Polarimetry at millimeter wavelengths with the NIKA camera: calibration and performance

Abstract

Magnetic fields, which play a major role in a large number of astrophysical processes can be traced via observations of dust polarization. In particular, Planck low-resolution observations of dust polarization have demonstrated that Galactic filamentary structures, where star formation takes place, are associated to well organized magnetic fields. A better understanding of this process requires detailed observations of galactic dust polarization on scales of 0.01 to 0.1 pc. Such high-resolution polarization observations can be carried out at the IRAM 30 m telescope using the recently installed NIKA2 camera, which features two frequency bands at 260 and 150 GHz (respectively 1.15 and 2.05 mm), the 260 GHz band being polarization sensitive. NIKA2 so far in commissioning phase, has its focal plane filled with ∼3300 detectors to cover a Field of View (FoV) of 6.5 arcmin diameter. The NIKA camera, which consisted of two arrays of 132 and 224 Lumped Element Kinetic Inductance Detectors (LEKIDs) and a FWHM (Full-Width-Half-Maximum) of 12 and 18.2 arcsec at 1.15 and 2.05 mm respectively, has been operated at the IRAM 30 m telescope from 2012 to 2015 as a test-bench for NIKA2. NIKA was equipped of a room temperature polarization system (a half wave plate (HWP) and a grid polarizer facing the NIKA cryostat window). The fast and continuous rotation of the HWP permits the quasi simultaneous reconstruction of the three Stokes parameters, I, Q, and U at 150 and 260 GHz. This paper presents the first polarization measurements with KIDs and reports the polarization performance of the NIKA camera and the pertinence of the choice of the polarization setup in the perspective of NIKA2. We describe the polarized data reduction pipeline, specifically developed for this project and how the continuous rotation of the HWP permits to shift the polarized signal far from any low frequency noise. We also present the dedicated algorithm developed to correct systematic leakage effects. We report results on compact and extended sources obtained during the February 2015 technical campaign. These results demonstrate a good understanding of polarization systematics and state-of-the-art performance in terms of photometry, polarization degree and polarization angle reconstruction.

Observing with NIKA2Pol from the IRAM 30m telescope : Early results on the commissioning phase

“…The most important one corresponds to a parasitic signal at all harmonics of the HWP rotation frequency ω. This systematic effect, which is common to all the pixels of the KID array, is called HWP Synchronous Signal (HWPSS) and it is corrected in the data analysis [10]. Furthermore, we also observe in the NIKA2 polarization data intensity to polarization leakage, which constitutes to date the most critical systematic effect.…”

“…From observations of Uranus, which can be considered as unpolarized, we have estimated a maximum fractional instrumental polarization varying from 0.45 % to 1.84 % in Stokes U and from 1.27 % to 2.16 % in Stokes Q [12]. A similar effect was observed for the NIKA camera and we developed a procedure to correct for it [10]. Although the intensity to polarization leakage in NIKA2 is about a factor 2 lower than in NIKA, it can not be corrected for using the NIKA procedure because it varies more significantly with elevation.…”

“…The rotation of the HWP modulates the incident polarization. As a consequence, the polarization signal is shifted at the fourth harmonic of the HWP rotation frequency ω and can be recovered by using a demodulation technique, which consists of a numerical lock-in around the 4ω [10] frequency. This detection strategy allows a quasi-simultaneous observation of the three Stokes parameters I, Q, and U although HWP-induced systematic effects are observed and need to be subtracted.…”

“…Work is in progress to account for this effect and it is the priority of the forthcoming commissioning campaigns. In this paper, we concentrate on extended sources for which the intensity to polarization leakage effect is reduced when averaging across the source (see [10,11]).…”

Probing changes of dust properties along a chain of solar-type prestellar and protostellar cores in Taurus with NIKA

“…Now the advent of the NIKA2 instrument on the IRAM 30 m telescope (Calvo et al 2016) will give us the chance to enlarge the observed sample of objects at early stages of star formation and characterize their physical properties at long wavelengths. Thanks to its polarization-sensitive capability at 1.2 mm (see Ritacco et al 2017), the NIKA2 camera will also allow us to gain insight into the magnetic field structure of star-forming regions, especially at the scales where filaments fragment into prestellar cores, providing the community with an exceptional opportunity to enlighten one of the most important but yet poorly explored aspects of the star formation process.…”