A mildly relativistic radio jet from the otherwise normal Type Ic Supernova 2007gr

TL;DR

This paper reports the discovery of a mildly relativistic radio jet in the normal Type Ic supernova SN 2007gr, revealing that such supernovae can produce low-energy relativistic outflows despite typical optical velocities.

Contribution

It provides the first evidence of a mildly relativistic jet in a normal Type Ic supernova, expanding understanding of supernova explosion mechanisms and jet formation.

Findings

Detected expansion velocity of at least 0.6c in SN 2007gr

Radio observations reveal a mildly relativistic bipolar jet

Optical data show typical supernova velocities (~6000 km/s)

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. 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.