Radio evolution of the remnant of Supernova 1987A

[Truncated] Radio supernovae result from the collision between a supernova (SN) shock and the progenitor’s circumstellar medium (CSM). Supernova 1987A in the Large Magellanic Cloud, as the only nearby core-collapse supernova observed with a telescope since its early stages, has allowed unique studies of the evolution of the SN−CSM interaction and the complex structure of the resulting emission. This thesis focuses on the radio evolution of the remnant of SN 1987A as the shock wave impacts the dense CSM in the equatorial ring (ER).

The radio flux density of the remnant as a function of time and frequency is a guide to the conditions at the shock front. The ongoing observing campaign of the supernova remnant (SNR) with the Australia Telescope Compact Array (ATCA), via flux density monitoring from 1 to 9 GHz, shows that the radio emission has been increasing at an exponential rate since day ∼5000, with an e−folding time of 2408 ± 227 days. This indicates that the propagating blast wave, and the increasing shock volume associated with that, is interacting with an increasingly dense region of the CSM, likely associated with the ER. Since the spectral index, α (Sν ∝ να), has become flatter over time, the shock compression ratio has increased by 14±3% from 2001 to 2009 and is currently estimated at 3.20±0.04. The exponential increase of the radio light curve is likely a consequence of the increased efficiency of the acceleration mechanism that generates synchrotron radiation at the shock front.

New ATCA high-resolution images at 18 and 44 GHz have been used to inspect the spectral index variations within the SNR and, thus, the spatial variations of the compression ratio, cosmic ray density, and magnetic field strength. A comparison with previous ATCA observations at 18 and 36 GHz highlights an asymmetric expansion of the remnant, with expansion velocities on the eastern lobe significantly higher than that measured on the western lobe. From the high-resolution spectral maps at 18−44 GHz, it appears that a higher injection efficiency can be associated with the eastern regions of the SNR, while regions of flatter spectral index align and partially overlap with the optically visible ejecta. A multi-wavelength comparison shows that the asymmetry direction of the radio emission is consistent with that seen in contemporaneous X-ray observations, but opposite to that of the Hα emission. This supports the hypothesis that the remnant morphology might be due to an asymmetric explosion, rather than to an asymmetric distribution of the CSM.