Radio and X-ray Observations of the Type Ic SN 2007gr Reveal an
  Ordinary, Non-relativistic Explosion
  We present extensive radio and X-ray observations of the nearby Type Ic SN
2007gr in NGC 1058 obtained with the Very Large Array and the Chandra X-ray
Observatory and spanning 5 to 150 days after explosion. Through our detailed
modeling of these data, we estimate the properties of the blastwave and the
circumstellar environment. We find evidence for a freely-expanding and
non-relativistic explosion with an average blastwave velocity, v~0.2c, and a
total internal energy for the radio emitting material of E ~ 2 x 10^46 erg
assuming equipartition of energy between electrons and magnetic fields
(epsilon_e=epsilon_B=0.1). The temporal and spectral evolution of the radio
emission points to a stellar wind-blown environment shaped by a steady
progenitor mass loss rate of Mdot ~ 6 x 10^-7 solar masses per year (wind
velocity, v_w=10^3 km/s). These parameters are fully consistent with those
inferred for other SNe Ibc and are in line with the expectations for an
ordinary, homologous SN explosion. Our results are at odds with those of Paragi
et al. (2010) who recently reported evidence for a relativistic blastwave in SN
2007gr based on their claim that the radio emission was resolved away in a low
signal-to-noise Very Long Baseline Interferometry (VLBI) observation. Here we
show that the exotic physical scenarios required to explain the claimed
relativistic velocity -- extreme departures from equipartition and/or a highly
collimated outflow -- are excluded by our detailed Very Large Array radio
observations. Moreover, we present an independent analysis of the VLBI data and
propose that a modest loss of phase coherence provides a more natural
explanation for the apparent flux density loss which is evident on both short
and long baselines. We conclude that SN 2007gr is an ordinary Type Ibc
supernova.
