The Pulsar and Nebula in SNR 0540-69.3
Sammanfattning: Supernova remnants (SNRs) are the end products of supernova (SN) explosions. These explosions happen when certain massive stars face the ends of their evolutionary cycle and create shock waves propagating in the surrounding medium by ejecting part of the star’s material. An SNR is the region defined by these powerful shocks. Due to the nature of SNRs spreading stellar material to their surroundings, they play a major part in the chemical evolution of the universe. Some SNRs are observed to contain a pulsar (PSR), a spinning neutron star emitting 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. This is the case for SNR 0540-69.3 (SNR0540), the so called twin of one of the most studied astronomical objects in the sky, the Crab Nebula. The attached paper (Paper I) is based on VLT observations of the central regions of SNR 0540 with the instruments MUSE and X-Shooter. The MUSE observations provide a possibility to study the optical spatial variations of SNR 0540 for the first time utilising spectroscopy, and are in general one of the few such studies of PWNe in the optical. On the other hand, earlier works focusing on the shape of the SNR 0540 continuum spectrum in the optical wavelengths have mostly utilised narrow band photometry, which has yielded conflicting results. The X-shooter observations of SNR 0540 providing the first near-infrared (NIR) spectrum with a good coverage in the optical can be used to tackle these problems. In Paper I, the continuum emission is modelled by power-law models to constrain the underlying conditions in the PSR and PWN in SNR 0540. An important parameter in these models is called the spectral index (α), which determines the slope of the spectrum. We find significant spatial variations in the spectral index that reveal a torus-jet structure around the PSR, confirming earlier results. Surprisingly, we also find that the spectral index decreases (from α ~ 1.7 to α ~ 0.5) toward the outer parts of the PWN and is the largest for the PSR (α1 ~ 1 in the low and α2 ~ 2 in the high frequencies), in contrast to theoretical expectations for the basic scenario of synchrotron cooling. Additionally, two spectral indices seem to be required to characterise both the PSR’s and PWN’s optical(-NIR) spectra. Future observations in the optical but also in the infrared and X-rays would help understanding the complex conditions in the central regions of SNRs. Most importantly, optical observations of other SNRs would shed light on whether SNR 0540 is a special case. The unexpected spectral index variations in SNR 0540 highlight the need for further theoretical work to better understand the origin of the optical synchrotron emission in PSRs and PWNe.
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