2026ApJ...998..186L

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 3σ 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.