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

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/Burst Alert Telescope and the wide bandwidth of Fermi/Gamma-ray Burst Monitor 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, such as the variation of the lag with energy and the pulse width with energy. The measurements also show an indication of an increase in the 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.