PhD defence: Mapping the Remains of a Stellar Explosion Optical Investigation of Supernova Remnant SNR 0540–69.3 and its Emission Components

by Linda Tenhu (KTH Physics)

Friday 24 Oct 2025, 10:00 → 13:00 Europe/Stockholm

FB53 (AlbaNova Main Building)

Opponent: Professor Parviz Ghavamian, Towson Universit

Supervisor: Josefin Larsson, Partikelfysik, astrofysik och medicinsk bildbehandling KTH

Abstract

Supernova remnants (SNRs) are the end products of supernova (SN) explosions, which occur when certain massive stars reach the end of their evolutionary cycles. When material from the progenitor star is ejected during the explosion a shock wave is created. These shocks propagate outwards from the explosion centre, sweeping up the surrounding medium, thereby forming a shell behind the shock front. While the outer regions are shaped by the blast wave shocks, the central regions of some SNRs host a pulsar, a spinning neutron star that emits electromagnetic radiation through its magnetic poles, and a pulsar-wind nebula (PWN), where relativistic particles and magnetic fields escaping the pulsar interact with the surroundings.

Many aspects concerning SNR physics await their discovery. Active areas of research include finding the link between the progenitor stars, SN explosions, and the resulting SNRs, as well as understanding of the complex conditions in SNR shocks. Furthermore, while the emission from the central pulsars and PWNe across the whole spectral energy distribution is broadly understood, the details of the underlying physical processes remain uncertain. This thesis contributes to advancing our understanding of these topics by presenting a detailed investigation of the optical emission from SNR 0540–69.3 (SNR 0540). Using the Very Large Telescope’s instruments MUSE and X-shooter, it provides, for the first time, an integral-field spectroscopic view of the entire optical SNR 0540, and access to its near-infrared spectrum.

Paper I focuses on studying the shape of the optical continuum emission of the pulsar and PWN in the central SNR 0540. The optical continuum emission from the PWN is analysed by fitting power-law models to the spectra, allowing the construction of a spatial map of spectral slopes, referred to as spectral indices, across the remnant. This spectral index map reveals distinct spatial variations, including a torus-jet structure and a general trend of shallower spectral indices (decreasing spectral index) towards the outer regions of the PWN. This so-called spatial spectral hardening toward the outer edges of the PWN is opposite to what has been previously observed (e.g., for the Crab Nebula), and therefore provides new insights into the particle distributions and acceleration in PWNe. The optical continuum emission of the pulsar is also analysed, revealing a softer pulsar spectrum (i.e. a greater spectral index) compared to the spatially integrated spectrum of the PWN. This is opposite to what has been observed for the Crab Nebula, despite the many similarities these two remnants share. The spectral difference between SNR 0540 and the Crab Nebula pulsars may propagate to their respective PWNe, potentially explaining the contrasting trends in spatial spectral index variations observed between these two nebulae.

Paper II adds SNR 0540 to the sample of SNRs, whose optical shock emission has been studied utilising modern integral-field spectroscopic instruments. Using the MUSE data, the spatial and spectral properties of shock-related emission lines are analysed by two different analytical approaches: one to study complete emission line spectra from selected spatial regions in the MUSE field of view (FOV), and another to obtain spatially resolved results for individual emission lines across the entire FOV. Specialised background, and emission line fitting algorithms are developed for both approaches. These methods reveal inhomogeneous and clumpy shock emission tracing the blast-wave shell previously observed in X-rays (as demonstrated with archival Chandra X- ray data), from lines such as [S II] 𝜆𝜆6716,6731 and [Fe XIV] 𝜆5303. The physical properties of the shock-interacting regions are estimated using various emission line ratios, highlighting the complex nature of the surrounding medium around SNR 0540 and confirming previous findings for this and similar remnants. Additionally, a spectrum extracted south of the PWN reveals emission lines whose identity and origins remain unknown. Possible explanations, such as Fe lines originating from a high-velocity ejecta clump, present challenges and call for further investigation.

SNR 0540 is a young Type II SNR in the Large Magellanic Cloud (LMC), hosting a highly energetic pulsar and a PWN. Observations indicate that this remnant is expanding within a cavity, with it’s blast wave interacting with the surrounding medium. The findings of this thesis suggest that the optical continuum emission from pulsars and their nebulae may be more complex than previously assumed. Existing models are largely based on the Crab Nebula, to which SNR 0540 is often considered as “twin”, yet this thesis identifies additional key differences between the two. These results call for further optical observations of continuum emission from pulsars and PWNe, as well as theoretical development in this area. In addition, this thesis supports similar studies of shock interaction in other SNRs in the LMC and Small MC, documenting the underlying shock conditions as well as showing that the blast wave evolution is strongly influenced by the surrounding medium. Finally, the methods developed in this thesis can be applied to other integral-field spectroscopic studies of SNR 0540 across different wavelengths, as well as to similar observations of other SNRs.