M1 PS spectral index posterior means. We note that the spectral indices have an offset of +1 introduced by the logarithmic binning of the energy dimension (see Eq. (4)). Left: map of spectral indices recovered for the PS pixels. The color scale is centered on the prior mean, −1.25, and ranges from two prior standard deviations above to two standard deviations below the prior mean. Right: histogram of posterior PS pixel spectral index means for switched on (blue) and switched off PS pixels (orange). The x-axis is centered on the prior mean, with the ticks indicating prior standard deviation sized steps from the prior mean. The y-axis shows counts on a logarithmic scale.