Low-frequency Radio Absorption in Tycho’s Supernova Remnant

Tycho's supernova remnant (SNR) is the remnant of the SN Ia explosion SN1572. In this work we present new low-frequency radio observations with the LOw Frequency ARray (LOFAR) Low-band and High-band Antennae, centered at 58 MHz and 143 MHz, and with an angular resolution of 41'' and 6'', respectively. We compare these maps to Very Large Array 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 the foreground and internal absorption from Tycho's unshocked ejecta. The external absorption could be due to an ionized 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-ionized molecular shell in the vicinity of the remnant. We note that possible ionizing 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 behavior of the region at unabsorbed radio frequencies. However, the observations are suggestive of free–free absorption from unshocked ejecta inside Tycho's reverse shock.