Mitigating point-source contamination in CMB polarization: a Generalized Point Spread Function fitting approach

TL;DR

The paper introduces GPSF, a new method for removing point-source contamination in CMB polarization maps, improving the accuracy of primordial gravitational wave measurements.

Contribution

GPSF models overlapping sources using the full pixel covariance, providing more accurate flux estimates and reducing bias in tensor-to-scalar ratio measurements.

Findings

GPSF reduces point-source bias from 1.67e-3 to 2.9e-4

GPSF maintains low impact on background signal and variance

Outperforms standard masking and inpainting methods

Abstract

Observations of Cosmic Microwave Background (CMB) B-mode polarization provide a way to probe primordial gravitational waves and test inflationary predictions. Extragalactic point sources become a major source of contamination after foreground cleaning and can bias estimates of the tensor-to-scalar ratio $r$ at the $10^{-3}$ level. We introduce Generalized Point Spread Function Fitting (GPSF), a method for removing point-source contamination in polarization maps. GPSF uses the full pixel-domain covariance, including off-diagonal terms, and models overlapping sources. This allows accurate flux estimation under realistic conditions, particularly for small-aperture telescopes with large beams that are more susceptible to source blending. We test GPSF on simulated sky maps, apply foreground cleaning using the Needlet Internal Linear Combination (NILC) method, and compare its performance with standard masking and inpainting. The results show GPSF reduces point-source contamination without significantly affecting the background signal, as seen in both the maps and their power spectra. For the constraint on $r$, GPSF reduces the bias from $1.67 \times 10^{-3}$ to $2.9 \times 10^{-4}$, with only a 2% increase in standard deviation. Compared to inpainting and masking, GPSF yields lower bias while maintaining comparable variance. This suggests that it may serve as a promising method for future CMB experiments targeting measurements of $r \sim 10^{-3}$.