Multiple Components in the Broadband γ-Ray Emission of the Short GRB 160709A

Abstract

GRB 160709A is one of the few bright short gamma-ray bursts detected by both the Gamma-ray Burst Monitor and the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The γ-ray prompt emission of GRB 160709A is adequately fitted by combinations of three distinct components: (i) a nonthermal component described by a power law (PL) with a highenergy exponential cutoff, (ii) a thermal component modeled with a Planck function, and (iii) a second nonthermal component shaped by an additional PL crossing the whole γ-ray spectrum. While the thermal component dominates during ∼ 0.12 s of the main emission episode of GRB 160709A with an unusually high temperature of ∼ 340 keV, the nonthermal components dominate in the early and late time. The thermal component is consistent with the photospheric emission resulting in the following parameters: the size of the central engine, R 0 = 3.8 +5.9 −1.8 × 10 8 cm, the size of the photosphere, R ph = 7.4 +0.8 −1.2 × 10 10 cm, and a bulk Lorentz factor, Γ = 728 +75 −93 assuming a redshift of 1. The slope of the additional PL spectrum stays unchanged throughout the burst duration; however, its flux decreases continuously as a function of time. A standard external shock model has been tested for the additional PL component using the relation between the temporal and spectral indices (the closure relation). Each set of spectral and temporal indices from two energy bands (200 keV-40 MeV and 100 MeV-10 GeV) satisfies a distinct closure relation. From the closure relation test we derived the index for the electron spectral distribution, p = 2.5 ± 0.1. The interaction of the jet with the interstellar environment is preferred over the interaction with the wind medium.

Neutron star mergers and how to study them

“…There are additional analyses that can be done that are quite useful. Examples include fitting multiple spectral functions simultaneously has provided evidence for additional components (e.g., Guiriec et al 2011;Tak et al 2019) and a potential spectro-temporal signature indicative of nearby BNS mergers (Burns et al 2018).…”

“…The broader energy coverage of Fermi has enabled the study of more complex spectral models. For example, Guiriec et al (2010) fit the prompt emission of three bright SGRB with multiple components, including a thermal component, the main non-thermal component, and an extra power law, which has been seen in additional bursts (e.g., Tak et al 2019). These components could originate from the three stages (photospheric, internal dissipation, external shock) which can have temporally overlapping signals given the enormous bulk Lorentz factors involved.…”

Prevalence of Extra Power-Law Spectral Components in Short Gamma-Ray Bursts

“…In addition, GRB 160709A is included in our analysis, which has been studied in detail and suggested to be a short hard GRB (Tak et al 2019). Note that, the bursting phase of GRB 160709A is selected in our analysis, such as 0.32 and ∼0.77 seconds from GBM trigger time, which is also present in Tak et al (2019).…”

“…Combining the observations of the Large Area Telescope (LAT) onboard Fermi, it is interesting to note that the keV-GeV spectra of some GRBs, including both LGRBs and SGRBs, consist of more than one * qwtang@ncu.edu.cn; kaiwang@hust.edu.cn; ryliu@nju.edu.cn component. For example, a Band component with a PL component for GRB 080916C, GRB 090510, GRB 0909026A and GRB 110731A (Ryde et al 2010;Ackermann et al 2010Ackermann et al , 2013, a CPL component with a PL component for GRB 090902B (time-integral), GRB 100414A and GRB 160709A (Ackermann et al 2013;Tak et al 2019), and a blackbody component (BB or multi-BB) with a PL component for GRB 081221, GRB 090902B (time-resolved), 110920A, GRB 160107A and GRB 160709A (Basak & Rao 2013;Ryde et al 2010;Iyyani et al 2015;Kawakubo et al 2018;Tak et al 2019).…”

“…There are 186 GRBs reported in the 10 year catalog of Fermi−LAT (Ajello et al 2019), from August 2008 to August 2018, among which 169 are LGRBs and 17 are SGRBs. In previous studies, only two SGRBs, namely, GRB 090510 and GRB 160709A are discovered to show the extra PL component in the spectrum (Ackermann et al 2010;Tak et al 2019). In order to further search for and explore the properties of the extra PL component, here we perform a comprehensive joint spectral analysis of Fermi−GBM and Fermi−LAT data for short GRBs in 8 keV-10 GeV detected between August 2008 and August 2020.…”