# Detection of low-redshift absorption in the luminous Seyfert galaxy
PG1211+143 - a slow, distant inflow feeding accretion or the gravitational
redshift from a ring of matter close to the SMBH ?

Ken Pounds and Kim Page
School of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH,
UK

(Accepted ; Submitted )

###### Abstract

The detection of a high velocity ($\sim$ 0.3c) inflow of highly ionized matter
during a long XMM-Newton observation of the luminous Seyfert galaxy PG1211+143
in 2014 offered the first direct observational evidence of a short-lived
accretion event, where matter approaching at a high inclination to the black
hole spin plane may result in warping and tearing of the inner accretion disc,
with subsequent inter-ring collisions producing shocks, loss of rotational
support and rapid mass infall. In turn, such accretion events provide an
explanation for the ultrafast outflows (UFOs) now recognised as a common
property of many luminous Seyfert galaxies. While the ultra-fast inflow in
PG1211+143 was detected in only one of 7 spacecraft orbits, a weaker (lower
column density) inflow, at a much lower redshift of $\sim$ 0.123, is revealed
in the soft x-ray spectrum by summing RGS data over the full 5-weeks XMM-
Newton campaign. Modelling of the simultaneous, stacked pn data finds evidence
for a similar low redshift absorption component in a previously unexplained
feature on the low energy wing of the Fe K emission line complex near 6 keV.

We briefly consider Doppler and strong gravity explanations for the observed
redshift, with the former indicating a distant inflow feeding off-plane
accretion, where an infall velocity of v$\sim$ 0.038c and (free-fall) radius
at 1400 Rg lies beyond the tearing radius for PG1211+143 but still within the
sphere of influence of the SMBH. An intriguing alternative, recently given
added credence, might be as the gravitational redshift of absorption in matter
orbiting the SMBH at a radius of $\sim$ 27 Rg. In the latter case the narrow
RGS absorption line spectrum constrains the thickness of the orbiting ring due
to the strong velocity shear so close to the hole.

###### keywords:

galaxies: active – galaxies: Seyfert: quasars: general – galaxies: individual:
PG1211+143 – X-ray: galaxies

††pagerange: Detection of low-redshift absorption in the luminous Seyfert
galaxy PG1211+143 - a slow, distant inflow feeding accretion or the
gravitational redshift from a ring of matter close to the SMBH
?–References††pubyear: 2023††pagerange: Detection of low-redshift absorption
in the luminous Seyfert galaxy PG1211+143 - a slow, distant inflow feeding
accretion or the gravitational redshift from a ring of matter close to the
SMBH ?–References

##  1 Introduction

XMM-Newton observations of the luminous Seyfert galaxy PG1211+143 have been
remarkably productive, perhaps rightly so as a target in the ’guaranteed
observing time’ awarded to the late Martin Turner, PI of the primary focal
plane camera, EPIC (Turner et al. 2001). An early observation showed a strong,
blue-shifted absorption line of highly ionized Fe, interpreted as an outflow
or wind, with a line-of-sight-velocity of 0.15$\pm$0.01$c$ (Pounds et al.
2003; Pounds & Page 2006). Archival data from XMM-Newton and the Japanese
x-ray observatory Suzaku subsequently showed that such ultra-fast, highly-
ionized outflows (UFOs) are relatively common in nearby, luminous AGN (Tombesi
et al. 2010, 2011; Gofford et al. 2013). While those archival searches
typically reported a single velocity, in the few cases where an AGN was
observed repeatedly, the wind velocity was often different. Mkn509 is the best
example from the XMM-Newton data archive, with wind velocities of
$\sim$0.173c, $\sim$0.139c and $\sim$0.196c, separated by 5 years and 6 months
respectively (Cappi et al. 2009). Looking back, it is interesting to note that
such winds were predicted to occur in luminous AGN when a region of the
accretion disc became ’super-Eddington’ (Shakura and Sunyaev 1973; SS73).
However, as a member of the original mission definition team for the X-ray
Multi-Mirror Observatory (Bleeker et al. 1984), I can confirm UFOs were not
part of the scientific case.

An extended XMM-Newton observation of PG1211+143 in 2014, covering 7
consecutive spacecraft orbits over 5 weeks, showed a more complex wind
structure, with three primary (high column density) outflow velocities of
$v\sim 0.066c$, $v\sim 0.13c$ and $v\sim 0.19c$ (Pounds et al. 2016a), none
being consistent with the velocity observed in 2001. In a review of continuum-
driven or ’Eddington’ winds, King and Pounds (2015) furthermore showed that
the observability of an individual, short-lived wind ejection may be of order
months or less, as an expanding shell presents a lower absorbing column to the
nuclear x-ray source.

Considering the simultaneous observation of multiple expanding shells of
absorbing gas in the context of super-Eddington accretion, Pounds, Lobban and
Nixon (2017) noted that short-lived winds might be a natural consequence of
the way in which matter accretes in an AGN between mergers, typically falling
from far outside the radius of gravitational influence of the SMBH and with
essentially random orientation. The resulting accretion stream will in general
orbit in a plane misaligned to the spin of the central black hole (King &
Pringle 2006, 2007), with the inner disc subject to Lense-Thirring precession
around the spin vector, and orbits at smaller radii precessing sufficiently
fast to cause the tearing-away of independent rings of gas.

Computer simulations by Nixon et al. (2012) show that - as each torn-off ring
precesses on its own timescale - two neighbouring rings will eventually
collide, with the shocked material losing rotational support and falling
inwards to a new radius defined by its residual angular momentum. A
substantial increase in the local accretion rate might then allow a previously
sub-Eddington flow to become briefly super-Eddington, with excess matter
ejected as a wind with velocity at or above the local escape velocity (as
predicted in SS73). For typical AGN disc parameters, Nixon et al. (2012) find
the ‘tearing radius’ is of order a few hundred gravitational radii from the
black hole - with predicted (escape) wind speeds in the range observed (King
and Pounds 2015; fig 4a).

An important bonus from the 2014 observation of PG1211+143 came with the
detection of red-shifted X-ray absorption spectra during the second XMM-Newton
orbit (rev 2659), revealing a substantial stream of matter approaching the
black hole at velocities up to $\sim$ 0.3c (Pounds et al. 2018) and offering
the first observational support for the random accretion scenario.
Simultaneous hard and soft X-ray spectra provided independent confirmation of
the high inflow velocity, with the soft X-ray absorption indicating less
ionized (higher density) matter embedded in the main highly ionized flow. In
addition to that co-moving inflow at redshift $z=0.48$, a stronger soft x-ray
absorption component was detected, at $z=0.189$, corresponding to an inflow
velocity $\sim$0.1c, and upstream location at 200 Rg.

We now report a weaker (lower column) and more persistent absorption component
at an observed redshift of $\sim$ 0.123, revealed in the combined RGS1 and
RGS2 data from the full 7-orbit observation, with a high on-target exposure of
1.24Ms. If interpreted as a Doppler shift,the observed redshift would
correspond to an inflow velocity of $\sim$0.038c and (free-fall) location at
1400 Rg, beyond the putative tearing radius but still inside the gravitational
’sphere of influence’ of the SMBH in PG1211+143. Supporting evidence for the
low redshift absorption is found in spectral modelling of the simultaneous
stacked pn data from 2014.

In the following analysis we assume a cosmological redshift for PG1211+143 of
$z=0.0809$ (Marziani et al. 1996), with a black hole mass of $4\times
10^{7}$$\rm~{}M_{\odot}$ (Kaspi et al. 2000) confirming the historical
bolometric luminosity close to Eddington. Spectral modelling is based on the
XSPEC package (Arnaud 1996) and includes absorption due to the line-of sight
Galactic column NH $\sim$ 3$\times$ 1020 cm-2 (Murphy et al. 1996). To allow
direct comparison with previous analyses of 2014 XMM-Newton data we again use
photoionized emission and absorption grids of pre-computed spectra based on
the XSTAR code of Kallman et al. (2006). Observed redshifts are converted to
an inflow velocity taking into account the cosmological redshift and
relativistic Doppler effect.

##  2 A low column, low redshift absorber in the stacked RGS spectrum from
2014

In an initial spectral analysis of the 2014 RGS data, Pounds et al.(2016b;
P16b) report a mean soft x-ray spectrum, with an array of photoionized
emission and absorption features superposed on an x-ray continuum modelled by
power law and black body components. In modelling the transient absorption
event identified with an ultrafast inflow in the second XMM-Newton orbit,
rev2659, the underlying continuum components and photoionised emission
features were retained, with only normalisations left free (Pounds et al.
2018). The blue-shifted absorption attributed to outflowing matter was also
retained, with the primary aim being to model and quantify the ultrafast
inflow. Here we follow a similar procedure for the stacked 2014 data to search
for weaker but more persistent absorption features.

We find two low-redshift absorption components: (1) at an observed redshift of
0.0889+/-0.0007, with ionization parameter log $\xi$ $\sim$ 0.83 and column
density NH $\sim$ 6$\times$ 1019 cm-2;
(2) at a redshift of 0.123+/-0.001, with ionization parameter log $\xi$ $\sim$
2.1, and column density NH $\sim$ 8$\times$ 1019 cm-2

Figure 1: Confidence contour maps of the low redshift absorber detected in RGS
data from 6 orbits of the 2014 XMM-Newton observation of PG1211+143.

The first component has an ionization parameter and redshift consistent with
the cosmological value for PG1211+143, suggesting a small change to the
adopted values for the line-of-sight ISM in our spectral model.

More interesting is the component with an observed redshift of $\sim$0.123,
indicating a rest-frame inflow velocity of 0.038+/-0.001c. Figure 1 shows
probability contour plots from the best-fit model parameters of the new
component, with the observed redshift as a function of column density and of
ionization parameter.

###  2.1 Supporting evidence from the stacked pn spectrum

The relatively high ionization parameter of the low redshift absorber
suggested it might also be seen in simultaneos, stacked pn data, extending to
higher photon energies. That proved to be the case, both in full spectral
modelling and in isolating a previously unidentified absorption line on the
low energy wing of the Fe K emission complex (Fig.2).

The photoionization model described in P16a was repeated for the stacked 2014
pn data, now allowing for additional redshifted absorption, finding
significant component at an observed redshift of 0.114+/-0.015, with
ionization parameter log $\xi$ $\sim$ 2, and column density NH $\sim$ 1022
cm-2.

Figure 2: Ratio of pn data to a simple double power law continuum fit to the
stacked 2014 pn spectrum of PG1211+143, showing a broad emission feature at
$\sim$6 keV, and complex absorption structure from $\sim$7–9 keV identified in
Pounds (2016a) with 3 outflow velocities. Figure 3: The FeK emission feature
in Fig.3 is modelled with 3 Gaussian lines at a small blue shift relative to
markers indicating the rest energies of Fe K lines of neutral, He-like and
H-like ions. Not previously discussed is the narrow absorption feature
observed at $\sim$ 5.8 keV.

In Fig.2 the strong emission feature near 6 keV is identified as a combination
of the neutral Fe-K fluorescence line, and the 1s-2p resonance emission lines
of He-like and H-like Fe, with markers at the source rest energies, revealing
(Fig.3) a strong absorption line on the red wing of the emission complex at
$\sim$5.84/-0.02 keV. Identifying that absorption line with the He-alpha
resonance transition of Fe (rest energy 6.70 keV) yields an observed redshift
of 0.128+/-0.003, consistent with the RGS value.

Interpreted as a Doppler redshift, an infall velocity of 0.038+/-0.001c would
correspond to a (free-fall) location at 1400 Rg, beyond the tearing radius but
within the sphere of influence of the SMBH in PG1211+143 suggesting a
potentially important role in maintaining off-plane accretion on a multi-year
timescale.

An intriguing alternative, discussed below, might be as the gravitational
redshift of absorption in matter orbiting close to the SMBH, although the
observed ’narrow’ absorption lines in the RGS data present an important
constraint to that alternative. Fig. 4 reproduces a section of the RGS soft
x-ray spectrum, but with the 0.123 redshifted absorber removed, revealing 9
absorption lines, identified with resonance K-shell transitions of O, Ne, and
Mg, and of Fe-L, with a common redshift of 0.123. The lines are plotted with
Gaussian profiles, with one-$\sigma$ width of 30 mÅ, consistent with the pre-
and early post-launch RGS resolution over the relevant waveband (den Herder et
al. 2001).

To estimate any additional line broadening in the observed spectrum, the set
of 9 lines was then re-modelled with a fixed 1$\sigma$ width, and the quality
of the fit determined as the common width was increased from 20mÅto 80mÅin
steps of 5 mÅ. That test confirmed a strong upper limit to an additional
physical line width of 20 mÅ, which we use below to limit the ring thickness.

Figure 4: RGS spectrum of the low redshift absorber detected in stacked data
from the 2014 XMM-Newton observation of PG1211+143. Nine absorption lines are
plotted with Gaussian profiles of $\sigma$ width 30 mÅ, with markers
indicating the wavelength of each identified line at a redshift of 0.123.

##  3 Discussion

The detection of ultra-fast highly ionized outflows (UFOs) in early x-ray
spectra of the narrow line Seyfert galaxy PG1211+143 (Pounds et al. 2003) and
the luminous QSO PDS 456 (Reeves et al. 2003), opened up a new field of study
of AGN, well matched to the high throughput of x-ray spectrometers on ESA’s
XMM-Newton, launched in late 1999. King and Pounds (2003) noted that such
winds are a natural result of a high accretion ratio, with excess matter being
driven off by radiation pressure when the accretion rate exceeds the local
Eddington limit, as predicted in the classic review of AGN accretion disc
physics by Shakura and Sunyaev (1973).

While that picture provides a satisfactory explanation of most UFOs, where a
single detection yielded a unique wind velocity, the extended study of
PG1211+143 in 2014 found a more complex outflow profile, with velocities of
$\sim$0.06c, $\sim$0.13c and $\sim$0.18c (P16a). Such complexity was clearly
inconsistent with a wind profile launched from a flat axi-symmetric disc
(SS73), suggesting some intrinsic disc instability or rapidly variable
accretion rate.

The detection of multiple red-shifted X-ray absorption lines during part of
the same 2014 XMM-Newton observation of PG1211+143 where a common redshift
($\sim$0.48) indicated absorption in a substantial inflow approaching the
black hole at a velocity of $\sim$ 0.3c (Pounds et al. 2018) offered the first
observational support for such variable accretion.

As noted in the Introduction, a highly variable accretion rate in th inner
disc could be a consequence of the way in which AGN accrete gas initially
falling towards the SMBH with essentially random orientation, and Lense-
Thirring precession causing misaligned orbits to precess around the black hole
spin vector, with the innermost ring(s) warping and potentially tearing off.
Collision between neighbouring rings, rotating at different rates, would then
shock, with loss of rotational support leading to matter falling freely to a
new radius defined by its residual angularmomentum, where - if it can cool
fast enough - it forms a new disc (Nixon et al. 2012). The present paper may
provide direct evidence for that disc.

We report here a weaker (lower column density), ionized inflow at an observed
redshift of $\sim$ 0.123, detected in both stacked RGS and pn spectral data
from the 2014 XMM-Newton observation and briefly consider two radically
different origins.

Interpreted in line with previous work, where all blue- and red-shifted
spectra were considered in terms of the Doppler effect, with relativistic
correction as appropriate, we find a slow, distant infall, perhaps feeding
off-plane accretion from beyond the tearing region discuused by Nixon et
al.(2012). An intriguing alternative, as noted above, might be the
gravitational redshift in absorption from a ring of matter circulating close
to the SMBH.

The RGS spectrum constrains that ’strong gravity’ explanation since the
gravitational redshift varies inversely with radial distance from the black
hole, with the ring thickness column depth limited by the intrinsic absorption
line widths. The upper limit to an intrinsic line width noted earlier limits
the thickness of of the orbiting matter to $\sim$1 Rg, or 6$\times$ 1012 cm
for PG1211+143, implying a particle density of nH $\sim$ 107 cm-3 to match the
observed column density.

For the pn data, which covers a higher spectral energy range, the observed
column density is substantially greater, but the depth constraint much weaker
than for the RGS data due to the more limited pn spectral resolution. Our
conclusion is that the ’narrow’ lines in Fig.4 do not be rule out a strong
gravity the origin of the 0.123 redshift.

The strong gravity interpration gained recent support from an orbit-by-orbit
analysis of the 2014 XMM-Newton data (Pounds and Nayakshin (2023), reporting
the launch of a new wind component in PG1211+143 some 2-3 weeks after the
rapid infall reported in Pounds et al. (2018). The infalling matter was
observed to within 20Rg of the SMBH, while the gravitational redshift for
matter in a circular orbit at 27Rg would match the 0.123 redshift of absorbing
matter reported in this paper. We note, finally, that Lens-Thirring precession
should move such a ring of matter across the line of sight in of order 3.5/a
yr, where 0$<$a$<$1 is the Kerr spin parameter, consistent with an observed
redshift prevailing over the 5-weeks long observation in 2014 - and perhaps
still evident today !

##  4 Data availability

The data underlying this article are available in the XMM archive at
http://nxsa.esac.esa.int/nxsa-web.

##  Acknowledgements

XMM-Newton is a space science mission developed and operated by the European
Space Agency. We acknowledge in particular the excellent work of ESA staff in
Madrid successfully planning and conducting the XMM-Newton observations.

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