2018ApJ...863..163N

We present radio observations and modeling of one of the nearest and brightest Type IIP supernova SN 2004dj exploded in the galaxy NGC 2403 at a distance of ∼3.5 Mpc. Our observations span a wide frequency and temporal range of 0.24-43 GHz and ∼1 day to 12 years since the discovery. We model the radio light curves and spectra with the synchrotron emission. We estimate the mass-loss rate of the progenitor star to be {dot}M ∼ 1 x 10^–6 M_☉yr^–1 for a wind speed of 10 km s^–1. We calculate the radio spectral indices using 1.06, 1.40, 5.00, and 8.46 GHz flux density measurements at multiple epochs. We witness steepening in the spectral index values for an extended period predominantly at higher frequencies. We explain this as a signature of electron cooling happening at the supernova shock in the plateau phase of the supernova. We estimate the cooling timescales for inverse Compton cooling and synchrotron cooling and find that inverse Compton cooling is the dominant cooling process.