A New Measurement of the Spectral Lag of Gamma-Ray Bursts and its Implications for Spectral Evolution Behaviors

TL;DR

This study systematically measures spectral lag in 50 gamma-ray bursts, revealing power-law energy dependencies that support relativistic geometric effects and linking spectral lag to pulse duration and spectral evolution behaviors.

Contribution

It introduces a new energy-independent measurement method for spectral lag and analyzes its relation to spectral evolution in GRB pulses.

Findings

Spectral lag follows a power-law energy dependence.

Shorter pulses tend to have negligible spectral lag and HIT behavior.

Longer pulses tend to have significant spectral lag and HTS behavior.

Abstract

We carry out a systematical study of the spectral lag properties of 50 single-pulsed Gamma-Ray Bursts (GRBs) detected by Fermi/GBM. By dividing the light curves into multiple consecutive energy channels we provide a new measurement of the spectral lag which is independent on energy channel selections. We perform a detailed statistical study of our new measurements. We find two similar power-law energy dependencies of both the pulse arrival time and pulse width. Our new results on the power-law indices would favor the relativistic geometric effects for the origin of spectral lag. However, a complete theoretical framework that can fully account for the diverse energy dependencies of both arrival time and pulse width revealed in this work is still missing. We also study the spectral evolution behaviors of the GRB pulses. We find that the GRB pulse with negligible spectral lag would usually have a shorter pulse duration and would appear to have a "hardness-intensity tracking" (HIT) behavior and the GRB pulse with a significant spectral lag would usually have a longer pulse duration and would appear to have a "hard-to-soft" (HTS) behavior.