The Input Spectrum

It is known that AGN have diverse intrinsic spectra. A key question is how the intrinsic spectrum affects the wind’s observable parameters. I use the distribution of X-ray photon indices (see Fig. 3.16) measured by Scott & Stewart (2014) from fitting 761 type 1 sources in the XMM-Newton serendipitous catalogue (2XMMi; Watson et al. 2009). As type 1 objects are generally less obscured, they should be a more reliable measure of the intrinsic distribution of photon indices. Also, the sources studied throughout this thesis are type 1 AGN. The range Γ = 1.6 to 2.4 is chosen. This corresponds to ∼ 80% of the sources, as seen in the shaded area in Fig. 3.16.

A power law is assumed to be a reasonable first order approximation of the intrinsic X-ray continuum of AGN. However, as already discussed (in section 1.3.1), it is likely to be much more complex. Fig. 3.17 presents five power laws with increasing photon index from 1.6 to 2.4.

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Parameter Values Description

log(L/Ledd) −0.08 The Eddington ratio used to define the accretion energy provided Rcor/Rg 41 The radius at which black body transitions to the Comptonised spectrum

kT/eV 468 The temperature of the soft X-ray component

τ 10 The optical depth of the soft X-ray component

Γ 2.4 The photon index of the hard X-ray power law

Fpl 0.09 The fraction of accretion energy emitted in the power law component

Table 3.4: The parameters of the optxagnf model which was used to define the optxagnf input spectrum. These parameters are taken from the fit of Matzeu et al. (2016) to one of the low-state 2013 Suzaku spectra which is modified by cold absorption. However, these values are comparatively equivalent for its purpose here to fits found for the high state in Matzeu et al. (2017).

Alongside the power laws there are two more complex models: a simple broken power law SED and optxagnf (Done et al. 2012), a Comptonised disk model. Both models are taken from Matzeu et al. (2016), who fit PDS 456 using photometric data from the XMM-Newton Optical Monitor along with X-ray spectra from Suzaku and NuSTAR. The use of these detectors provides data from ∼2 eV to 50 keV, allowing for a realistic input spectrum over a wide energy range. The broken power law has a fixed break energy of 0.5 keV, which is the lowest energy of the Suzaku spectrum used. The broken power law model has a steep photon index of Γ=3.3 between the UV and the soft X-ray and a harder Γ=2.4 above a fixed break of 0.5 keV. The input parameters for the optxagnf disk–corona model are listed in table 3.4, presented in Matzeu et al. (2016). In both models the hard X-ray component is a power law with a photon index of 2.4 – however optxagnf adds a high energy cut-off (100 keV) which causes a roll over at higher energies.

In Fig. 3.17 the integrated 2 − 10 keV fluxes of the input spectra are normalised to unity. The fraction of luminosity radiated above 9.28 keV compared to the hardest (Γ = 1.6) power law is then calculated for each of the input continua in Fig. 3.17. This can be used to understand the expected ionisation state of the wind, although rather simplistically. Harder spectra have a higher luminosity above the ionisation potential of H-like iron relative to the softer photon indices. The subsequent effects of the harder spectral energy distribution on

the disk wind model can be seen in the top panel of Fig. 3.18, where the power law input spectrum with a Γ of 1.6 produces a very weak absorption feature from H-like Fe with an equivalent width EW= −18 eV, as most of the iron becomes fully ionised and therefore does not produce any opacity. As the distribution softens there are fewer ionising photons and therefore the population of fully ionised Fe XXVII is reduced in favour of lower ionisation states such as He-like and H-like iron. In Fig. 3.18 it is shown that as the photon index increases the softer input spectrum produces a stronger lower energy feature, as the average Fe ionisation state changes from fully ionised to H-like, and subsequently to He-like. A similar effect can be seen in the strength of the broad emission feature, which is barely visible in the top panel but grows in strength as the input spectrum becomes softer – a similar effect is seen in Fig. 3.10. The increase in strength of the absorption can be seen in the corresponding equivalent widths presented in table 3.5, particularity when looking at the correlation of the ratio of the line strengths to the photon index.

It is important to note that the absolute values of the equivalent width vary strongly with other parameters (notably the mass outflow rate). The observed values from the AGN outflow samples in Tombesi et al. (2010) and Gofford et al. (2013) vary typically from −10 to −90 eV for Fe XXV and from −10 to −150 eV for Fe XXVI. Whilst the mass outflow rate could be scaled to match these, as shown in Fig. 3.14, a more detailed object by object approach is favourable. A large equivalent width feature has been observed in the PDS 456 low flux state 2013 observation which will discuss in the next chapter has a equivalent width of ∼ 500 eV; the fitting of this feature will be explored in later chapters.

A key result from the dependence of the absorption line strength with the photon index is that soft X-ray continuum sources such as in PDS 456 and the NLS1 1H 0707 − 495 are prime candidates for finding disk winds. They are indeed sources accreting at least close to the Eddington limit (Reeves et al. 2009; Done & Jin 2016) therefore have the physical requirements for launching a wind, and are X-ray weak meaning they have a relatively low 2–10 keV luminosity compared to the UV (Reeves et al. 2009). They have a steep soft X-ray spectrum meaning that any wind launched would not be over-ionised and therefore

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Figure 3.17: The illuminating continuum represented as five power laws with increasing photon indices from 1.6 to 2.4, alongside the more realistic input spectra, the first of which is a broken power law and the second is a disk corona Comptonisation model (optxagnf). The spectra have been normalised to 1 in the 2–10 keV band for comparison. Note that as the input spectrum becomes steeper, the number of photons above the threshold of ionisation for iron decreases therefore, lowering average charge of iron within the outflow. The labels provide the percentage of integrated flux above H-like iron’s K edge (9.28 keV) relative to the power law with Γ = 1.6 - as photons are required to be above this energy to ionise iron, it is this band which will have the largest effect on the populations of iron species.

more visible. So it may not be surprising that they may be expected to produce strong observable wind features over the X-ray band. Moving to more realistic spectra such as the optxagnf model does not lead to a large modification to the observed Fe Kα line profile when compared to the equivalent hard X-ray spectrum (Γ = 2.4) in Fig. 3.18. This is because of the similarities in flux above 9 keV, which is identical in the case of the broken power law and only lower by 2% in the case of the optxagnf model.

Equivalent width of features/eV

Input Spectrum Fe XXV Heα Fe XXVI Lyα Fe XXV/Fe XXVI

Γ = 1.6 − −112 −

Γ = 1.8 − −201 −

Γ = 2.0 −071 −304 0.23

Γ = 2.2 −199 −389 0.51

Γ = 2.4 −395 −423 0.93

Broken power law −337 −476 0.71

optxagnf −387 −486 0.79

Table 3.5: The equivalent width of the absorption feature with varying photon index (Γ). This shows how the distribution of photons to higher energies decreases the ratio of the absorption features.

However, the increased soft flux from the SED fitting will be important for the presence of lower ionisation lines as observed in the XMM-Newton RGS for both PDS 456 (Reeves et al. 2016) and PG1211 + 143 (Pounds et al. 2016a). The presence of soft X-ray absorption features in the model was first noted in Sim et al. (2010) in relation to PG1211 + 143, who attempted a simple comparison between the wind spectra and the observed soft X-ray absorption features.

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