Characteristics of Two-episode Emission Patterns in Fermi Long Gamma-Ray Bursts

Abstract

Two-episode emission components separated by quiescent gaps in the prompt emission of gamma-ray bursts (GRBs) have been observed in the Swift era, but there is a lack of spectral information due to the narrow energy band of the Swift/Burst Alert Telescope. In this paper, a systematic analysis of the spectral and temporal properties of the prompt emission of 101 Fermi/Gamma-ray Burst Monitor detected long GRBs show the existence of two-episode emission components in the light curves, with quiescent times of up to hundreds of seconds. We focus on investigating the differences of those two emission episodes. We find that the light curves of the two emission components exhibit different behavior, e.g., a soft emission component that either precedes or follows the main prompt emission or that the intensity of the two emission episodes are comparable with each other. No statistically significant correlation in the duration of the two emission episodes can be claimed. We define a new parameter ε as the ratio of the peak flux of the first and second emission episodes and find that a higher ε corresponds to a larger fluence. The preferred spectral model in our analysis is a cutoff power-law model for most GRBs. The distribution of E p for episodes I and II range from tens of keV to 1000 keV with a lognormal fit and there are no significant differences between them. Moreover, we do not find significant relationships between ε and E p for the two emission episodes. Those results suggest that these two-episode emission components likely share the same physical origin.Subject headings: gamma-ray burst: general-methods: statistical 1 The continuous time (CTIME) data include eight energy channels and have a finer time resolution of 64 ms. The continuous spectroscopy (CSPEC) data include 128 energy channels, and a time resolution of 1.024 s. The timetagged event (TTE) data consist of individual detector events, each tagged with arrival time, energy (128 channels), and detector number (Paciesas et al. 2012).2 http://fermi.gsfc.nasa.gov/ssc/data/. 3 http://sourceforge.net/projects/gtburst/.4 In order to obtain the arrival time of different energy photons, we separate the NaI and BGO detectors into two energy bands, respectively, e.g., [8, 50] keV and [50, 1000] keV for NaI; [250, 1000] keV and >1000 keV for BGO.

Temporal Analysis of GRB Precursors in the Third Swift-BAT Catalog

“…Because the energy is accumulated during the quiescent time, the subsequent pulse is stronger if the quiescent time is longer. However, we cannot find the relation between the energy release and the quiescent time in our sample, and the relation was also not obtained by Lan et al (2018), Charisi et al (2015), and Coppin et al (2020), respectively. In the propeller model, it is assumed that both the central engine and the accretion flow have similar physical parameters to explain all the GRBs.…”

“…By comparing some temporal parameters of the pre/post-peak event to those of the main burst, they did not find the correlation between the pre/post-peak event and the main burst. Lan et al (2018;labeled as "L18" in Table 1) used a Bayesian block algorithm to search the two-episode GRB emission in the Fermi-GBM data. They did not find any significant correlation between the duration of the first episode and that of the second episode.…”

Gamma-ray burst precursors as observed by Fermi-GBM

“…Given that precursors are defined to be dimmer than the prompt emission, it is possible that, if a burst is located too far away, the precursor will be indistinguishable from the detector background. To mitigate this effect, previous precursor analyses [5][6][7][8] have sometimes restricted their GRB sample to 'sufficiently bright' long bursts. These studies did indeed also find the highest fraction of GRBs with precursors, up to 20% [5].…”

“…They are observed as a slight increase in the gamma-ray emission, but dimmer than that of the prompt phase. Depending on the exact definition, 3% to 20% of GRBs are estimated to have a precursor [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

Electromagnetic Precursors of Short Gamma-Ray Bursts as Counterparts of Gravitational Waves

“…Precursors were initially identified as weak signals in long GRBs (e.g., [33][34][35][36][37][38][39]). Later precursors of SGRBs were found in the Swift/Burst Alert Telescope (BAT) data [40].…”