Bayesian evidence for flux scale errors in Galactic synchrotron maps

TL;DR

This study uses Bayesian analysis to identify significant flux scale errors in the 408 MHz Haslam map, revealing potential overestimation of brightness and questioning its reliability as a reference for Galactic synchrotron emission.

Contribution

It provides the first Bayesian evidence for flux scale errors in the Haslam map and assesses the impact of spectral curvature models on synchrotron emission analysis.

Findings

Potential 60% flux overestimation in Haslam map

Models with spectral curvature are statistically disfavored

Flux scale errors may vary across the sky

Abstract

The 408 MHz Haslam map is widely used as a low-frequency anchor for the intensity and morphology of Galactic synchrotron emission. Multi-frequency, multi-experiment fits show evidence of spatial variation and curvature in the synchrotron frequency spectrum, but there are also poorly-understood multiplicative flux scale disagreements between experiments. We perform a Bayesian model comparison across a range of scenarios, using fits that include recent spectroscopic observations at $\sim 1$ GHz by MeerKAT as well as a reference map from the OVRO-LWA at 73 MHz. In the few square degrees that we analyzed, a large uncorrected flux scale factor potentially as large as 1.6 in the Haslam data is preferred, indicating a 60\% overestimation of the brightness. This partly undermines its use as a reference map. We also find that models with nonzero spectral curvature are statistically disfavored. Given the limited sky coverage here, we suggest a similar analysis across many more regions of the sky to determine the extent and variation of flux scale errors, and whether they should be treated as random or systematic errors in analyses that use the Haslam map as a template.