Panchromatic observations of SN2011dh point to a compact progenitor star

We report the discovery and detailed monitoring of X-ray emission associated with the TypeIIbSN2011dh using data from the Swift and Chandra satellites, placing it among the best-studied X-ray supernovae (SNe) to date. We further present millimeter and radio data obtained with the Submillimeter Array, the Combined Array for Research in Millimeter-wave Astronomy, and the Expanded Very Large Array during the first three weeks after explosion. Combining these observations with early optical photometry, we show that the panchromatic data set is well described by non-thermal synchrotron emission (radio/mm) with inverse Compton scattering (X-ray) of a thermal population of optical photons. In this scenario, the shock partition fractions deviate from equipartition by a factor, (εe/εB) 30. We derive the properties of the shock wave and the circumstellar environment and find a time-averaged shock velocity of and a progenitor mass-loss rate of (for an assumed wind velocity, vw = 1000 km s-1). We show that these properties are consistent with the sub-class of TypeIIbSNe characterized by compact progenitors (TypecIIb) and dissimilar from those with extended progenitors (Type eIIb). Furthermore, we consider the early optical emission in the context of a cooling envelope model to estimate a progenitor radius of R * 1011cm, in line with the expectations for a TypecIIbSN. Together, these diagnostics are difficult to reconcile with the extended radius of the putative yellow supergiant progenitor star identified in archival Hubble Space Telescope observations, unless the stellar density profile is unusual. Finally, we searched for the high-energy shock breakout pulse using X-ray and gamma-ray observations obtained during the purported explosion date range. Based on the compact radius of the progenitor, we estimate that the shock breakout pulse was detectable with current instruments but likely missed due to their limited temporal/spatial coverage. Future all-sky missions will regularly detect shock breakout emission from compact SN progenitors enabling prompt follow-up observations with sensitive multi-wavelength facilities. © 2012. The American Astronomical Society. All rights reserved.