  The class of type Ic supernovae have drawn increasing attention since 1998
owing to their sparse association (only four so far) with long duration
gamma-ray bursts. Although both phenomena originate from the core collapse of a
massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit
mostly in soft gamma-rays or hard X-rays. Though the GRB central engine
generates ultra-relativistic jets, which beam the early emission into a narrow
cone, no relativistic outflows have hitherto been found in type Ib/c supernovae
explosions, despite theoretical expectations and searches. Here we report radio
(interferometric) observations that reveal a mildly relativistic expansion in a
nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days
apart, we detect expansion of the source and establish a conservative lower
limit for the average apparent expansion velocity of 0.6c. Independently, a
second mildly relativistic supernova has been reported. Contrary to the radio
data, optical observations of SN 2007gr indicate a typical type Ic supernova
with ejecta velocities ~6000 km/s, much lower than in GRB-associated
supernovae. We conclude that in SN 2007gr a small fraction of the ejecta
produced a low-energy mildly relativistic bipolar radio jet, while the bulk of
the ejecta were slower and, as shown by optical spectro-polarimetry, mildly
aspherical.
