PANCHROMATIC OBSERVATIONS OF SN 2011dh POINT TO A COMPACT PROGENITOR STAR

Abstract

We report the discovery and detailed monitoring of X-ray emission associated\nwith the Type IIb SN 2011dh using data from the Swift and Chandra satellites,\nplacing it among the best studied X-ray supernovae to date. We further present\nmillimeter and radio data obtained with the SMA, CARMA, and EVLA during the\nfirst three weeks after explosion. Combining these observations with early\noptical photometry, we show that the panchromatic dataset is well-described by\nnon-thermal synchrotron emission (radio/mm) with inverse Compton scattering\n(X-ray) of a thermal population of optical photons. In this scenario, the shock\npartition fractions deviate from equipartition by a factor, (e_e/e_B) ~ 30. We\nderive the properties of the shockwave and the circumstellar environment and\nfind a shock velocity, v~0.1c, and a progenitor mass loss rate of ~6e-5\nM_sun/yr. These properties are consistent with the sub-class of Type IIb SNe\ncharacterized by compact progenitors (Type cIIb) and dissimilar from those with\nextended progenitors (Type eIIb). Furthermore, we consider the early optical\nemission in the context of a cooling envelope model to estimate a progenitor\nradius of ~1e+11 cm, in line with the expectations for a Type cIIb SN.\nTogether, these diagnostics are difficult to reconcile with the extended radius\nof the putative yellow supergiant progenitor star identified in archival HST\nobservations, unless the stellar density profile is unusual. Finally, we\nsearched for the high energy shock breakout pulse using X-ray and gamma-ray\nobservations obtained during the purported explosion date range. Based on the\ncompact radius of the progenitor, we estimate that the breakout pulse was\ndetectable with current instruments but likely missed due to their limited\ntemporal/spatial coverage. [Abridged]\n