Ionising luminosity

As photon packets propagate through the wind they interact with the material, heating and cooling of the cells as the go this sets the ionisation state of the wind. The packets are drawn from the input spectrum, allowing any spectral input to be adapted. The input spectrum is then scaled to the model parameter L2−10 keV (the luminosity between 2 and

3.2 Input parameters 63

used as it is covered by most X-ray detectors so is obtainable from the observed spectrum. As it sets the amplitude of the ionising continuum, this parameter acts as a proxy for the ionisation state of the wind. To explore the effect of the ionisation state of the wind on the observed spectrum I collect the output spectrum with increasing 2–10 keV luminosity, ranging from 0.03% to 0.8% Ledd. These can be seen in Fig. 3.10 which shows the spectra

from an intermediate inclination angle of ∼ 51◦. I note that L2−10 keV is normalised to Ledd

Figure 3.10: Spectral changes with increasing luminosity from 2–10 keV with ˙M = 0.68 ˙Medd.

As ionising luminosity increases more elements become more highly ionised. This causes a reduction in the line strengths of these ions as their population becomes smaller; this can be seen between 1 and 2 keV. It is interesting to note that the top panel corresponds to Fig. 3.11 while the bottom panel corresponds to Fig. 3.12. The change in wind ionisation can also be seen in the absorption feature around 8 keV in the reduction of the depth of the line and profile (of He-like Fe) and the development of a second line (H-like Fe). This ionisation change can be seen in the profile of the Fe Kα emission as a reduction in the emission line strength.

3.2 Input parameters 65

Moving to higher luminosities (top to bottom in Fig. 3.10) the atoms are stripped of their electrons making the wind more transparent. This is seen in the reduction of the (bound–free) curvature in the direct spectra – specifically between 1–5 keV. The shaded range in Fig. 3.10, which shows the energy range where the Fe L absorption from mildly ionised iron (FeXVII−XXIV) would be expected after applying the velocity shift due to the

winds motion. The top panel will also have contributions from the K-shell absorption of lighter elements such as (Mg,Si,Ne,S,Ca) which introduces much of the opacity along the wind above 1 keV. Fig. 3.11 shows the spatial distribution of the ions of each element. For the lowest ionisation (0.03% Ledd) simulation it can be seen that most elements are only

partially ionised except for the lightest elements C and N, which have already been fully ionised except at the very base of the wind.

As the L2−10 keVincreases further, moving to lower panels in Fig. 3.10 the outer electrons

of most elements are stripped meaning that the lighter elements become over-ionised. In Fig. 3.12 it can be seen that all atoms below iron are fully ionised except at the base of the wind. This is seen in the corresponding spectrum in the bottom panel of Fig. 3.10 where only He-like and H-like iron absorption remains.

This change in ionisation can also be seen in the equivalent width of Fe XXV Heα and Fe XXVI Lyα absorption features. As the luminosity increases, the mildly ionised iron ions move into a more highly ionised population (as is seen by comparing Fig. 3.11 to Fig. 3.12), so the equivalent width of He-like Fe increases as is seen in table 3.2. The Heα absorption from the H-like iron is not detected until L2−10 keV = 0.08% Ledd when the population of Fe

XXVI becomes large enough to produce a measurable feature. As the luminosity increases further so does the average ionisation state of iron; this can be seen in the decrease of the ratio (FeXXV/FeXXVI) and therefore the relative strength of the H-like line increases with

L2−10 keV. Another effect is the weakening of the Fe Kα emission feature in the scattered

spectrum, as the scattering medium becomes more highly ionised and therefore becomes closer to full transparency.

Pro duction of syn th et ic sp ectra

Figure 3.11: The ionisation colour maps of average ionisation state (right hand side scale) within each cell for different elements within the outflow. This shows that the ionisation structure is the same for all elements. This is a low ionisation example with L_{2−10 keV} = 1043.5erg s−1 (0.03% L_edd) and ˙M = 25 ˙M yr−1 (68% ˙Medd).

It can be seen that the base is of lower ionisation. This is due to the wind being denser at the base but also the side of the wind which faces the source acts to shield the far side of the wind from much of the intrinsic luminosity, allowing lower ionisation species to survive.

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Input

parameters

67

Figure 3.12: High ionisation example of Fig. 3.11 with L2−10 keV = 1045.0 erg s−1 (0.8% Ledd) and M = 25 ˙˙ M

yr−1 (68% ˙Medd). It can be seen that all elements below iron are completely ionised everywhere but at the

base of the wind. This means that most of the lines observed in Fig. 3.10’s top panel are lost, as observed in the bottom panel.

Equivalent width of features/eV

% L2−10 keV/Ledd Fe XXV Heα Fe XXVI Lyα Fe XXV/Fe XXVI

0.03 −199 − − 0.04 −237 − − 0.08 −260 −064 4.04 0.14 −274 −167 1.65 0.25 −268 −238 1.13 0.45 −265 −264 1.00 0.80 −228 −253 0.90 1.40 −149 −220 0.68 2.40 −061 −169 0.36 4.10 −018 −107 0.17 7.10 −002 −060 0.03

Table 3.2: The equivalent width of the absorption features with varying 2–10 keV luminosity; this can be seen from the ratio of Fe XXV to Fe XXVI. Initially as the ionisation increases, the mildly ionised iron becomes He-like as can be seen by the increase in equivalent width. However as the ionisation increases further, the H-like line first appears and then growths in depth. These values are based on simulations with a mass outflow rate of 68% ˙Medd.

It is important to note that moving from L2−10 keV = 0.03% to 7.1% Ledd in table 3.2

decreases the equivalent width of both the He-like and H-like iron lines as the wind becomes more transparent. However the former decreases more rapidly leading to a change in the observed change in the Fe xxv/Fe xxvi ratio, which is a sensitive measure of the wind’s overall ionisation.

As mentioned earlier this grid was built based upon the parameters of PDS 456. The observed L2−10 keV range is from 0.11 to 0.54 % Ledd. I can compare this to the expected

intrinsic value for L2−10 keV using the 2–10 keV to Lbol correction factors of Marconi et al.

(2004). This paper compiles the L2−10 keV/Lbol ratios for a sample of AGN at different

luminosities. For the luminosity of Lbol = 1047 erg s−1 ∼ Ledd Reeves et al. (2000) give a

correction factor Lbol/L2−10 keV = 79. Therefore an expected L2−10 keV/Lbol for PDS 456 is

3.2 Input parameters 69

in Fig. 3.10.

From Fig. 3.11 and Fig. 3.12 it can be seen once again that inclination is important, as the inner edge of the wind has the highest ionisation because it sees the full unattenuated flux of the X-ray continuum. So inclinations which intercept more of the inner edge of the wind will not only be faster but will also have a higher ionisation state and therefore a reduced opacity. For larger inclinations we see through larger optical depths through the wind, therefore I will see lower ionisation gas due to shielding of the layers at lower physical depths.

This means that larger depths see a reduced flux. This effect is strongest at the base of the wind where the increased column density increases the scattering so lower ionisation species can survive at the larger optical depths. Even with the self shielding an ionising luminosity of 0.8% Ledd is enough to over-ionise all atoms below iron except at the base of

the wind. It can also be seen that the ionisation decreases further out showing that the flux is falling off quicker than the density, once again due to the self shielding.