Testing for spectral index variations in polarised CMB foregrounds

Abstract

We present a Bayesian parametric component separation method for polarised microwave sky maps. We solve jointly for the primary cosmic microwave background (CMB) signal and the main Galactic polarised foreground components. For the latter, we consider electron-synchrotron radiation and thermal dust emission, modelled in frequency as a power law and a modified blackbody respectively. We account for inter-pixel correlations in the noise covariance matrices of the input maps and introduce a spatial correlation length in the prior matrices for the spectral indices beta. We apply our method to low-resolution polarised Planck 2018 Low and High Frequency Instrument (LFI/HFI) data, including the SRoll2 re-processing of HFI data. We find evidence for spatial variation of the synchrotron spectral index, and no evidence for depolarisation of dust. Using the HFI SRoll2 maps, and applying wide priors on the spectral indices, we find a mean polarised synchrotron spectral index over the unmasked sky of beta-sync = -2.833 +- 0.620. For polarised dust emission, we obtain beta-dust = 1.429 +- 0.236. Our method returns correlated uncertainties for all components of the sky model. Using our recovered CMB maps and associated uncertainties, we constrain the optical depth to reionization, tau, using a cross-spectrum-based likelihood-approximation scheme (momento) to be tau = 0.0598 +- 0.0059. We confirm our findings using a pixel-based likelihood (pixlike). In both cases, we obtain a result that is consistent with, albeit a fraction of a sigma higher than, that found by subtracting spatially uniform foreground templates. While the latter method is sufficient for current polarisation data from Planck, next-generation space-borne CMB experiments will need more powerful schemes such as the one presented here.

12 pages, 11 figures

References in corpus (14)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data
• Three Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis
• Planck intermediate results. LVII. Joint Planck LFI and HFI data processing
• CMB-S4: Forecasting Constraints on Primordial Gravitational Waves
• Planck intermediate results. XXII. Frequency dependence of thermal emission from Galactic dust in intensity and polarization
• Maximum Likelihood algorithm for parametric component separation in CMB experiments
• Polarised radio filaments outside the Galactic plane
• Spatial variations in the spectral index of polarized synchrotron emission in the 9 yr WMAP sky maps
• Planck intermediate results. L. Evidence for spatial variation of the polarized thermal dust spectral energy distribution and implications for CMB $B$-mode analysis
• Inference of the optical depth to reionization from low multipole temperature and polarisation Planck data
• Evidence for Line-of-Sight Frequency Decorrelation of Polarized Dust Emission in $Planck$ Data
• Characterization of the polarized synchrotron emission from Planck and WMAP data
• Spectral indices of Galactic radio loops between 1420, 820 and 408 MHz
• Improved large scales interstellar dust foreground model and CMB solar dipole measurement

Cited by in corpus (5)

• QUIJOTE scientific results -- IV. A northern sky survey in intensity and polarization at 10-20GHz with the Multi-Frequency Instrument
• Dispu$\tau$able: the high cost of a low optical depth
• Morphological Analysis of the Polarized Synchrotron Emission with WMAP and Planck
• Impact of Galactic dust non-Gaussianity on searches for B-modes from inflation
• Cosmoglobe DR1. III. First full-sky model of polarized synchrotron emission from all WMAP and Planck LFI data