


JR acknowledges financial support through grants 
NNX17AC38G, NNX17AD56G and HST-GO-14477.001-A. AL acknowledges support via 
the STFC consolidated grant ST/K001000/1. EN is funded by the EU Horizon 2020 Marie Sklodowska-Curie 
grant no. 664931.
Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.



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