[Paper Review] NIKA 150 GHz polarization observations of the Crab nebula and its spectral energy distribution

A. Ritacco, J. F. Macías–Pérez+46 more|arXiv (Cornell University)|Apr 25, 2018

Astrophysics and Cosmic PhenomenaPhysics and Astronomy47 references27 citations

TL;DR

This paper presents the first high-resolution (18′′ FWHM) 150 GHz polarization observations of the Crab nebula using the NIKA camera on the IRAM 30 m telescope. It reconstructs the spatial distribution of polarization degree and angle, confirming a nearly constant polarization angle of −87.7°±0.3° across 20–353 GHz and finding a polarization spectral index βpol = −0.347 ± 0.026, consistent with the total intensity spectral index β = −0.323 ± 0.001, confirming synchrotron emission as the dominant mechanism.

ABSTRACT

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. We report in this paper high resolution (18 arcsec FWHM) observations of the Crab nebula in total intensity and linear polarization at 150 GHz with the NIKA camera. NIKA, operated at the IRAM 30 m telescope from 2012 to 2015, is a camera made of Lumped Element Kinetic Inductance Detectors (LEKIDs) observing the sky at 150 and 260 GHz. From these observations we are able to reconstruct the spatial distribution of the polarization degree and angle of the Crab nebula, which is found to be compatible with previous observations at lower and higher frequencies. Averaging across the source and using other existing data sets we find that the Crab nebula polarization angle is consistent with being constant over a wide range of frequencies with a value of -87.7$^\circ$ +- 0.3 in Galactic coordinates. We also present the first estimation of the Crab nebula spectral energy distribution polarized flux in a wide frequency range: 30-353 GHz. Assuming a single power law emission model we find that the polarization spectral index $\beta_{pol}$ = - 0.347 +- 0.026 is compatible with the intensity spectral index $\beta$ = - 0.323 +- 0.001.

Motivation & Objective

• To obtain high angular resolution (18′′ FWHM) polarization observations of the Crab nebula at 150 GHz using the NIKA camera.
• To reconstruct the spatial distribution of polarization degree and angle across the nebula at millimeter wavelengths.
• To characterize the spectral energy distribution (SED) of both total intensity and polarized emission from the Crab nebula over a wide frequency range (30–353 GHz).
• To test the consistency of the Crab nebula's polarization angle and spectral index across frequencies, particularly for use as a CMB calibration standard.
• To assess discrepancies between existing data sets and improve the accuracy of polarization calibration for future CMB experiments.

Proposed method

• Observations were conducted with the NIKA camera, using Lumped Element Kinetic Inductance Detectors (LEKIDs), at 150 GHz on the IRAM 30 m telescope.
• Stokes I, Q, and U maps were produced to derive polarization degree and angle across the nebula.
• Data were combined with existing polarization measurements from WMAP (23–94 GHz), XPOL (90 GHz), Planck (30–353 GHz), POLKA (345 GHz), and other instruments.
• A single power law model was fitted to the SED of total intensity and polarized flux using χ² minimization to determine spectral indices β and βpol.
• Systematic uncertainties, including absolute calibration errors and source fading over time, were accounted for in the final flux and spectral index estimates.
• The polarization angle was analyzed across frequency bands to assess its constancy, with Galactic coordinates used as reference.

Experimental results

Research questions

• RQ1 Is the polarization angle of the Crab nebula constant across the 20–353 GHz frequency range, as required for its use as a CMB calibration standard?
• RQ2 What is the spectral energy distribution (SED) of the polarized emission from the Crab nebula at millimeter and submillimeter wavelengths?
• RQ3 How do the polarization spectral index βpol and total intensity spectral index β compare, and is their consistency consistent with synchrotron emission?
• RQ4 Are there spatial variations in polarization degree and angle across the nebula at 150 GHz, and how do they compare to lower or higher frequency observations?
• RQ5 What are the systematic uncertainties in polarization measurements at high angular resolution, and how do they affect calibration accuracy?

Key findings

• The polarization angle of the Crab nebula is consistent with being constant across the 20–353 GHz range, with a value of −87.7°±0.3° in Galactic coordinates.
• The polarization degree is found to be constant at 6.95%±0.03% across the observed frequency range, indicating stable emission properties.
• The polarization spectral index is measured as βpol = −0.347 ± 0.026, which is statistically consistent with the total intensity spectral index β = −0.323 ± 0.001.
• The NIKA 150 GHz data are consistent with the best-fit single power law model for the SED of total intensity and polarized flux, with a total flux density of 1010.2 ± 3.8 Jy at 150 GHz.
• High-resolution observations from NIKA (150 GHz) and POLKA (345 GHz) show polarization angles slightly lower than the average by ∼3° and ∼5°, respectively, though uncertainties are high due to systematics.
• The results confirm that the Crab nebula's emission is consistent with a single population of relativistic electrons producing synchrotron radiation, supporting its use as a standard calibrator for CMB polarization experiments.