Low-frequency radio absorption in Tycho's supernova remnant

TL;DR

This study uses low-frequency radio observations to detect and analyze free-free absorption in Tycho's supernova remnant, exploring external and internal origins of absorption and their implications for the remnant's environment.

Contribution

First detailed low-frequency radio maps of Tycho's SNR at 58 and 143 MHz are presented, revealing absorption effects and proposing models for their origins.

Findings

Detection of low-frequency absorption in some regions of Tycho's SNR.

Possible external absorption from a thin ionized cavity or molecular shell.

Internal absorption likely from unshocked ejecta inside the remnant.

Abstract

Tycho's SNR is the remnant of the type Ia supernova explosion SN1572. In this work we present new low-frequency radio observations with the LOFAR Low-Band and High-Band Antennae, centred at 58 MHz and 143 MHz, and with an angular resolution of 41'' and 6'' respectively. We compare these maps to VLA maps at 327 MHz and 1420 MHz, and detect the effect of low-frequency absorption in some regions of the remnant due to the presence of free electrons along the line-of-sight. We investigate two origins for the low-frequency free-free absorption that we observe: external absorption from foreground, and internal absorption from Tycho's unshocked ejecta. The external absorption could be due to an ionised thin, diffuse cavity surrounding the SNR (although this cavity would need to be very thin to comply with the neutral fraction required to explain the remnant's optical lines), or it could be due to an over-ionised molecular shell in the vicinity of the remnant. We note that possible ionising sources are the X-ray emission from Tycho, its cosmic rays, or radiation from Tycho's progenitor. For the internal absorption, we are limited by our understanding of the spectral behaviour of the region at unabsorbed radio frequencies. However, the observations are suggestive of free-free absorption from unshocked ejecta inside Tycho's reverse shock.