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

Abstract

Positive lags in gamma-ray bursts (GRBs), where hard photons anticipate softer ones, provide a unique window into the temporal evolution of their prompt emission. Negative lags, when hard photons are delayed, are instead more enigmatic to interpret. Disentangling the effects that produce both kinds of lags is critical for identifying the physical mechanisms at work in the prompt and early afterglow phases of GRBs. We investigate the potential of time lags for distinguishing different emission components at different energy bands. Using data from the Fermi Gamma-ray Burst Monitor (GBM) and the LAT Low Energy(LLE) technique, we perform a time-resolved joint spectral analysis in the range 10 keV-100 MeV for two exceptionally bright bursts, GRB 160625B and GRB 190114C. Time lags between the lowest-energy band (10-100 keV) and progressively higher-energy bands up to 30-100 MeV were computed across their distinct emission episodes via the cross-correlation function. For GRB 160625B, the spectra are described by a single component with clear hard-to-soft evolution, and the time lags are always positive. Analysis of the high-energy exponential cutoff, likely originating above the photosphere, yields bulk Lorentz factor estimates of $\Gamma \sim 120-250$. GRB 190114C exhibits negative lags in the 30-100 MeV band, coinciding with a delayed high-energy powerlaw component that dominates the LLE range after ~2.5 s. Comparison with multi-wavelength observations shows some compatibility with the early afterglow, though its origin remains open, leaving room for external shocks or internal dissipation. Time lags are effective diagnostic tools for the spectral evolution of GRBs: positive lags trace the softening of the prompt emission, whereas negative lags indicate the appearance of a new, independent high-energy spectral component.