The Identification of Infrared Synchrotron Radiation from Cassiopeia A

We report the discovery of polarized flux at 2.2 micron from the bright shell of the approximately 320 year old supernova remnant Cas A. The fractional polarizations are comparable at 6 cm and 2.2 micron, and the polarization angles are similar, demonstrating that synchrotron radiation from the same relativistic plasma is being observed at these widely separated wavebands. The relativistic electrons radiating at 2.2 micron have an energy of ~ 150 GeV, (gamma ~ 3e5), assuming an ~500 microGauss magnetic field. The total intensity at 2.2 micron lies close to the power law extrapolation from radio frequencies, showing that relativistic particle acceleration is likely an ongoing process; the infrared emitting electrons were accelerated no longer than ~80 years ago. There is a small but significant concave curvature to the spectrum, as expected if the accelerating shocks have been modified by the back pressure of the cosmic rays; given calibration uncertainties, this conclusion must be considered tentative at present. The 2.2 micron polarization angles and the emission-line filaments observed by HST are both offset from the local radial direction by 10 - 20 degrees, providing evidence that the magnetic fields in Cas A are generated by Rayleigh-Taylor instabilities in the decelerating ejecta.