NIKA 150 GHz polarization observations of the Crab nebula and its spectral energy distribution

Abstract

The Crab nebula is a supernova remnant exhibiting a highly polarized synchrotron radiation at radio and millimetre wavelengths. It is the brightest source in the microwave sky with an extension of 7 by 5 arcminutes, and is commonly used as a standard candle for any experiment which aims to measure 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 observations (18 FWHM) 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 • ± 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 β pol = -0.347 ± 0.026 is compatible with the intensity spectral index β= -0.323 ± 0.001.

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

“…The polarization intensity flux integrated over the extension of the Crab nebula is 12.3 ± 0.1 Jy (statistical uncertainties only). This value is consistent with the expectation from the polarization intensity SED given in [11]. For the Stokes I map we obtain a total flux of 158.04±0.18 Jy (statistical uncertainties only).…”

“…As a consequence, we expect the Crab nebula degree and polarization angle to be constant at radio and millimeter wavelengths. This has been shown by [11] using intensity and polarization observations of the Crab nebula from 23 to 217 GHz. However, other emission contributions are expected and they could be investigated using NIKA2 among other experiments.…”

“…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]).…”

“…For the NIKA2 commissioning, we concentrated on observations of the Crab Nebula, which were taken in December 2018. The Crab nebula is a supernova remnant that is generally considered as a good polarization angle calibrator for polarization experiments ( [11,13]). The spectral energy distribution in both intensity and polarization of the Crab nebula is well described by a single power law spectrum, as expected from synchrotron emission powered by a single population of relativistic electrons [11].…”

“…The Crab nebula is a supernova remnant that is generally considered as a good polarization angle calibrator for polarization experiments ( [11,13]). The spectral energy distribution in both intensity and polarization of the Crab nebula is well described by a single power law spectrum, as expected from synchrotron emission powered by a single population of relativistic electrons [11]. As a consequence, we expect the Crab nebula degree and polarization angle to be constant at radio and millimeter wavelengths.…”