Spectra of HB 21 supernova remnant: Evidence of spectra flattening at the low frequencies

TL;DR

This study analyzes the radio spectra of the HB 21 supernova remnant across multiple frequencies, revealing spectral flattening at low frequencies likely caused by thermal plasma absorption.

Contribution

It provides new measurements of the spectral index of HB 21 at low radio frequencies and identifies spectral flattening phenomena.

Findings

Spectral flattening observed below 408 MHz.

Thermal plasma absorption likely causes low-frequency spectral flattening.

Mean spectral index estimated for HB 21.

Abstract

We use observations of the continuum radio emission at 1420, 820, 408, 34.5 and 22 MHz to estimate the mean brightness temperatures of the HB 21 supernova remnant (SNR) at five frequencies. We also presented mean spectral index of HB 21. The spectra of HB 21 are estimated for mean temperatures versus frequency for 1420, 820, 408, 34.5 and 22 MHz. We also presented $T-T$ plots of three frequency pairs: between 1420--34.5, 1420--22, 34.5--22 MHz. We noticed flatter spectral indices at frequencies below 408 MHz. Probably this is due to the absorption by thermal plasma at low frequencies.