Fingerprint of Galactic Loop I on polarized microwave foregrounds

TL;DR

This study investigates the polarized microwave emission from Galactic Loop I, demonstrating it as a major local foreground component and providing insights into its E-mode dominance, which impacts primordial gravitational wave detection.

Contribution

The paper offers the first geometrical evidence linking Loop I polarization to local supernova activity and clarifies its role in microwave foregrounds affecting CMB B-mode studies.

Findings

Loop I's polarization is predominantly E-mode.

Galactic Loop I significantly contributes to microwave foregrounds.

Evidence suggests a local origin for a large part of the polarized emission.

Abstract

Context: Currently, detection of the primordial gravitational waves by the B-mode of Cosmic Microwave Background (CMB) is primarily limited by our knowledge of the polarized microwave foreground emissions. Thus improvements of the foreground analysis are necessary. As revealed in~\cite{2018arXiv180410382L}, the E-mode and B-mode of the polarized foreground have noticeable different properties, both in morphology and frequency spectrum, suggesting that they arise from different physical processes, and need to be studied separately. Aims: I will study the polarized emission from Galactic loops, especially Loop I, and mainly focus on the following issues: Does it contribute predominantly to the E-mode or B-mode? In which frequency bands and in which sky regions can it be identified? Methods: Based on a well known result about the magnetic field alignment in supernova explosions, a theoretical expectation is established that the loop polarizations should be predominantly E-mode. In particular, the expected polarization angles of Loop I are compared with those from the real microwave band data of WMAP and Planck. Results and conclusions: The comparison between model and data shows remarkable consistency between data and expectation at all bands and for a large area of the sky. This result suggests that the polarized emission of Galactic Loop I is a major polarized component in all microwave bands from 23 to 353 GHz, and a considerable part of the polarized foreground is likely originated from a local bubble associated with Loop I, instead of the far more distant Galactic emission. The result also provides a possible way to explain the reported E-to-B excess~\citep{2016A&A...586A.133P} by contribution of the loops. Finally, this work may also provide the first geometrical evidence that the Earth was hit by a supernova explosion.