PhD Defense Eva Ratti
Accurate mass measurements of stellar-scale black holes (BHs) and neutron stars (NSs) are challenging to obtain, but important to address a number of astrophysical questions. For example, tracing the currently poorly constrained mass distribution of NSs and BHs can provide constraints to theoretical models about binary evolution. Similarly, measurements of massive NSs can rule out theories about the composition of these objects (which is unknown), as a different composition implies a different value for the maximum possible mass.
A reliable method to measure masses is based on the observation of X-ray binary systems (XRBs), where the NS or BH accretes matter from a companion star. The accretion process is an efficient source of radiation, largely emitted in the X-ray waveband. When the accretion rate is low, it is possible for some XRBs to observe the mass donor star and follow its orbital motion through phase-resolved optical spectroscopy. Under certain conditions, the mass of the binary components can be inferred from those observations based on Kepler's laws.
During my PhD research I performed multi-wavelenght studies of X-ray sources in a low-accretion state, searching for suitable candidates for dynamical mass measurements. For three targets, I performed a complete dynamical study. I found that XRBs can show significant activity and variability even when accreting at a low rate, complicating the observation of the companion star and the estimate of the masses. I also found that variable spectral features from binaries hosting a white dwarf (WD) accretor can at times mimic NS or BH XRBs in a low-accretion state. The dynamical study of one of the new X-ray sources I analyzed revealed an accreting binary system with an extreme ratio between the mass of the donor star and that of the WD accretor. This result was intriguing in the framework of binary evolution theory. By studying simultaneous X-ray and radio observations of a known black hole candidate performed in a low-accretion state, I also found an interesting result regarding the coupling of X-rays and radio emission in XRBs.
Location: Agnietenkapel, Oudezijds Voorburgwal 231, Amsterdam