Spectral Evolution Responsible for the Transition from Positive Lags to Negative Lags in Gamma-Ray Bursts

Abstract

It is well known that most gamma-ray bursts (GRBs) are dominated by positive spectral lags, while a small fraction of GRBs show negative lags. However, Wei et al. first identified a well-defined transition from positive lags to negative lags in GRB 160625B, and then got robust limits on a possible violation of Lorentz invariance (LIV) based on the observation. Recently, such a transition has been found in three different emission episodes in GRB 190530A by Gunapati et al., which provides us a great opportunity to investigate whether the transition results from LIV-induced observed spectral lags. Our analysis shows that the LIV model cannot be compatible with the current observations, whereas, only the spectral evolution induced spectral lags could responsible for the transition. So, spectral evolution can also explain the positive to negative lag in GRB 190530A.