Accretion disc tomography in Low-Mass X-Ray Binaries: A multi-wavelength approach

X-ray binaries systems where a compact object accretes matter from a companion star, offer exceptional opportunities to explore astrophysical phenomena, including the General Relativity and Magneto-Hydrodynamics, which cannot "yet" be reproduced in laboratories. Particularly, the study of accretion discs in low-mass X-ray binary systems not only offers pivotal clues to the physics of accretion and the processes releasing a huge amount of electromagnetic radiation in X-rays and gamma-rays, but also can unravel the geometry of these systems. The core of thisthesis encompasses a multi-wavelength temporal and spectral analysis of LMXBs, stretching from optical to X-ray wavelengths. My first work presented in Chapter 5 delves into the spectral analysis of the low-mass X-ray pulsar 4U 1822-371. This study reveals for the first-time significant evidence of a reflection component in the x-ray spectrum of a high inclination source and provides updated orbital ephemerides. The analysis con-tributes to understanding the system’s geometry proposing a unique configuration for the source. With my second project (Chapter 6) I broadened the scope of my analysis to the optical range, analysing the double-peaked H𝛽 line profile in the optical spectrum of Swift J1357.2-0933, a transient LMXB hosting one of the galaxy most massive stellar-mass black hole candidates. This investigation not only estimated the black hole’s systemic and radial velocity but also uncovered compelling evidence of a narrow core within the H𝛽 emission profile.Building on these findings, my final project (Chapter 7) introduces an innovative method that applies X-ray spectral models to optical data. I applied a non-relativistic version of the diskline model to describe the optical H𝛼 and H𝛽 line profiles. Applying this method to two sample sources, Swift J1357.2-0933 and MAXI J1305-704, yielded estimates for the inner and outer radii of the disc’s emitting region. Remarkably, the method also determined the disc’s inclination angle with a precision surpassing existing literature values.Overall, this comprehensive approach advances our understanding of LMXB geometries. It constrains critical parameters such as the inclination angle, size of the accretion disc, and the varying temperature and ionization states across different disc regions. These insights are not only pivotal for unraveling the nature of the accretion process but also for creating a ’topographic map’ of the disc, shedding light on the entire structure of the disc and the physics governing it.