Isotropic energy (computed in the XRT energy range) of XRF versus their rest frame peak time. The right-hand vertical axis shows the values of α_Γ (rescaled by 1) corresponding to the values of E_iso through Eq. (2). Different symbols and colours refer to different samples collected from the literature. Orange points, green asterisks, and cyan crosses are associated with XRF (only those with a measured redshift) from Yi et al. (2016), Bernardini et al. (2011), and Chincarini et al. (2010), respectively. The solid (dashed) red line shows the behaviour of the isotropic energy vs. flare peak time predicted considering two shells emitted with a delay Δ T = 30 (1000) s with respect to the start of the prompt emission and with a separation Δ R = 7 × 10¹⁰ (7 × 10¹²) cm.