2025ApJ...991..230G

The prompt phase X-ray and gamma-ray light curves of gamma-ray bursts (GRBs) exhibit erratic and complex behaviors, often with multiple pulses. The temporal shape of the individual pulses is often modeled as “fast rise exponential decay” (FRED). Here, we introduce a novel fitting function to quantify the pulse asymmetry. We conduct a light-curve and a time-resolved spectral analysis on 61 pulses from 22 GRBs detected by the Fermi Gamma-ray Burst Monitor. Contrary to previous claims, we find that only ∼50% of the pulse light curves in our sample show a FRED shape, while about 25% have a symmetric light curve, with the other 25% having a mixed shape. Furthermore, our analysis reveals a clear trend: in multipulse bursts, the initial pulse tends to exhibit the most symmetric light curve, while subsequent pulses become increasingly asymmetric, adopting a more FRED-like shape. Additionally, we correlate the temporal and spectral shapes of the pulses. By fitting the spectra with the classical “Band” function, we find a moderate positive Spearman's correlation index of 0.23 between the pulse asymmetry and the low-energy spectral index ${\alpha }_{\max }$ (the maximum value across all time bins covering an individual pulse). Thus, during GRB light curves, the pulses tend to get more asymmetric and spectrally softer with time. We interpret this as a transition in the dominant emission mechanism, from photospheric (symmetric-like and hard) to nonthermal emission above the photosphere, and show that this interpretation aligns with a GRB jet Lorentz factor of the order of a few tens in many cases.