23 GHz VLBI observations of SN 2008ax

June 2009 • 2009A&A...499..649M

Authors • Martí-Vidal, I. • Marcaide, J. M. • Alberdi, A. • Guirado, J. C. • Pérez-Torres, M. A. • Ros, E. • Shapiro, I. I. • Beswick, R. J. • Muxlow, T. W. B. • Pedlar, A. • Argo, M. K. • Immler, S. • Panagia, N. • Stockdale, C. J. • Sramek, R. A. • van Dyk, S. • Weiler, K. W.

Abstract • We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8 ± 0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48 ± 0.12 mJy, compatible with the flux densities measured with the VLA at epochs close in time to our VLBI observations. The lowest average expansion velocities derived from our observations are (1.90 ± 0.30) × 10⁵ km s^-1 (case of a double source) and (5.2 ± 1.3) × 10⁴ km s^-1 (taking the weaker source component as a spurious, close by, noise peak, which is the more likely interpretation). These velocities are 7.3 and 2 times higher, respectively, than the maximum ejecta velocity inferred from optical-line observations.

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Schuyler Van Dyk

Senior Scientist