QUIJOTE scientific results - XII. Characterization of the polarized synchrotron emission at the power spectrum level in the QUIJOTE-MFI wide survey

We present an analysis of the polarized synchrotron emission at the power-spectrum level in the multipole range $30 < \ell < 130$, using the recent QUIJOTE-MFI wide survey (10-20GHz) in combination with Planck and WMAP data. The EE and BB power spectra are well described by a simple power-law model, with no evidence for the need of additional parameters. The best-fit slopes are $α_{EE} = -3.24 \pm 0.12$ and $α_{BB} = -3.81 \pm 0.34$ for $|b|>20^\circ$, hinting toward a steepening of the BB spectrum relative to EE at the $\sim 1.6σ$ level. The synchrotron frequency spectral indices are found to be $β_s^{EE} = -3.26 \pm 0.05$ and $β_s^{BB} = -3.25 \pm 0.10$ for the same cut, in agreement with earlier real-space analyses. We find no evidence for spatial variations of $β_s$ across different Galactic latitude cuts, within current uncertainties. The synchrotron BB/EE power ratio is approximately 0.2 over the considered multipole range, lower than typical values for thermal dust, and consistent with previous measurements. We also measure a dust-synchrotron spatial correlation in the range 0.1-0.25 over $30 < \ell < 130$. Using these results, we estimate the level of synchrotron contamination in future CMB B-mode measurements, finding an average power in the considered sky masks equivalent to $r \sim 0.003$ at 90GHz and $r \sim 0.0002$ at 150GHz for the relevant angular scales.