Time lags as a proxy of the spectral evolution in gamma-ray bursts

Context. Positive lags in gamma-ray bursts (GRBs) provide a unique window into the temporal evolution of their prompt emission, where hard photons precede softer ones. Negative lags, when hard photons are delayed, are instead harder to interpret. It is therefore critical to separate the different effects that produce the two types of lags for identifying the physical mechanisms at work in the prompt and early afterglow phases of GRBs. Aims. We investigate the potential of time lags for distinguishing different emission components at different energy bands. Considering data from the Fermi Gamma-ray Burst Monitor (GBM) and the LAT Low Energy (LLE) technique, we establish a connection between lag behavior and high-energy spectral properties. Methods. We performed a time-resolved joint spectral analysis in the range 10 keV–100 MeV for two exceptionally bright bursts, GRB 160625B and GRB 190114C. We computed the time lags between the lowest-energy band (10–100 keV) and progressively higher-energy bands up to 30–100 MeV for their distinct emission episodes by means of the cross-correlation function. Results. For GRB 160625B, the spectra are described by a single component with clear hard-to-soft evolution, and the time lags are always positive. The analysis of the high-energy exponential cutoff, likely originating above the photosphere, yielded bulk Lorentz factor estimates of Γ ∼ 120 − 250. GRB 190114C exhibits negative lags in the 30–100 MeV band, coinciding with a delayed additional high-energy power-law component that begins to dominate the LLE range after ∼2.5 s. The comparison with multiwavelength observations showed some compatibility with the early afterglow, although its physical origin remains an open question that leaves room for interpretations such as external shocks or internal dissipation. Conclusions. Time lags are effective diagnostic tools for investigating the spectral evolution of GRBs. Positive lags trace the softening of the prompt emission, whereas negative lags, although more difficult to interpret, indicate the appearance of a new, independent high-energy spectral component.