A complete model of helium-like line and continuum emission in astrophysical plasmas has been incorporated into the plasma simulation code CLOUDY. All elements between He and Zn are treated, any number of levels can be considered, and a full treatment of radiative and collisional processes is included. This includes photoionization from all levels, line transfer including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the nebula. The model helium atom is described and compared to a second standalone helium atom in the low-density case. The effects of the mixing of singlet and triplet terms, the truncation of the physical system, and the convergence of the predicted line intensities as a function of the number of quantum levels explicitly included are considered. New Case-B emissivities are calculated for the helium atom at a range of electron temperatures and densities common in planetary nebulae. Observations of the Orion Nebula are analyzed and compared with predictions of the model helium atom. Observations of low-metallicity extragalactic objects by other authors are analyzed. The methods and details of the model helium-like ions are described. The standard X-ray diagnostics of these ions are revisited and augmented with semi-analytical and numerical calculations of ultraviolet line diagnostics. Finally, a new interface between CLOUDY and the X-ray spectral analysis tool XSPEC is discussed.

We utilize a Poisson likelihood in a maximum likelihood statistical analysis to analyze X-ray spectragraphic data. Specifically, we examine four extragalactic supernova remnants (SNR). IKT 5 (SNR 0047-73.5), IKT 25 (SNR 0104-72.3), and DEM S 128 (SNR 0103-72.4) which are designated as Type Ia in the literature due to their spectra and morphology. This is troublesome because of their asymmetry, a trait not usually associated with young Type Ia remnants. We present \emph{Chandra X-ray Observatory} data on these three remnants, and perform a maximum likelihood analysis on their spectra. We find that the X-ray emission is dominated by interactions with the interstellar medium. In spite of this, we find a significant Fe overabundance in all three remnants. Through examination of radio, optical, and infrared data, we conclude that these three remnants are likely not "classical" Type Ia SNR, but may be examples of so-called "prompt" Type Ia SNR. We detect potential point sources that may be members of the progenitor systems of both DEM S 128 and IKT 5, which could suggest a new subclass of prompt Type Ia SNR, Fe-rich CC remnants. In addition, we examine IKT 18. This remnant is positionally coincident with the X-ray point source HD 5980. Due to an outburst in 1994, in which its brightness changed by 3 magnitudes (corrsponding to an increase in luminosity by a factor of 16) HD 5980 was classified as a luminous blue variable star. We examine this point source and the remnant IKT 18 in the X-ray, and find that its non-thermal photon index has decreased from 2002 to 2013, corresponding to a larger proportion of more energetic X-rays, which is unexpected.

The presented thesis deals with theoretical modeling of the X-ray emission from active galactic nuclei. It studies spectral and polarisation properties of local radiation reflected from the surrounding accretion disc, which is being illuminated by a hot corona above, as well as global observational perspectives at infinity for unobscured radio-quiet sources. Modeling of this kind could then serve for observational fitting of spin of the central supermassive black hole, constraining the accretion disc's or coronal properties, or de- termining observer's inclination towards the systems. A radiative transfer Monte Carlo simulation code STOKES [Goosmann and Gaskell, 2007, Marin et al., 2012, 2015, Marin, 2018] is used for local computations. Its performance is compared to results of other attempts already existing in literature and analytical approximations. The local scheme is discussed mostly in terms of emergent polarisation that has been for the first time simulated for these types of objects with high accuracy. Integration over the accretion disc and superposition with the primary radiation in the so-called lamp-post or extended coronal model, including all general relativistic effects in the vicinity of the central super- massive black hole, is then performed on the basis of already existing routine...