Radio Studies of Supernovae 1979C, 1986J, and 2006X with LOFAR

Abstract We present LOw Frequency ARray (LOFAR) studies of supernovae SN 1979C, SN 1986J, and SN 2006X, focusing on new observations from the LOFAR Two-metre Sky Survey (LoTSS) and the International LOFAR Telescope (ILT). For Type Ia supernovae (SNe Ia) SN 2006X, we derive a 3 σ upper limit of 0.7 mJy at 0.146 GHz, and using radio emission models based on the CS15DD2 explosion model, we constrain the circumstellar density to n H ≲ 10 cm −3 for the microphysical parameters ϵ rel = ϵ B = 0.01. SN 1979C is clearly detected in the LoTSS image with a flux density of 4.6 ± 0.36 mJy nearly 40 yr postexplosion. Modeling its radio evolution suggests a steep flux decay ( F ν ∝ t −2.1 ) between 22 and 42 yr, a break in the spectrum near 1.5 GHz possibly due to synchrotron cooling, a progenitor mass of ∼13 M ⊙ , and a progressive steepening with velocity for the density slope of the supernova ejecta. Our findings for SN 1979C contradict scenarios involving central compact object emission, and we obtain X-ray temperatures close to those derived from recent observations. For SN 1986J, we present the first ILT image showing a flux density of 6.77 ± 0.2 mJy at 0.146 GHz. The spectral index of the shell emission is found to be 0.66 ± 0.03, consistent with previous estimates, although variations at low frequencies warrant further investigation. Our results highlight the power of LOFAR for studying long-term radio evolution in supernovae.