Rest frame duration vs. occurrence time for XRFs. The right-hand vertical axis shows the values of α_Γ (rescaled by 1) corresponding to the values of Δt^rest through Eq. (10). Different symbols and colours refer to different sources found in the literature for these parameters. 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 red solid line shows the expected relation between the flare width and the time of the flare in the IS shock model (it is obtained by adopting Δ T = 30 s and Δ R = 7 × 10¹⁰ cm).