2003ApJ...592..299R

Recent high-energy observations of Cassiopeia A suggest the presence of synchrotron radiation, implying acceleration of cosmic rays by young supernova remnants. We detect synchrotron emission from Cas A in the near-infrared using Two Micron All-Sky Survey (2MASS) and Palomar 200 inch (5.1 m) PFIRCAM observations. The remnant is detected in the J, H, and K_s_bands using 2MASS: the K_sband is the brightest, H is moderate, and J is faint. In the J and H bands, bright [Fe II] lines (1.24 and 1.64 µm) are detected spectroscopically. The Palomar observations include K_s-continuum, narrowband 1.64 µm (centered on [Fe II]) and 2.12 µm [centered on H₂(1-0)] images. While the narrowband 1.64 µm image shows filamentary and knotty structures, similar to the optical image, the K_simage shows a relatively smooth, diffuse shell, remarkably similar to the radio image. The H₂image is identical to the K_s-continuum image, with surface brightness reduced as expected for the ratio of filter bandwidths, showing no contribution of H₂lines to the K_s-band image. The broadband near-infrared fluxes of Cas A are generally consistent with, but a few tens of percent higher than, an extrapolation of the radio fluxes. The hardening to higher frequencies is possibly due to nonlinear shock acceleration and/or spectral index variation across the remnant. We show evidence of spectral index variation across Cas A using the ``spectral tomography'' technique. The presence of near-infrared synchrotron radiation requires the rolloff frequency to be higher than 1.5x10¹⁴ Hz, implying that electrons are accelerated to energies of at least E=0.3B^–1/2_mGergs, or 0.2 TeV. The morphological similarity in diffuse emission between the radio and K_s-band images implies that synchrotron losses are not dominant, or we would expect to see a greater concentration in knots. We also show that dust continuum is not significant in the near-infrared emission of Cas A. Our observations show unambiguous evidence that the near-infrared K_s-band emission of Cas A is from synchrotron emission by accelerated cosmic-ray electrons.