2025MNRAS.544.2419N

The Haslam 408 MHz all-sky map is widely used as a template to model the diffuse Galactic synchrotron emission at radio and microwave frequencies. Recent studies have suggested that there are large uncorrected flux scale errors in this map, however. We investigate the possibility of statistically recalibrating the Haslam map using absolutely calibrated (but low angular resolution) radio experiments designed to measure the 21cm global signal at low frequencies. We construct a Gibbs sampling scheme to recover the full joint posterior distribution of ∼50 000 parameters, representing the true sky brightness temperature field, as-yet uncorrected flux scale factors, and synchrotron power-law spectral indices. With the idealized full-sky simulated data, we perform a joint analysis of a $1^\circ$ resolution diffuse map at 408 MHz and multiband 21cm global signal data with $30^\circ$ resolution under different assumptions about (1) noise levels in the maps, (2) sky coverage, and (3) synchrotron spectral index information. For our fiducial scenario in which the global signal experiment has a 50 mK noise rms per coarse pixel in each of 20 frequency bins between 50 and 150 MHz - the typical range for a global signal experiment, we find that the notional Haslam flux scale factors can be recovered in most (but not all) sub-regions of the sky to an accuracy of $\pm 2~{{\ \rm per\ cent}}$. In all cases we are able to rectify the sky map to within ∼5 K of the true brightness temperature. Our method can be used to correct the Haslam map once maps obtained from global experiments are available.