Tales of dual dancers: observational studies of accretion flows in compact binaries

Abstract

This thesis presents an observational study of accreting compact binaries, whose radiation is emitted across the entire electromagnetic spectrum. Each wavelength probes a different binary component, providing insights into the physical processes at play. Ultraviolet (UV) emission, in particular, has been proven crucial in understanding the structure and dynamics of accretion discs, as well as tracing inflow and outflow processes. This work focuses on studying two spectacular objects: the dwarf nova WZ Sge and the black hole X-ray transient MAXI J1820+070, using primarily time-resolved observations from the Hubble Space Telescope (HST).

For WZ Sge, my research presents the first UV characterisation of the source during three distinct stages of its 2001 superoutburst. The system exhibits an unusual dip in its light curve, and a key focus of this study is to investigate whether this dip results from its transition to a magnetic propeller state. Instead of finding evidence of magnetic propeller signatures, this work proposes the existence of a veiling curtain - material along the line-of-sight - surrounding the system. The properties of the material are characterised by a moderate-temperature, high-density, low-velocity absorbing profile. This result is of high importance as it highlights a previously unknown physical component, always present around the source.

For MAXI J1820+070, I present the first multi-epoch UV spectral and temporal characterisation during its outburst, covering both hard and soft states. Using HST and complementary AstroSat observations, this work constitutes a unique opportunity to define the UV response to luminosity changes and state transitions in black hole binaries. It is shown how the UV spectrum and the stochastic and aperiodic variability change among these states. The HST spectra provide meaningful insights about the physical conditions of the accretion disc and the possible emergence of outflows. This research also quantifies the reprocessing efficiency in the outer disc using irradiated disc models. These avenues are explored due to the low extinction to the system, which is also determined.

The findings of this thesis significantly advance our understanding of accreting compact binaries. The discovery of the veiling curtain in WZ Sge opens new routes for investigating similar features in other systems, while the detailed UV characterisation of MAXI J1820+070 enhances our understanding of state transitions and disc dynamics in black hole binaries. These results signify the importance of UV observations in unraveling the complex processes occurring in these systems.

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Identifiers

ORCID for Maria Georganti: orcid.org/0000-0002-3776-9652

ORCID for Diego Altamirano: orcid.org/0000-0002-3422-0074

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