6533b859fe1ef96bd12b7ff7

RESEARCH PRODUCT

Multi-Wavelength VLA Observations of the Young Supernova SN 1993J

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We present continuum VLA measurements of SN1993J in M81 taken on 17 and 21 December 2000, about 2820 days after the supernova explosion, spanning the frequency range from 0.32 to 14.9 GHz. These observations aimed at discerning the main absorbing processes acting in SN1993J at late epochs. We discuss three models for the late radio emission of SN1993J: (i) a simple power-law spectrum, (ii) a power-law spectrum free-free absorbed by a screen of homogeneously distributed ionized gas, and (iii) a power-law spectrum free-free absorbed by a “clumpy” medium. We find that the best fit to the data is yielded by a power-law spectrum free-free absorbed by either a homogeneous, or a clumpy, distribution of ionized gas. The radio spectrum between 0.32 and 14.9 GHz is well characterized by α = −0.67 ± 0.02 (Sv α vα), typical of an optically thin radio supernova, and is significantly different from that obtained at epochs ≲1000 days between 1.4 and 14.9 GHz, which indicates an ongoing spectral evolution in the radio emission from SN1993J. Since the spectral index γ of the electron distribution (N E = NoE −γ), is related to the observed spectral index by γ = 1–2 α, our value of α translates into γ = 2.34 ± 0.04, which agrees with that predicted by Fransson & Bjornsson (1998, ApJ, 509, 861) for t ≳ 1000 days. We also fit the available radio continuum data of SN1993J for the period from ∼ 70 up to 2820 days since the explosion, using a power-law spectrum free-free absorbed by a screen of homogeneously distributed ionized gas. The fit shows that the observed spectral index of SN1993J has been slowly evolving since t ∼ 1000 days on, with α increasing from a value close to −1 to −0.67. This spectral evolution seems to suggest that, in addition to the radiative (synchrotron) losses, adiabatic cooling and ionization (Coulomb) losses at the lowest frequencies might be contributing significantly to the integrated electron spectrum.