Dependence of the power-law index α on the burst flux for all bursts from 3A 1820–303 (all of these bursts are hard-state PRE bursts). The data are shown in a two-dimensional histogram, where the colour of a bin shows the frequency of occurrences (normalised to the peak value) according to the colour bar on the right. The solid blue line is the average of the data and the blue dashed lines show the 1σ limits. The solid green line shows the one-zone model with a column depth of 10⁸ g cm^-2, and the cyan, magenta, and yellow lines show the three multi-zone models with ignition depths of 10⁸, 10⁹, and 10¹⁰ g cm^-2, respectively, based on the work of Cumming & Macbeth (2004).