title: A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr.
abstract: 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 (GRBs). 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 approximately 6,000 km s(-1), 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 spectropolarimetry, mildly aspherical.
authors.affiliation: Joint Institute for VLBI in Europe (JIVE), Postbus 2, 7990AA Dwingeloo, The Netherlands. zparagi@jive.nl
authors.forename: Z
authors.initials: Z
authors.lastname: Paragi
authors.forename: G B
authors.initials: GB
authors.lastname: Taylor
authors.forename: C
authors.initials: C
authors.lastname: Kouveliotou
authors.forename: J
authors.initials: J
authors.lastname: Granot
authors.forename: E
authors.initials: E
authors.lastname: Ramirez-Ruiz
authors.forename: M
authors.initials: M
authors.lastname: Bietenholz
authors.forename: A J
authors.initials: AJ
authors.lastname: van der Horst
authors.forename: Y
authors.initials: Y
authors.lastname: Pidopryhora
authors.forename: H J
authors.initials: HJ
authors.lastname: van Langevelde
authors.forename: M A
authors.initials: MA
authors.lastname: Garrett
authors.forename: A
authors.initials: A
authors.lastname: Szomoru
authors.forename: M K
authors.initials: MK
authors.lastname: Argo
authors.forename: S
authors.initials: S
authors.lastname: Bourke
authors.forename: B
authors.initials: B
authors.lastname: Paczyński