Polarized Mid-Infrared Synchrotron Emission in the Core of Cygnus A

Abstract

We present high-angular (~0.4") resolution mid-infrared (MIR) polarimetric observations in the 8.7 $\mu$m and 11.6 $\mu$m filters of Cygnus A using CanariCam on the 10.4-m Gran Telescopio CANARIAS. A highly polarized nucleus is observed with a degree of polarization of 11$\pm$3% and 12$\pm$3% and position angle of polarization of 27$\pm$8 degrees and 35$\pm$8 degrees in a 0.38" (~380 pc) aperture for each filter. The observed rising of the polarized flux density with increasing wavelength is consistent with synchrotron radiation from the pc-scale jet close to the core of Cygnus A. Based on our polarization model, the synchrotron emission from the pc-scale jet is estimated to be 14% and 17% of the total flux density in the 8.7 $\mu$m and 11.6 $\mu$m filters, respectively. A blackbody component with a characteristic temperature of 220 K accounts for >75% of the observed MIR total flux density. The blackbody emission arises from a combination of (1) dust emission in the torus; and (2) diffuse dust emission around the nuclear region, but the contributions of the two components cannot be well constrained in these observations.

arXiv comment8 pages, 2 figures (Accepted for publication in ApJ)

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