VLBI Observations of Supernova 1993J in M81

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Session 31 -- Supernovae
Oral presentation, Wednesday, January 12, 2:15-3:45, Salon V Room (Crystal Gateway)

[31.04] VLBI Observations of Supernova 1993J in M81

M. Rupen, J. Conway (NRAO), N. Bartel, M. Bietenholz (York U.), T. Beasley, R. Sramek, J. Romney (NRAO), M. Titus (Haystack), D. Graham (MPIfR), V. Altunin, D. Jones (JPL), A. Rius (IAG), T. Venturi (IRA-Bologna), G. Umana (IRA-Noto), R. Francis, M. McCall, M. Richer, C. Stevenson (York U.), K. Weiler, S. Van Dyk (NRL), N. Panagia (STScI), W. Cannon (ISTS), J. Popelar (EMR), R. Davis (NRAL)

Global VLBI observations of SN1993J with 9 to 14 antennas for up to 18 hours show clearly that the supernova is expanding in a way consistent with being uniform. The radii (HWHM in microarcseconds) of circular Gaussians used in model fits to the visibility data, together with the observing wavelengths and epochs are: $57\pm4$ (1.3cm, Apr. 26), $91^{+17}_{-10}$ (1.3cm, May 16), $82\pm7$ (3.6cm, May 16), $165^{+8}_{-17}$ (2cm, Jun. 26), $161^{+5}_{-6}$ (3.6cm, Jun. 26). A weighted least-squares linear fit gives an angular expansion velocity of $1.74\pm0.22$ microarcseconds per day and an explosion date of Mar. $27.0\pm3.5$ UT, the latter in agreement with results from optical data. A power-law fit constrained to go through a more precise optical zero point of Mar. 28.0 (Traummell et al., 93 ApJL 414, L21) gives an exponent of $0.97\pm0.04$. Early optical spectra, e.g. measured with the INT on La Palma and the DDO near Toronto, indicate maximum expansion velocities of $17,500\pm500$~km/s (measured at the blue edge of the H$\alpha$ absorption trough). With the assumptions that a) the transverse velocity of the radio emitting region is $10\pm15\%$ larger than the above optical velocity and that b) the brightness distribution of the supernova can be approximated by any model between a uniform disk and a very thin shell, we obtain a distance to M81 of $4.3\pm0.8$ Mpc. All uncertainties are 1 standard deviations. We have continued the observations in intervals of about 6 weeks and should later be able to obtain detailed images of the supernova.

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