The initial evolution of SN 2011dh: The importance of inhomogeneities

Abstract

SN 2011dh is rather unique in that it offered detailed observations of the initial phase in the radio as well as optical regimes. This makes possible a comparison between models used to deduce properties of the outer envelope of the supernova ejecta. It is shown that a consistent description suggests the forward shock to have started in the piston phase with constant velocity, and only later, around 50 days, transitioned to the standard model, which is independent of initial conditions. In addition, observations imply that the radio source is inhomogeneous with a covering factor of, approximately, 50%. It is emphasised that the deduced properties of the synchrotron source are very sensitive to the presence of inhomogeneities; for example, a covering factor of 50% increases the ratio of the energy densities of relativistic electrons and magnetic field by several orders of magnitude as compared to a homogeneous source. The shallow density gradient in the envelope causes substantial deceleration of the forward shock. This is used to argue that the magnetic field strength scales inversely with radius rather than inversely with time; this is similar to SN 1993J. Attention is also drawn to the similarities between the flat spectra of compact, extragalatic radio sources and the evolution of radio supernovae; e.g., the scaling of the magnetic field and the constant brightness temperature.