VLBI observations of SN 2008iz

I. Expansion velocity and limits on anisotropic expansion

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: brunthal@mpifr-bonn.mpg.de
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ UC Berkeley, 601 Campbell Hall, Astronomy Department & Radio Astronomy Lab, Berkeley, CA 94720, USA
⁴ Department of Astrophysics, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands
⁵ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
⁷ Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA

Received: 26 January 2010
Accepted: 25 March 2010

Abstract

We present observations of the recently discovered supernova 2008iz in M 82 with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array at frequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory. The supernova was clearly detected in two VLBI images separated by 11 months. The source shows a ring-like morphology and expands with a velocity of ~23 000 km s^-1. The most likely explosion date is in mid-February 2008. The measured expansion speed is a factor of ~2 higher than expected under the assumption that synchrotron self-absorption dominates the light curve at the peak, indicating that this absorption mechanism may not be important for the radio emission. We find no evidence for an asymmetric explosion. The VLA spectrum shows a broken power law, indicating that the source was still optically thick at 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emission from SN 2008iz and a second radio transient recently discovered by MERLIN observations.