Correlation of flux and persistent spectral parameters for 4U 1636–536. On the y-axis, σ is the significance of hard X-ray shortage, kT_e the electron temperature, and Γ the photon index; on the x-axis Flux is the persistent emission flux of 40–70 keV. Left panel: points corresponding to the pre-burst continuum spectrum of the observation in which each burst of 4U 1636–536 is located. Right panel: bursts stacked in the hard state. The points (from left to right) correspond to the stacking of the bursts in the four regions 1, 2, 3, and 4 in Fig. 2. The spectral parameters are the result of the fit after combining the pre-burst continuum spectra of the observations where these bursts are located.