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Issue A&A
Volume 394, Number 1, October IV 2002
Page(s) 71 - 76
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021134



A&A 394, 71-76 (2002)
DOI: 10.1051/0004-6361:20021134

The late-time radio spectrum of SN 1993J

M. A. Pérez-Torres1, A. Alberdi2 and J. M. Marcaide3

1  Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
    e-mail: torres@ira.cnr.it
2  Instituto de Astrofísica de Andalucía, CSIC, Apdo. Correos 3004, 18080 Granada, Spain
    e-mail: antxon@iaa.es
3  Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Spain
    e-mail: J.M.Marcaide@uv.es

(Received 16 May 2002 / Accepted 30 July 2002 )

Abstract
We present VLA radio continuum measurements of SN 1993J in M 81 at the frequencies of 0.32 ( P-band), 1.3 and 1.7 ( L-band), 4.9 ( C-band), 8.5 ( X-band), and 14.9 ( U-band) GHz carried out on December 17 and 21, 2000, about 2820 days after the supernova explosion. We find that a power-law spectrum, free-free absorbed by an homogeneous, or clumpy, distribution of ionized gas yields the best fit to the radio data. A combined homogeneous-clumpy model is not favored, but neither totally excluded. This result contrasts with the modeling of the early ( $t\,\lesssim\,230$ days) radio emission from SN 1993J, where a mixture of homogeneous and clumpy absorbers appeared to be necessary to adequately describe the behavior of the light curves. The radio spectrum of supernova SN 1993J between 0.32 and 14.9 GHz is well characterized by $\alpha = -0.67 \pm 0.02 $ ( $S_\nu \propto \nu^\alpha$), typical of an optically thin radio supernova. A fit to the radio spectra of SN 1993J from ~70 up to 2820 days shows that the observed spectral index of SN 1993J has been slowly evolving since $t\sim1000$ days, with the observed spectral index changing from $\alpha \approx -1$ to $\alpha=-0.67$. 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.


Key words: techniques: interferometric -- supernovae: individual: SN 1993J -- ISM: supernova remnants -- radio continuum: stars -- galaxies: individual: M 81

Offprint request: M. A. Pérez-Torres, torres@ira.cnr.it

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