MERATE (LC) – Master – X-Ray Binaries – A study of the accretion process in active black hole and neutron stars across a state transition

Background – X-ray binaries:
Throughout the Universe the combination of a deep potential well and an accretion disc, (which forms when matter is gravitationally captured by a celestial body) leads to the generation of fast, collimated outflows called jets.
This process, still poorly understood, occurs in proto-planetary discs and at the centre of galaxies alike, but around black hole (BHs) and neutron stars (NSs) it is taken to the extreme.
In a process known as feedback these so-called compact objects contrive to feed back to the surrounding space a large fraction of the energy and matter they could have swallowed, thereby acting to heat their environment rather than behaving only as sinks.
Feedback is important across a range of scales: from stellar-mass BHs and NSs in X-ray binaries (XRBs), to super-massive BHs powering the AGN, which via this process regulated the growth of massive galaxies.
The AGN and XRBs hosting BHs and NSs provide us with the best tests of General Relativity, but while the former evolve over decades to millenia, XRBs evolve rapidly, offering us the opportunity to probe on humanly accessible time-scales the energy and matter input/output around accreting objects.
The knowledge gained from studying XRBs can then be directly applied to AGN, where the inflow/outflow processes follow the same basic principles as around stellar-mass BHs.

Using observations from across the entire electromagnetic spectrum, and employing various techniques best-suited to extract the information stored in the data we investigate the physics of the accretion and outflow generation processes in X-ray binaries, with the aim of understanding the nature of such processes and the link between them both on stellar-mass scales, and on super-massive scales.

The thesis – Essentially all accreting black hole and neutron star X-ray binaries show a number of accretion states, characterized by specific features in terms of their luminosity, spectral properties, fast time-variability, and outflows production.
The canonical accretion states include a hard and a soft state, and two intermediate states.
These are always crossed according to the same cyclical pattern: hard state > hard intermediate state > soft intermediate state > soft state > soft-intermediate state > hard intermediate state > hard state.
Sources spend a large amount of time (several months) in the hard and soft states, during which the spectral and timing properties of a source remain relatively constant, but the luminosity varies significantly.
Instead, the two intermediate states are crossed relatively rapidly (days or faster), and the spectral and timing properties vary dramatically, while the luminosity remains relatively stable.
There appears to be a correlation between the amount of time it takes for a source to go transition from the hard state to the soft state and the luminosity at which such transition occurs.

The aim of this thesis is to verify the existence of such correlation by means of spectral and timing analysis, and interpret it in the light of the most recent theoretical models describing the physics of accreting compact objects.
We will use X-ray data from the Rossi X-ray Timing Explorer satellite on a sample of Galactic black hole and neutron star X-ray binaries.
The results of this project will reveal important information of the accretion processes around compact objects, and will be relevant not only in the context of stellar mass compact objects, but will be also contrasted with and extended to the case of super-massive black holes powering the Active Galactic Nuclei.