2009ApJ...694L...6B

We report on two epochs of very-long-baseline interferometry (VLBI) observations of the Type Ib/c supernova, SN 2008D, which was associated with the X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days, at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the source's size and position at each epoch, and thus constrain its expansion velocity and proper motion. We find the source at best marginally resolved at both epochs, allowing us to place a 3σ upper limit of ∼ 0.75 c on the expansion velocity of a circular source. For an elongated source, our measurements are compatible with mildly relativistic expansion. However, our 3σ upper limit on the proper motion is 4 µas/day, corresponding to an apparent velocity of less than 0.6 c, and is consistent with a stationary flux centroid. This limit rules out a relativistic jet such as a gamma-ray burst jet away from the line of sight, which would be expected to show apparent proper motion of greater than c. Taken together, our measurements argue against the presence of any long-lived relativistic outflow in SN 2008D. On the other hand, our measurements are consistent with the nonrelativistic expansion velocities of less than 30,000 km/s and small proper motions (< 500 km/s) seen in typical supernovae.