A Decade of SN1993J: Discovery of Wavelength Effects in the Expansion Rate

Abstract

We have studied the growth of the shell-like radio structure of supernova SN1993J in M81 from September 1993 through October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. For this purpose, we have developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure like SN1993J. The source structure of SN1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J through approximately day 1500 after explosion with an expansion parameter $m=0.845\pm 0.005$ ($R\propto t^m$). However, from that day onwards the expansion is different when observed at 6 and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same $m$ as before day 1500, while at 6cm the expansion appears more decelerated, and is characterized by another expansion parameter, $m_6=0.788\pm 0.015$. Therefore, since about day 1500 on, the radio source size has been progressively smaller at 6cm than at 18cm. These findings are in stark contrast to previous reports by other authors on the details of the expansion. In our interpretation the supernova expands with a single expansion parameter, $m=0.845\pm 0.005$, and the 6cm results beyond day 1500 are due to physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6cm radiation, and (b) a radial decrease of the magnetic field in the emitting region. (Long abstract cut. Please, read full abstract in manuscript).

21 pages, 19 figures, accepted in A&A

References in corpus (5)

• The massive binary companion star to the progenitor of supernova 1993J
• Hubble Space Telescope and Ground-Based Observations of SN 1993J and SN 1998S: CNO Processing in the Progenitors
• Long Term Radio Monitoring of SN 1993J
• SN 1993J VLBI (IV): A Geometric Determination of the Distance to M81 with the Expanding Shock Front Method
• Synchrotron aging and the radio spectrum of SN 1993J

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