Characterization of the polarized synchrotron emission from Planck and WMAP data

Abstract

The purpose of this work is to characterize the diffuse Galactic polarized synchrotron, which is the dominant CMB foreground emission at low frequency. We present EE, BB, and EB power spectra estimated from polarization frequency maps at 23 and 30 GHz as observed respectively by the WMAP K-band and the Planck lowest frequency channel, for a set of six sky regions covering from 30% to 94% of the sky. We study the synchrotron polarization angular distribution and spectral energy distribution (SED) by means of the so-called pseudo-$C_{\ell }$ formalism, provided by the NaMaster package, in the multipole interval 30 $\le$ $\ell$ $\le$ 300. Best results are obtained cross-correlating Planck and WMAP data. The EE and BB angular power spectra show a steep decay of the spectral amplitude as a function of multipole, approximated by a power law $C^{EE,BB}\propto {\ell }^{{\alpha }_{EE,BB}}$, with ${\alpha }_{EE}=-2.95\pm 0.04$ and ${\alpha }_{BB}=-2.85\pm 0.14$. The B/E power asymmetry is proved with a B-to-E ratio, computed as the amplitude ratio at the pivot multipole $\ell =80$, of 0.22$\pm$0.02. The EB cross-component is compatible with zero at 1$\sigma$, with an upper constraint on the EB/EE ratio of 1.2% at the 2$\sigma$ level. We show that the EE and BB power-law model with null EB cross-correlation describes reasonably well the diffuse synchrotron polarization emission for the full sky if the bright Galactic center and point sources are masked. The recovered SED shows power-law spectral indices ${\beta }_{EE}=-3.00\pm 0.10$ and ${\beta }_{BB}=-3.05\pm 0.36$ compatible between themselves, in the frequency range 23-30 GHz. Results also seem to indicate that the SED gets steeper from low to high Galactic latitude.

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