Measuring and calibrating Galactic synchrotron emission

TL;DR

This paper discusses the challenges and methods for accurately measuring and calibrating the large-scale Galactic synchrotron emission, emphasizing the importance of zero-level calibration for interpreting polarization data.

Contribution

It introduces improved calibration techniques for all-sky surveys of Galactic synchrotron emission, crucial for accurate polarization analysis and understanding Galactic magnetic fields.

Findings

Identified the importance of zero-level calibration for polarization data.

Compared different all-sky survey datasets at various frequencies.

Highlighted the impact of calibration on interpreting polarization structures.

Abstract

Our position inside the Galaxy requires all-sky surveys to reveal its large-scale properties. The zero-level calibration of all-sky surveys differs from standard 'relative' measurements, where a source is measured in respect to its surroundings. All-sky surveys aim to include emission structures of all angular scales exceeding their angular resolution including isotropic emission components. Synchrotron radiation is the dominating emission process in the Galaxy up to frequencies of a few GHz, where numerous ground based surveys of the total intensity up to 1.4 GHz exist. Its polarization properties were just recently mapped for the entire sky at 1.4 GHz. All-sky total intensity and linear polarization maps from WMAP for frequencies of 23 GHz and higher became available and complement existing sky maps. Galactic plane surveys have higher angular resolution using large single-dish or synthesis telescopes. Polarized diffuse emission shows structures with no relation to total intensity emission resulting from Faraday rotation effects in the interstellar medium. The interpretation of these polarization structures critically depends on a correct setting of the absolute zero-level in Stokes U and Q.