Measuring the pulse of GRB 090618: A Simultaneous Spectral and Timing Analysis of the Prompt Emission

TL;DR

This paper introduces a new method for simultaneous spectral and timing analysis of GRB prompt emission, applied to GRB 090618, revealing correlations useful for standard candle applications.

Contribution

A novel approach for combined spectral and timing analysis of GRB pulses, applied to a bright GRB to improve understanding of pulse properties and their cosmological implications.

Findings

Pulse width increases at low energies for some pulses.

Model parameters correlate with luminosity indicators.

Spectral and timing properties are consistent with empirical relations.

Abstract

We develop a new method for simultaneous timing and spectral studies of Gamma Ray Burst (GRB) prompt emission and apply it to make a pulse-wise description of the prompt emission of GRB 090618, the brightest GRB detected in the Fermi era. We exploit the large area (and sensitivity) of Swift/BAT and the wide band width of Fermi/GBM to derive the parameters for a complete spectral and timing description of the individual pulses of this GRB, based on the various empirical relations suggested in the literature. We demonstrate that this empirical model correctly describes the other observed properties of the burst like the variation of the lag with energy and the pulse width with energy. The measurements also show an indication of an increase in pulse width as a function of energy at low energies for some of the pulses, which is naturally explained as an off-shoot of some particular combination of the model parameters. We argue that these model parameters, particularly the peak energy at the beginning of the pulse, are the natural choices to be used for correlation with luminosity. The implications of these results for the use of GRBs as standard candles are briefly described.