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 spectral features with the help of low frequency observations plays an important role for understanding the evolution of the radio source through the propagation of synchrotron emission to observers through the SNR environment and the interstellar medium. Aims. In this paper we present measurements of the integrated spectrum of Cas A to characterize the properties of free-free absorption towards this SNR. We also add new measurements to track its slowly evolving and decreasing integrated flux density. Methods. We use the Giant Ukrainian radio telescope (GURT) for measuring 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 has been measured on 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 towards in Cassiopeia A. Generally, their values are close to the ones suggested by Arias et al. (2018), although for some there are slight differences. In the absence of clumping we find the unshocked ejecta of M = 2.61 solar mass 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 solar mass the 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.