Dissertation defence (Astronomy): MSc Anna Bobrikova
MSc Anna Bobrikova defends the dissertation in Astronomy titled “Studying weakly magnetized neutron stars via polarized X-rays” at the University of Turku on 2 October 2026 at 12.00 (University of Turku, Agora, XXII Auditorium, Vesilinnantie 3, Turku).
Opponent: Professor Manuel Linares Alegret (Norwegian University of Science and Technology (NTNU), Norway)
Custos: Professor Juri Poutanen (University of Turku)
Summary of the Doctoral Dissertation:
Neutron stars are among the most extreme objects in the Universe: they contain more mass than the Sun while being only about the size of the city of Turku, and the matter inside them is compressed to densities greater than those found inside atomic nuclei. Their enormous gravity and powerful magnetic fields strongly affect the matter and light around them, which makes neutron stars unique natural laboratories for studying physical conditions that cannot be reproduced on Earth. Consequently, neutron stars are important for fundamental physics, not only for astronomy.
Unlike the Sun, many stars are not alone but belong to systems in which two or more stars orbit around a common centre of mass. In my doctoral research, I studied neutron stars that are part of binary systems with an ordinary companion star of relatively low mass. In these systems, the enormous gravity of the neutron star pulls matter away from the surface of the companion in a process called accretion. As this material moves towards the neutron star, it becomes extremely hot and produces large amounts of X-ray radiation, which allows us to investigate the physical processes taking place very close to the neutron star. The exact geometry of the accretion flow, however, remains unknown.
For decades, astronomers have studied neutron stars mainly by measuring the spectrum of their X-ray radiation and by observing how their brightness changes with time. Although these methods have provided a great deal of information about the physical conditions around neutron stars, they are less sensitive to the actual shape and orientation of the regions where the radiation is produced.
X-ray polarimetry provides a way to investigate this structure by measuring the preferred orientation of the vibrations in X-ray light. As radiation is produced and scattered by matter close to the neutron star, its polarization can change in ways that depend on the geometry of the surrounding material. Polarization measurements can therefore reveal information about structures that are far too distant to be directly resolved by telescopes.
This type of observation became much more powerful after the launch of the Imaging X-ray Polarimetry Explorer in late 2021, the first space mission dedicated specifically to measuring X-ray polarization. Since its launch, the mission has opened a new window onto neutron-star binary systems and made it possible to investigate their structure in a way that was previously largely unavailable.
During my doctoral research, I worked on observations of more than fifteen weakly magnetized neutron-star systems, as well as on the first X-ray polarimetric observation of an accreting millisecond pulsar. I also developed theoretical models describing how polarized X-ray radiation should appear when it is produced in different regions around these neutron stars, allowing the observed polarization to be compared with predictions for different physical configurations.
The main aim of my doctoral research was to improve our understanding of how matter behaves and produces radiation as it falls onto neutron stars, with particular emphasis on the geometry of the regions responsible for the observed X-rays. By combining new polarization measurements with theoretical modelling, my work contributed to addressing long-standing questions about how radiation from accreting pulsars is directed through space and about the structure and origin of the X-ray emission from weakly magnetized neutron stars.
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Portrait: Vadzim Krautsou
