The spectral analysis and study of GRB 120709A, a burst with three distinct emission episodes

Abstract

The reason for quiescent intervals and isolated emission episodes in gamma-ray burst (GRB) light curves is still not well understood. Probing these questions further requires the study of the spectral characteristics and properties of the outflow of such GRBs. GRB 120709A is a burst with three distinct emission episodes, separated by quiescent periods lasting several seconds. The episodes are comparable with each other in terms of peak flux and duration. The GRB was observed by the Fermi LAT and GBM instruments, covering over six orders of magnitude of energy. In this work, we carry out the time-integrated and time-resolved spectral analysis of GRB 120709A. We use these results to study the temporal variation of its parameters. In particular, we find that the peak energy of the spectrum of each episode traces the flux of those episodes. We also study and parameterise the correlation between the isotropic luminosity and intrinsic peak energy. Finally, we determine the parameters of the outflow based on gamma-gamma opacity calculations. We find that if we require the MeV emission to occur above the photosphere, then the ratio of the GeV and MeV emission radii can not be greater than ∼1.2. This implies an internal origin of the GeV emission. This also allows us to estimate the average value of the Lorentz factor to be in the ∼100–200 range. We also evaluate the photospheric and MeV emission radii and find them to be in the (4.5–5.3) ×1012 cm and (2.1–5.2) ×1014 cm ranges, respectively. We note the limitations of this exercise due to the relative faintness of the GRB which prevents us from doing a very fine time-resolved analysis or fitting the spectra with multi-component models. We also discuss GRB 120709A in the context of models for distinct emission episodes and quiescent intervals.