GRB 090618: different pulse temporal and spectral characteristics within a burst

TL;DR

This study analyzes the temporal and spectral differences between pulses within GRB 090618, revealing distinct behaviors in pulse width, asymmetry, spectral evolution, and lag, indicating complex emission mechanisms.

Contribution

It provides a detailed pulse-by-pulse analysis of GRB 090618, highlighting differences in properties between early and later emission episodes, which was not previously characterized.

Findings

Pulse width and asymmetry are energy-dependent.

Pulse A exhibits hard-to-soft spectral evolution, unlike pulses in episode B.

Different pulses show varying lag and symmetry behaviors.

Abstract

GRB 090618 was simultaneously detected by Swift-BAT and Fermi-GBM. Its light curve shows two emission episodes consisting of four prominent pulses. The pulse in the first episode (episode A) has a smoother morphology than the three pulses in the second episode (episode B). Using the pulse peak-fit method, we have performed a detailed analysis of the temporal and spectral characteristics of these four pulses and found out that the first pulse (pulse A) exhibits distinctly different properties than the others in episode B (pulses B1, B2 and B3) in the following aspects. (i) Both the pulse width ($w$) and the rise-to-decay ratio of pulse ($r/d$, pulse asymmetry) in GRB 090618 are found to be energy-dependent. The indices of the power-law correlation between $w$ and $E$ for the pulses in episode B however are larger than that in episode A. Moreover the pulses B1, B2 and B3 tend to be more symmetric at the higher energy bands while the pulse A displays a reverse trend. (ii) Pulse A shows a hard-to-soft spectral evolution pattern, while the three pulses in the episode B follow the light curve trend. (iii) Pulse A has a longer lag than the pulses B1, B2 and B3. The mechanism which causes the different pulse characteristics within one single GRB is unclear.