Panchromatic Observations of SN 2011dh Point to a Compact Progenitor Star

We report the discovery and detailed monitoring of X-ray emission associated with the Type IIb SN 2011dh 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, (epsilon(e)/epsilon(B)) similar to 30. We derive the properties of the shock wave and the circumstellar environment and find a time-averaged shock velocity of (v) over bar approximate to 0.1c and a progenitor mass-loss rate of (M) over dot approximate to 6x10(-5) M-circle dot yr(-1) (for an assumed wind velocity, v(w) = 1000 km s(-1)). We show that these properties are consistent with the sub-class of Type IIb SNe characterized by compact progenitors (Type cIIb) 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-* approximate to 10(11) cm, in line with the expectations for a Type cIIb SN. 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.