Abstract:
Supersoft X-ray sources (SSSs) represent a fascinating class of astrophysical objects characterised by their extremely soft thermal X-ray spectra (< 1keV), with effective temperatures typically ranging from 15 − 80 eV and luminosities approaching the Eddington limit (~10^36 − 10^38 erg s^-1). They are now understood to be the accreting white dwarfs (WDs) in binary systems that undergo high mass accretion rates, leading to quasi-steady thermonuclear burning on their surface. These systems, first identified in the Magellanic Clouds (LMC and SMC) and later in external galaxies and the Milky Way, are important for understanding binary star evolution, accretion physics, and the progenitors of Type Ia supernovae.
SSSs challenge traditional spectral modelling techniques. Blackbody approximations often fail to capture their complex physics, overestimating luminosities and underestimating temperatures due to insufficient treatment of atmospheric composition and radiative processes. In this dissertation, a comprehensive theoretical and observational investigation is carried out to explore the properties of SSSs using a novel grid of Local Thermodynamic Equilibrium (LTE) model atmospheres for hot WDs. These models span effective temperatures from 100 kK to 1000 kK and incorporate a range of surface gravities and chemical compositions – including solar, LMC/SMC-like, and those specific for the SSS stage of Classical Novae (CNe). Publicly released in XSPEC format, the models enable efficient spectral fitting while maintaining physical rigour, offering a practical alternative to computationally intensive Non-LTE (NLTE) approaches. Validation against NLTE results revealed only minor discrepancies, affirming their utility for parameter estimation across diverse SSS populations.
The primary objective is to interpret X-ray spectra obtained from cutting-edge observatories such as Chandra, XMM-Newton, and eROSITA. By applying these models to classical SSSs – including well-studied sources like CAL 83 and RX J0513.9 − 6951, as well as to the SSS phase of the CN AT 2018bej – the work derives key physical parameters such as effective temperature, surface gravity, and elemental abundances, while also unravelling their distinct evolutionary pathways.
For CAL 83, the parameters found agree with previous NLTE estimates, emphasising the applicability of our models for analysing SSS spectra. It was also confirmed that the X-ray off-states of this source are linked to the cessation of thermonuclear burning, and the WD mass estimate supports this scenario.
For RX J0513.9 − 6951, which exhibits anti-correlated optical/X-ray variability typically attributed to the contraction and expansion of the WD, time-resolved analysis of the X-ray spectra reveals WD parameters that favour an alternative scenario: optically thick clouds above the accretion disc modulate the optical flux by reprocessing variable X-ray irradiation, thereby producing the observed X-ray on/off states.
For AT 2018bej, a CN in its post-outburst SSS phase, a minor evolution was traced over a half-year timescale, accompanied by a decrease in the carbon abundance. This result supports the conclusion that LTE model atmospheres can be effectively used to analyse the available X-ray spectra of CNe during their SSS state.
Overall, the work significantly enhances our understanding of the complex interplay between accretion processes, nuclear burning, and radiative transfer in SSSs. The new model grids, made available for community use in XSPEC, pave the way for future observational and theoretical investigations aimed at refining our knowledge of WD atmospheres and the physics of X-ray binaries.
Classical Novae