Free–free absorption parameters of Cassiopeia A from low-frequency interferometric observations

Abstract

Context. Cassiopeia A is one of the most extensively studied supernova remnants (SNRs) in our Galaxy. The analysis of its continuum spectrum through low-frequency observations plays an important role for understanding the evolution of the radio source and the propagation of synchrotron emission to observers through the SNR environment and the ionized interstellar medium. Aims. In this paper we present measurements of the integrated spectrum of Cas A to characterize the properties of free–free absorption toward this SNR. We also add new measurements to track its slowly evolving and decreasing integrated flux density. Methods. We used the Giant Ukrainian Radio Telescope (GURT) to measure the continuum spectrum of Cassiopeia A within the frequency range of 16–72 MHz. The radio flux density of Cassiopeia A relative to the reference source of the radio galaxy Cygnus A was measured in May–October 2019 with two subarrays of the GURT, used as a two-element correlation interferometer. Results. We determine magnitudes of emission measure, electron temperature, and an average number of charges of the ions for both internal and external absorbing ionized gas toward Cassiopeia A. Generally, their values are comparable, albeit with slight differences, to recently presented values. In the absence of clumping, we find that the unshocked ejecta of M = 2.61 M⊙ at the electron density of 15.3 cm−3 has a gas temperature of T ≈ 100 K. If the clumping factor is 0.67, then the unshocked ejecta of 0.96 M⊙ has an electron density of 18.7 cm−3. Conclusions. The integrated flux density spectrum of Cassiopeia A obtained with the GURT interferometric observations is consistent with the theoretical model within measurement errors and also reasonably consistent with other recent results in the literature.