X-ray modelling of accreting neutron stars

Neutron stars residing in binary systems can attract the outer gaseous layers of their companion star. As this material spirals towards the neutron star, it heats up and starts to emit in the X-ray band. However, material doesn't always reach the surface of the neutron star unencumbered: if the neutron star is sufficiently magnetic, its magnetic field may start to interact with the inflowing gas. Various odd types of behaviour may be the result: pulsations seen from the poles of the neutron star as material is magnetically channeled onto them; propeller-like outflows; or 'trapped-disks' where the inflowing gas gets stuck when it encounters the magnetic field. In much of my PhD work, I focused on understanding these interactions via different approaches to study of the X-ray emission: (1) reflection spectroscopy, where we treat the accretion flow as a funhouse mirror and try to deduce its properties from the manner in which it reflects light; (2) timing studies, where we search for pulses from the poles; and (3) high-resolution X-ray spectroscopy, searching for blueshifted signatures of outflowing material driven by, for instance, the propeller mechanism.

Disappearing reflection in an accreting neutron star

The disk of material around a neutron star can act as a funhouse mirror: it reflects X-rays towards us but distorts the spectrum based on its own geometry and properties. Most clearly, reflection adds a broad emission line around the iron compex at ~6.4 keV, whose shape is set by Doppler shifts, special and general relativistic effects, and even the composition and density of the disk. In this study, we observed a neutron star where, during an earlier period of activity, a very clear broad iron emission line was seen. To our surprise, however, at ten times lower accretion rate, no evidence for any line could be observed. The absence of reflection did not stop us from inferring information about the mirror, however: we determined that this lack of iron signal was not due to poor data quality, but that instead, the disk may have become more ionised. Physically, we associated this change with the disk 'puffing up', as expected when the accretion rate drops.

X-raying a faint accreting neutron star

Some accreting neutron stars are always X-ray faint, but the origin of their faintness remains poorly known. In this in depth study, we explored data from XMM-Newton, Swift, and NuSTAR, via various techniques, to understand why the particular source IGR J17062-6143 is always faint. We conclude that the two proposed reasons, a strong magnetic field trapping the disk or a small orbit fitting only a tiny disk, may both be at play; in fact, they may enhance each others effects. Interestingly, later studies by Strohmayer et al. (2018) confirmed this interpretation. Excitingly, we will perform a large-scale observing campaign including XMM-Newton, the Hubble Space Telescope, ATCA, VLT, and NuSTAR, to answer our final questions about the source: what type of donor star does it harbour, and does it launch a radio jet?

Origin of Type-II bursts revealed by reflection

The interaction between infalling gas and neutron star magnetic fields can lead to rare behaviour: only 2 sources show so-called Type-II bursts, where the accretion rate briefly increases more then tenfold. Using reflection spectroscopy, we find evidence that the these bursts are caused by a magneto-centrifugal barrier trapping the disk, which is only overcome when enough material has build up at the barrier.