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

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.