X-ray emission and thermonuclear bursts in neutron star X-ray binaries

PhD ceremony: Mr. G.B. Zhang, 14.30 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: X-ray emission and thermonuclear bursts in neutron star X-ray binaries

Promotor(s): prof. R.M. Mendez

Faculty: Mathematics and Natural Sciences

Neutron stars are the most compact objects that can be directly observed. Due to their extreme densities, neutron stars are unique laboratories for particle physics. In this thesis, I study the X-ray emission from neutron stars accreting from a companion star in a binary system, focusing on their accretion states, surface emission and thermonuclear X-ray bursts. In the first part of my thesis, I analyze an observation from the system EXO 0748-676, when the source underwent a transition to quiescence, and I find that the compact object in this X-ray binary is likely a neutron star rather than a strange quark star. In the second part of my thesis I examine the thermonuclear explosions (X-ray bursts) on the surface of the accreting neutron star in the system 4U 1636-53. I find that the properties of these bursts depend upon mass accretion rate onto the neutron star at the time of the onset of the burst. I discovered a very rare triple-peaked X-ray burst in this source; I further discuss the possible origin of such a burst. I show that the average relation between bolometric flux and temperature during the cooling phase of the X-ray bursts in 4U 1636-53 is significantly different from the canonical Stefan-Boltzmann law (flux proportional to temperature to the fourth power). I explain this in terms of the chemical composition of the neutronstar atmosphere. Finally, I found a correlation between the X-ray spectral and timing parameters in the decaying phase of radius-expansion bursts in 4U 1636-53.