Infrared and optical polarization of Markarian-421

Abstract

Observations of the polarization of Mrk 421 in the V, J, and K bands are presented. The infrared radiation shows variable polarization with a range of at least 2-4%. The spectrum of the nonthermal source in the VJK regions approximates to a power law with a spectral index in the range alpha = 0.5-7; at the time of observation, the nonthermal source contributed at least 73% of the V flux in a 12-arcsec aperture. The IR polarization in the infrared is found to be substantially lower than that in the optical. The drop in polarization is too large to be explained as due solely to dilution by the unpolarized infrared flux of the host galaxy, and thus indicates a true wavelength dependence in the polarization of the nonthermal source.