Submillimeter Polarimetry with PolKa, a reflection-type modulator for the APEX telescope

摘要

Imaging polarimetry is an important tool for the study of cosmic magneticfields. In our Galaxy, polarization levels of a few up to $\sim$10\% aremeasured in the submillimeter dust emission from molecular clouds and in thesynchrotron emission from supernova remnants. Only few techniques exist toimage the distribution of polarization angles, as a means of tracing theplane-of-sky projection of the magnetic field orientation. At submillimeterwavelengths, polarization is either measured as the differential total power ofpolarization-sensitive bolometer elements, or by modulating the polarization ofthe signal. Bolometer arrays such as LABOCA at the APEX telescope are used toobserve the continuum emission from fields as large as $\sim0\fdg2$ indiameter. %Here we present the results from the commissioning of PolKa, apolarimeter for Here we present PolKa, a polarimeter for LABOCA with areflection-type waveplate of at least 90\% efficiency. The modulationefficiency depends mainly on the sampling and on the angular velocity of thewaveplate. For the data analysis the concept of generalized synchronousdemodulation is introduced. The instrumental polarization towards a pointsource is at the level of $\sim0.1$\%, increasing to a few percent at the$-10$db contour of the main beam. A method to correct for its effect inobservations of extended sources is presented. Our map of the polarizedsynchrotron emission from the Crab nebula is in agreement with structuresobserved at radio and optical wavelengths. The linear polarization measured inOMC1 agrees with results from previous studies, while the high sensitivity ofLABOCA enables us to also map the polarized emission of the Orion Bar, aprototypical photon-dominated region.