Engines in the Ashes

QC 2026-10-05

Abstract

Core-collapse supernova (CCSN) explosions mark the final stage in the lives of massive stars, leaving behind compact remnants while also launching powerful shock waves into their surroundings. Long before the final explosion, the massive progenitor stars continuously shed material through stellar winds and binary interactions, sculpting the circumstellar environment into which the supernova (SN) eventually explodes. The properties of this material therefore carries information about the progenitor’s mass-loss history prior to explosion. When the ejecta collide with this circumstellar material (CSM), forward and reverse shocks are formed, heating both the ejecta and the CSM to X-ray emitting temperatures.

Hidden at the centre of the expanding ejecta is the newly formed compact remnant. Some of these remnants are rapidly rotating, highly magnetised neutron stars known as pulsars. As the pulsar spins down, its rotational energy is converted into electromagnetic radiation and relativistic particles, powering an associated pulsar wind nebula (PWN). This emission extends into the X-ray band.

Some CCSNe are considerably more luminous than the rest. At the extreme end of the luminosity distribution lie the superluminous supernovae (SLSNe). Due to the higher energy explosions, the conventional powering mechanisms of CCSNe are not enough, and they are believed to require an additional energy source. Two favoured models are shock interaction with the CSM and a highly magnetised millisecond pulsar, often referred to as a magnetar. However, it remains uncertain whether either mechanism alone can account for the observed emission, whether both contribute, or whether additional powering mechanisms are involved.

Many open questions remain regarding how the properties and evolution of massive progenitor stars shape the resulting SN, their circumstellar environment, and their compact remnants. This thesis addresses these questions through systematic X-ray studies of large CCSN samples, using the X-ray emission to probe the compact-remnant properties and progenitor mass-loss history. In addition, it expands the very limited sample of X-ray detected SLSNe, providing new constraints on their powering mechanisms.

Late-time X-ray observations of nearby CCSNe provide constraints on the birth properties of compact remnants. The results favour pulsar populations with relatively long initial spin periods, while simultaneously demonstrating that late-time X-ray emission is predominantly produced by CSM interaction. We also find that the hard X-ray emission of SN 1979C is consistent with emission from a young pulsar, under the assumption of little-to no ejecta absorption.

We expand the very limited sample of X-ray detected SLSNe through a multi-epoch study of SN 2018bsz, one of the nearest hydrogen-poor SLSNe discovered to date. The observed X-ray evolution is difficult to reconcile with standard magnetar models, particularly when ejecta absorption is taken into account, whereas interaction with CSM naturally reproduces both the luminosity and temporal evolution. These results provide new evidence that CSM interaction may contribute significantly to the powering of at least a subset of hydrogen-poor SLSNe and highlight the diversity of physical mechanisms within this class.

Finally, through a comprehensive X-ray study of Type II SNe, we add to the X-ray detected SN sample, investigate early CSM absorption, and map the mass-loss history of the progenitor stars. We find that for some SNe the inferred mass loss evolves with time before explosion in a way that deviates from the expectations of a steady progenitor wind.

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