The Connection between Spectral Evolution and Gamma‐Ray Burst Lag

Abstract

The observed delay between the arrival times of high- and low-energy photons in gamma-ray bursts (GRBs) has been shown by Norris et al. to be correlated to the absolute luminosity of a burst. Despite the apparent importance of this distance indicator, there has yet to be a full explanation to its origin. Any attempt at explaining this relation must take into consideration that the observed lag is the direct result of spectral evolution. In particular, as the energy at which the GRB's νFν spectra is a maximum (Epk) decays through the four BATSE channels, the photon flux peak in each individual channel will inevitably be offset, producing what we measure as lag. In order to produce a robust relationship between the observed spectral evolution and spectral lag, we measure the rate of Epk decay (Φ0) for a sample of clean single-peaked bursts with measured lag. We use this data to provide an empirical relation that expresses the GRB lag as a function of the burst's spectral evolution rate. This implies that the luminosity of a GRB is directly related to the burst's rate of spectral evolution, which we believe begins to reveal the underlying physics behind the lag-luminosity correlation. We discuss several possible mechanisms that could cause the observed evolution and examine their connections to the burst's luminosity.