We present multi-epoch K _ { s } band imaging of the supernova remnant Cassiopeia A ( Cas A ) .
The morphology of the emission in this band is generally diffuse and filamentary , consistent with synchrotron radiation observed at radio wavelengths .
However , in one region to the southwest of the remnant , compact knots of emission appear to be entrained in the ejecta and have the same proper motion as ejecta observed at similar projected radii .
The presence of these knots suggests that material with high magnetic field strength contributes significantly to synchrotron emission at these wavelengths .
We analyze these knots at J , H , and K _ { s } bands as well as in 3.5 - 8 ~ { } \text { micron } emission and at 6 ~ { } \text { cm } where synchrotron emission is dominant and we find that the K _ { s } band emission falls along the expected synchrotron spectrum .
Using multi-epoch data , we calculate the magnetic field strength and electron density for a population of near-infrared synchrotron-emitting electrons .
We find electron densities from 1 , 000 - 15 , 000 ~ { } \text { cm } ^ { -3 } and magnetic field strengths from 1.3 - 5.8 ~ { } \text { mG } .
These magnetic field strengths are an order of magnitude higher than inferred from the much lower angular resolution gamma-ray observations toward Cas A .
We conclude that dense knots of post-shock material behind the Cas A shock front are emitting synchrotron emission in a compressed and enhanced magnetic field .