During my PhD, I worked on studying accretion disc wind outflows using multi-wavelength observations. In particular, we detected a transient ultraviolet outflow in the short period (P = 110.97 minutes) low-mass X-ray binary UW CrB (Fijma et al. 2023), consistent with a disc wind. We showed that the hot black body emission from the neutron star surface creates the conditions to launch a thermally-driven accretion disc wind in UW CrB, and thereby in much shorter period (and hence smaller) neutron star X-ray binaries than previously realised. 

We performed a follow-up study using a comprehensive multi-wavelength campaign on UW CrB, and showed that the detected ultraviolet outflow is likely a transient signature of a persistent disc wind outflow (Fijma et al. in press), suggesting that mass transfer in low-mass X-ray binaries can potentially be non-conservative down to short orbital periods. This, in turn, is important to consider for binary evolution prescriptions.

I am also interested in the accretion/jet connection in low mass X-ray binaries. Phenomenological differences between the accretion/jet coupling of black hole and neutron star low-mass X-ray binaries have been observed, yet the underlying reasons for these differences remain unclear. Therefore, we performed a study to disentangle the individual X-ray spectral components and evaluated their coupling with the radio jet emission. Based on this analysis, we show that there is no evidence of a significant thermal contribution in Swift/XRT spectra that could cause scatter in the radio/X-ray coupling, i.e., that emission from the neutron star surface cannot account for the observed differences (Fijma et al., 2023). 

We also evaluate the scenario of neutron stars displaying powerful ballistic jet ejections similar to black holes, based on the detection of a strong thermal X-ray emission component coinciding with the brightest radio detection, which we are currently aiming to follow-up.

Ultimately, studying the outflows of X-ray binaries is highly important for improving our understanding of their evolution.

In order to better understand their formation as well, we can probe their evolutionary histories using ultraviolet and optical spectroscopy. Through studying the composition and properties of their companion stars, we can obtain information about their formation channels. One example is for the low-mass X-ray binary Swift J1858.6-0814 (Castro Segura et al., 2024). The donor appears to have undergone CNO-processing, and based on its properties, we were able to constrain possible evolutionary tracks.

I am currently working on studying velocity kicks in the formation of neutron star low-mass X-ray binaries as well, as this can give us vital information about their formation channels, as well as binary evolution prescriptions and supernova physics.

During my studies and career, I got some opportunities to conduct my own observations. This was with the Mercator telescope (30 nights, of which 10 remotely), and with our local Anton Pannekoek Observatory (15 nights). I also got to participate in remote Designated Visitor Mode observations with the VLT (3 nights).

I am also interested in transient (radio) phenomena. During my studies, I worked on developing a method to efficiently detect image-plane radio transients on short time scales in MeerKAT radio data, as part of the ThunderKAT commensal searches program. We conducted one of the first image-plane searches down to 8 second time scales, and found a promising candidate in the 2 minute regime (Fijma et al. 2024). Aspects of the developed method were used in follow-up transient searches using MeerKAT data as well (Rowlinson et al. 2022).