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The NIKA polarimeter on science targets: Crab nebula observations at 150 GHz and dual-band images of Orion Molecular Cloud OMC-1


  • Dr. Alessia RITACCO

Auteurs principaux


NIKA, the NIKA2 path nder, was equipped of a room temperature polarization system (a half wave plate and a grid polarizer facing the cryostat window). This polarization system was the test bench for the NIKA2 polarimeter and was used during few observational/technical campaigns between 2014 and 2015. In particular during the last observing week of NIKA at the IRAM 30m telescope, in February 2015, we could demonstrate the ability of such a technology in detecting the polarization of the sky. Among the different targets used to constrain the instrumental polarization and to verify the calibration of the polarization degree and angle, we observed the Crab nebula and Orion Molecular Cloud region OMC-1. The molecular cloud OMC-1 represents a typical astrophysical target for future NIKA2Pol observations, aiming at tracing magnetic elds structures in star forming regions. The comparison with sub-millimetre and mm instruments showed a good reconstruction of the polarization. Furthermore thanks to the dual-band capability of NIKA we could trace the spatial distribution of the spectral index which showed interesting results along the bar, in the southern part of the source. In polarization the main fi lament is detected and it shows a well ordered magnetic fi eld following the total intensity structure of the source. The Crab nebula is a supernova remnant exhibiting a highly polarized synchrotron radiation at radio and millimeter wavelengths. It is the brightest source in the microwave sky with an extension of 7 by 5 arcminutes and commonly used as a standard candle for any experiment which aims at measuring the polarization of the sky. Though its spectral energy distribution has been well characterized in total intensity, polarization data are still lacking at millimetre wavelengths. Using the high resolution observation at 150 GHz and the observations of CMB satellites, WMAP and Planck, we could trace for the first time the Spectral Energy Distribution in polarization in the frequency range: 30-353 GHz. The analysis highlights a single population of electrons responsible for both the total power and polarization radiation. Moreover averaging across the source by using aperture photometry techniques we fi nd that the Crab nebula polarization angle is consistent with being constant over a wide range of frequencies with a value of -87.3+-0.3 in Galactic coordinates. This analysis provides a very good calibration for the next generation of ground-based instruments and space missions that aim at measuring the polarization radiation at these wavelengths. This contribution aims at giving an overview on the most interesting results obtained with the NIKA Polarimeter. The measurements performed with NIKA, together with the developed techniques to deal with systematics, opened the way to the current observations with NIKA2 that will provide important advances in the fi eld of galactic and extra-galactic emission and interactions with the magnetic field.