Variability in the Near-Infrared Synchrotron Emission From Cassiopeia A

TL;DR

This study analyzes near-infrared synchrotron emission in Cassiopeia A, revealing dense knots with high magnetic fields and electron densities, and compares these findings with gamma-ray observations to understand the remnant's magnetic environment.

Contribution

It provides the first detailed measurement of magnetic field strength and electron density in near-infrared synchrotron-emitting knots of Cas A, highlighting the role of dense post-shock material.

Findings

Magnetic fields in knots are 1.3-5.8 mG, much higher than gamma-ray estimates.

Electron densities range from 1,000 to 15,000 cm$^{-3}$.

Ks band emission aligns with expected synchrotron spectrum.

Abstract

We present multi-epoch Ks 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 Ks bands as well as in 3.5-8 micron emission and at 6 cm where synchrotron emission is dominant and we find that the Ks 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 cm$^{-3}$ and magnetic field strengths from 1.3-5.8 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.