A bright, high rotation-measure FRB that skewers the M33 halo

We report the detection of a bright fast radio burst, FRB\,191108, with Apertif on the Westerbork Synthesis Radio Telescope (WSRT). The interferometer allows us to localise the FRB to a narrow 5\arcsec\times7\arcmin ellipse by employing both multibeam information within the Apertif phased-array feed (PAF) beam pattern, and across different tied-array beams. The resulting sight line passes close to Local Group galaxy M33, with an impact parameter of only 18\,kpc with respect to the core. It also traverses the much larger circumgalactic medium of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute $\sim$10\% of its dispersion measure of 588\,pc\,cm$^{-3}$. FRB\,191108 has a Faraday rotation measure of +474\,$\pm\,3$\,rad\,m$^{-2}$, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetised plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with $\Delta\nu\approx40$\,MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data

Young hidden pulsars

Massive stars end their lives in supernova explosions, leaving behind compact remnants that enclose the ember of the star, often a pulsar. The supernova remnant (SNR) and pulsar harbor vital information on the explosion mechanism that destroyed their predecessor. Studying these systems helps us to unravel e.g., the violent fates of massive stars and the formation of neutron stars. The youngest and most energetic pulsars create a pulsar wind nebulae (PWNe), which acts as a calorimeter. Finding younger and more energetic systems will thus lead us to pin down the initial energetics of pulsars, also elucidating the young-pulsar/Fast Radio Burst (FRB) connection. This thesis focuses on finding young pulsars and understanding their environments. First, in Chapter 2 we conclude on the nature of the high-energy source HESS J1943+213, compatible with being either a PWN or a BL Lac object. Next, in Chapter 3 we explore to what extent PWNe and SNRs can negatively impact pulsar signal propagation. We find that for Galactic pulsars this effect is minimal. In Chapter 4 we aim to find young pulsars in SNRs and PWNe and report on a pulsar candidate in PWN G141.2+5.0. Finally, in Chapter 5 we report on the flux calibration verification we carried out as part of the scientific commissioning of ALERT, the Apertif LOFAR Exploration of the Radio Transient Sky

Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars

Context. Oxygen sequence Wolf-Rayet (WO) stars are a very rare stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive stars, and their remaining lifetimes and the expected properties of their supernovae. Aims. We aim to homogeneously analyze the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state. Methods. We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and to deduce the atmospheric parameters. We calculate dedicated evolutionary models to determine the evolutionary state of the stars. Results. The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of MHe,ini = 15−25 M⊙ that exhibit a fairly strong (a few times 10-5M⊙ yr-1), homogeneous (fc> 0.3) stellar wind. Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of Mini = 40−60 M⊙. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years