Evolution of spectral parameters for GRB 150309A. Top panel: Peak energy evolving with time and following an intensity-tracking trend during the first episode. Middle panel: Evolution of low energy spectral index α_pt using Fermi GBM data. During the first episode, α_pt seems to follow a hard-to-soft trend. The two horizontal lines are lines of death for synchrotron fast cooling (α_pt = −3/2, dotted dashed line) and synchrotron slow cooling (α_pt = −2/3, solid red line). Bottom panel: Evolution of high-energy spectral index β_pt using Fermi GBM data.