As discussed earlier, the mass within the flow is controlled by the mass outflow rate ( ˙M ); increasing ˙M scales the mass density in each cell. Fig. 3.13 shows the output spectra of several runs with increasing ˙M (from 14 to 115% ˙Medd) with a constant 2–10 keV luminosity
of 0.8% Ledd at an inclination of ∼ 51◦.
The increase in mass outflow rate has the effect of increasing the optical depth directly; this can be seen as the flux becomes suppressed. As interactions (scatterings) change the direction of photons into different lines of sight, the relative strength of the reflection component increases proportionally to ˙M , which can also be seen in (green spectra) Fig. 3.13.
As ˙M increases the column density intercepted goes from 3.1 × 1023cm−2 in the top panel to 2.5 × 1024cm−2 in the bottom panel, corresponding to an increase in the attenuation of the direct power law of 18% and 81% respectively.
Figure 3.13: The spectra of six runs which have increasing mass outflow rates from top to bottom in units of Eddington mass loss rate ˙M . Moving from top to bottom there is an increase in the strength of the 8.7 keV absorption feature along with the emergence of the 8.3 keV feature. Here the ionising luminosity is constant at 0.8% Ledd. Note the relative increase in strength in the lower ionisation species, this is due to the increase of density, which in turn decreases the ionisation of the wind. The increase in density also serves to suppress the overall continuum whilst also increasing the strength of the Fe Kα emission. The inclination for all spectra is 51◦.
3.2 Input parameters 71
Equivalent width of features/eV ˙
M / ˙Medd Fe XXV Heα Fe XXVI Lyα Fe XXV/Fe XXVI
0.14 −008 −063 0.13 0.19 −019 −094 0.21 0.26 −056 −145 0.38 0.36 −117 −195 0.60 0.49 −181 −233 0.78 0.68 −228 −253 0.90 0.88 −247 −258 0.96 1.20 −253 −250 1.01
Table 3.3: The equivalent widths of the He and H-like Fe absorption features with varying mass outflow rates showing that while the depth of the features increase, the ionisation state of the wind decreases. This is shown in the fourth column where the Fe XXV line becomes stronger relative to the Fe XXVI line with increasing outflow rate as the L2−10 keV is kept constant at 0.8% Ledd (and thus equivalent to the bottom panel of
Fig. 3.10) as well as the geometry with the values in table 3.1.
The depth of the 8.7 keV absorption line also increases as seen in the right hand side panel of Fig. 3.13; this line corresponds to the H-like Fe Lyα line. This can also be seen in table 3.3. Where the column density increases, so does the equivalent width. An interesting side effect of increasing ˙M is that the ionisation state of the wind decreases. This can be seen in the right hand side panel of Fig. 3.13, as indicated by the increase in depth of the lower ionisation line at ∼ 8.4 keV (corresponding to the He-like iron line). This is also seen in the ratio between the two lines in table 3.3, where the ratio of Fe xxv to Fe xxvi increases with
˙
M . The relative strength of the Fe XXV Heα line increases, because there are fewer photons compared to the number of atoms due to the increase in density of the wind as ˙M becomes larger — whereas the 2–10 keV luminosity remains constant causing the ionisation to fall.
L2−10 keVand ˙M have similar effects yet opposite on the spectra. While larger L2−10 keV
increases the ionisation, larger ˙M decreases it (whilst also suppressing the observed flux). Therefore it is combination of ˙M and L2−10 keV that sets the ionsiation state of the elements
Figure 3.14: The distribution of equivalent widths over different ˙M and L2−10 keV values for
both Fe XXV Heα and Fe XXVI Lyα lines showing the interplay between the two parameters.
sets the species observed and the observable depth of the lines. In Fig 3.14, the equivalent width (EW) of the Fe XXV Heα and Fe XXVI Lyα in the left and right hand side panels respectively is shown in ˙M vs L2−10 keV. At low ˙M and high L2−10 keV neither line is visible
(as the wind is over-ionised), whereas at low ionisation and high mass outflow rate the line strength (largest absolute EW) is still not maximised as there are not enough atoms in the He/H-like state (i.e. the wind is under-ionised). This is most apparent in the H-like Lyα line, which is not visible at the lower ionisations.
The relative strengths of the Heα and Lyα lines can be seen in Fig. 3.15, which presents the ratio between the EWs of the two lines. This shows the shift from He-like iron to H-like and then to completely ionised iron as both lines become very weak as L2−10 keV
increases and ˙M decreases (upper left of Fig. 3.15). Conversely the white region of Fig. 3.15 is due to the wind being under-ionised with a lack of Fe ions in the H-like state. As L2−10 keV
increases more, similar values for EW (and ratios of Heα and Lyα lines) can be produced by increasing ˙M to compensate. This can be seen both in Fig. 3.14 and Fig. 3.15 in the diagonal bands. This was alluded to in section 3.2.6. The degeneracy between these parameters is important as discussed in section 1.4.1: the mass outflow rate (alongside the outflow velocity) is important for determining the kinetic luminosity of the wind. The kinetic luminosity is a
3.2 Input parameters 73
Figure 3.15: The distribution of the comparative line strengths based on the ratio of the Fe XXV Heα and Fe XXVI Lyα lines. This shows the interplay between the L2−10 keV and
˙
M parameters. The white region is where the Lyα line is too weak to be detected (Heα/Lyα) > 4.2.
diagnostic parameter if the wind plays a role in feedback.
However, as the 2–10 keV luminosity is directly observed or can at least be constrained by the observed spectra, this is a relatively easy degeneracy to break – or at least limit its effect. Disentangling the effects changing ionising luminosity and changing mass outflow rate is important for parameters such as the amount of mass carried by a flow over its lifetime, which is used to infer the influence on the host galaxy. Therefore a physically meaningful anchor for L2−10 keV or ˙M is required to limit the effect of the degeneracy. This is discussed
Figure 3.16: The distribution of photon indices from Fig. 1 in Scott & Stewart (2014) of 761 type 1 AGN. The shaded area (Γ = 1.6 to 2.4) corresponds to ∼ 80% of the sources within the sample.