Spectral Lags and the Lag-Luminosity Relation: An Investigation with Swift BAT Gamma-ray Bursts

TL;DR

This study systematically analyzes spectral lags in 31 Swift GRBs to investigate their correlation with peak luminosity, confirming a significant but scattered lag-luminosity relation across multiple energy bands.

Contribution

It provides the largest Swift-based analysis of spectral lag and its correlation with luminosity, using multiple energy bands and redshift corrections.

Findings

Spectral lag correlates with peak luminosity with a mean correlation coefficient of -0.68.

The power-law index of the lag-luminosity relation is approximately 1.4.

The correlation shows large scatter but is statistically significant.

Abstract

Spectral lag, the time difference between the arrival of high-energy and low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et al. 2000 reported a correlation between the spectral lag and the isotropic peak luminosity of GRBs based on a limited sample. More recently, a number of authors have provided further support for this correlation using arbitrary energy bands of various instruments. In this paper we report on a systematic extraction of spectral lags based on the largest Swift sample to date of 31 GRBs with measured redshifts. We extracted the spectral lags for all combinations of the standard Swift hard x-ray energy bands: 15-25 keV, 25-50 keV, 50-100 keV and 100-200 keV and plotted the time dilation corrected lag as a function of isotropic peak luminosity. The mean value of the correlation coefficient for various channel combinations is -0.68 with a chance probability of ~ 0.7 x 10^{-3}. In addition, the mean value of the power-law index is 1.4 +/- 0.3. Hence, our study lends support for the existence of a lag-luminosity correlation, albeit with large scatter.