# Spectral Lag for a Radiating Jet Shell with a High Energy Cut-off Radiation Spectrum

## TL;DR

This paper investigates how spectral lag varies with photon energy in GRB jet shells with high-energy cut-offs, explaining observed behaviors including positive, negative lags, and reproducing the spectral lag in GRB~160625B.

## Contribution

It provides a theoretical model for spectral lag behavior in GRB jet shells with high-energy cut-offs, explaining observed lag patterns and their dependence on spectral shape.

## Key findings

- Spectral lag increases monotonically with photon energy for certain spectra.
- A turn-over from positive to negative lag occurs at high energies in Bandcut spectra.
- The model reproduces the spectral lag observed in GRB~160625B.

## Abstract

Recent research shows that the spectral lag is closely related to the spectral evolution in GRBs. In this paper, we study the spectral lag for a radiating jet shell with a high-energy cut-off radiation spectrum. For the jet shell with a cut-off power-law spectrum, the spectral lag monotonically increases with the photon energy and levels off at a certain photon energy. It is the same for the jet shell with a Band cut-off spectrum (Bandcut). However, a turn-over from the positive lags to negative lags appears in the high-energy range for the jet shell with Bandcut, which is very similar to that observed in GRB~160625B. The dependence of the spectral lags on the spectral shape/evolution are studied in details. In addition, the spectral lag behavior observed in GRB~160625B is naturally reproduced based on our theoretical outcome.