Research Note

23 GHz VLBI observations of SN 2008ax

¹ Dpt. Astronomia i Astrofísica, Universitat de València, C/ Dr. Moliner 50, 46100 Burjassot, Spain e-mail: I.Marti-Vidal@uv.es
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Instituto de Astrofísica de Andalucía (CSIC), C/ Camino bajo de Huétor 50, 18008 Granada, Spain
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 51, Cambridge, MA 02138, USA
⁵ Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
⁶ Department of Imaging and Applied Physics, Curtin University of Technology, Bentley, WA 6845, Australia
⁷ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 662, Greenbelt, MD 20771, USA
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
⁹ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123, Catania, Italy
¹⁰ Supernova Ltd, OYV #131, Northsouth Rd., Virgin Gorda, British Virgin Islands
¹¹ Department of Physics, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
¹² National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
¹³ University of California, Astronomy Department, Berkeley, California 94720, USA
¹⁴ Naval Research Laboratory, Code 7210, Washington, DC 20375-5320, USA

Received: 10 March 2009
Accepted: 24 March 2009

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.