Bayesian evidence for uncorrected gain factors in Galactic synchrotron template maps

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 gain factors 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. A large uncorrected gain factor of about 60\% in the Haslam data is strongly preferred, partly undermining its use as a reference template.