AUTORIZACION Y SUMINISTRO DE OXIGENO

A variety of X-ray spectral models have been implemented to characterise the spectra of X-ray sources. These spectral models are drawn from a pool of 4 base models which are used in multiple combinations and iterations to simulate the emission of various types of AGN. A brief summary of these models follows:

(i) Simple power-law (Figure 2.12): The most basic model used in the spectral analysis, the POWERLAW model of the standard XSPEC library is designed to replicate a power-law X-ray continuum produced by inverse Compton scattering. This is the only model which is redshift independent and there are only two parameters to be calibrated; Photon index (Γ) of the power-law and the normalisation. A spectrum that is best fit by power-law emission is most likely an unobscured AGN.

(ii) Spherical obscuration (Figure 2.13): Custom additive XSPEC model designed by Murray Brightman (see Brightman et al. 2011) simulating the emission profile of an inverse Compton scattering power-law being obscured by a spherical distribution of matter. The obscuring matter is assumed to be a ‘warm absorber’, with photoexcita- tion of Fe, C, N, and O atoms acting as the dominant obscuration processes, produc- ing absorption edge features. The model also features self-consistent line emission from Fe Kα transitions. There are potentially 5 parameters that can be calibrated in the model; the obscuring column density (NH), the elemental and Fe abundances

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are held at solar levels and the Γ is fixed at 1.9 (typical AGN synchrotron emission profile; see Nandra et al 1997) throughout all spectral fitting analyses. This model spectrum is thought to be the best analogue for AGN whose emission is dominated by galactic absorption.

(iii) Toroidal obscuration (Figure 2.14): Custom additive XSPEC model designed by Murray Brightman (see Brightman et al 2012) this model is designed to simulate the emission profile of an inverse Compton scattering power-law that is being absorbed by toroidally distributed obscuring material. The obscuring matter is assumed to be a ‘warm absorber’ where photoexcitation is the dominant process. As with spheroidal obscuration this model also features self consistent Fe Kα line emission. This model requires 7 input parameters: NH, the elemental and Fe abundances, Γ, the normal-

isation, the opening angle of the dust torus and the viewing angle of the observer. As with the spheroidal obscuration model, elemental and Fe abundances are held at solar levels and Γ=1.9. The choice of viewing angle can fundamentally alter the nature of the observed emission. If the viewing angle is greater than the opening angle then the spectrum will simply resemble obscured inverse Compton scattering power-law emission. If, however, the viewing angle is less than the opening angle then the central engine is observed directly and the absorption effects are greatly reduced. This will not default to a simple power-law though, as strong reflection components (ie Iron Kα line complexes) will still be present within the spectrum. Throughout this work different combinations of torus opening angles and viewing angles are utilised, although they are always fixed prior to fitting to avoid rampant model degeneracy. A secondary POWERLAW model is always used in conjunction with this model to simulate inverse Compton scattering power-law emission from the central source that has been scattered into the line of sight by hot electrons in the ionisation cone. The scattered power-law is fixed to the same Γ as the central source (1.9) but the normalisation is left free. This is a reasonable assumption for low S/N data, but is should be noted that scattered emission from the ionisation cone is thought to produce the soft excess observed in local Type 2 AGN (Kinkhabwala et al., 2002; Brinkman et al., 2002). Overlooking this additional complexity could adversely affect spectral fits to more detailed spectra. This scattered component is included regardless of the viewing angle. This model spectrum is representative of an AGN which has a torus of obscuring dust surrounding it. The obscuring dust torus is in close proximity to the central source, just beyond the accretion disc (1-10 parsecs) and is an intrinsic structural component of the AGN (ie Antonucci 1993).

(iv) Reflection dominated (Figure 2.15): Custom additive model developed by Nandra et al. (2007b) that is now part of the XSPEC package extended library simulating the profile of a reflected inverse Compton scattering power-law. The PEXMON model is based on the neutral reflection model of Magdziarz and Zdziarski (1995) which describes reflection off of a slab geometry. Nandra et al. (2007b) then added Fe Kα

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line emission with strengths calibrated according to the Monte Carlo calculations of George and Fabian (1991). Additionally Fe Kβ (7.05 keV) and Ni Kα (7.47 keV) emission lines, with fluxes 11.3% and 5% of the Fe Kα flux respectively, are included (Nandra et al., 2007b). The model has 7 parameters that must be calibrated; Γ, the cutoff energy, the scaling factor for reflected component relative to the source, elemental and Fe abundances, the inclination angle and the normalisation. Elemental and Fe abundances are fixed to solar values and Γ is fixed as 1.9 and the cutoff energy is fixed at the default of 1000keV. The scaling factor is fixed at -1. This is equivalent to there being no direct component in the observed spectrum (ie PEXMON is the pure reflection component). The inclination angle is fixed at 60 degrees throughout because it has negligible impact on the final C-statistic value when left free. As with the toroidal obscurer a secondary POWERLAW model is appended, in this case to serve as a substitute for a scattered power-law spectrum; once again assumed to originate from scattering off of hot electrons in the ionisation cone. The Γ of the direct POWERLAW is tied to that used for the PEXMON model (Γ = 1.9) and the normalisation is left free. The normalisation of the PEXMON component (representative of pure reflection from the central source) is representative of the true source intensity. The scattered component is only a fraction of the emission from the central source, with its intensity, and hence the normalisation factor, scaling linearly with that of the central source. Thus the ratio of the normalisations for the reflected and scattered components (henceforth R-value) can be used as an indicator of the severity of the obscuration. For heavily obscured AGN the normalisation of the scattered component will be much lower than that of the PEXMON component, and hence will exhibit a high R-value. There is a great deal of uncertainty when scaling the R-value to obscuration, however, thus it is only used to sort reflection dominated AGN into two broad categories; Compton thick and Compton thin AGN. Compton thick AGN (NH ≥ 1024cm−2) typically exhibit an R-value≥30, whereas

for R-value<30 the AGN is assumed to be Compton thin (1022 <NH < 1024cm−2).

This model specialises in identifying AGN whose spectra are reflection dominated.