Simulated spectropolarimetry of accretion disc winds

Abstract

Accretion powers the most luminous systems across the Universe, from close Cataclysmic Variable (CV) stars to distant quasars. Ubiquitous accretion disc winds (ADW), imprint spectral signatures of broad, blue-shifted absorption lines across the emitted continuum from accreting objects. These mass loaded outflows may modify their surroundings, offering a unique window into these dynamic accreting environments. Polarization may help to reveal the geometry and kinematics of asymmetric winds and spectropolarimetry could help us to unify these fascinating systems.

In this thesis, we implement polarization for the first time in the Monte Carlo radiative transfer (MCRT) code, Python, to create simulated spectropolarimetry of biconical disc wind models. We explore spectropolarimetry results for a fiducial CV ADW and an enhanced model for optical wavelengths. We investigate how scattering varies across ADW spectral signatures, and we vary physical parameters, wind velocity laws and clumping. Our results are qualitatively similar to observations of a Nova-like CV suggesting that spectropolarimetry of ADW can help constrain CV parameters.

We applied our techniques to Broad Absorption Line Quasi Stellar Object (BALQSO) wind models and we investigated polarization across UV resonance lines and optical wind signatures. Polarization levels in our simulations qualitatively match observations.

Our work suggests that spectropolarimetry is a powerful tool to probe ADW, revealing more about the underlying accretion continuum and how the wind kinematics and geometry may provide additional independent evidence to the spectrum from accreting objects. Thereby, enhancing knowledge about their driving mechanisms, mass-loss rates and ionization structure: highly important goals for the astronomical community.