Systematic Study of Spectral Diversity in Gamma-Ray Burst Prompt Emissions

概要

The Gamma-Ray Burst (GRB) is an explosive phenomenon producing enormous gamma-ray photons in a cosmological distance. GRBs are radiation from jets which are compact region moving relativistically. In the basis of the relativistic fireball model, which is a widely accepted working hypothesis for GRB emission, enormous energy (E ～ 1053 erg) is injected into the compact region with r ～ 100 km. Hence, an optically thick “fireball” expanding along the jets is expected to produce large amount of both thermal and non-thermal radiation. Spectra of GRBs’ prompt emission, however, show power-law shape, which implies non-thermal emission. Band(1993) reported that the spectra were fitted well by the phenomenological function which was described as two power laws (∝Eα and ∝Eβ) joined smoothly at a break energy (α - β)E0. The parameter Epeak Ξ (2 - α)E0 which indicates the peak energy in VFv = VEN(E) spectrum is distributed over a broad energy band from keV to MeV. However, the mechanism generating the observational diversity of E peak remains largely a mystery.
In this thesis, we discuss two questions on GRB spectra: “existence of thermal component” and “spectral diversity of peak energies”.
To search thermal component in GRB spectra, we focused on the energy dependence of the time constants in the exponential decaying phase of the prompt emission, which reflects radiation processes in general (Rybicki & Lightman, 1979). In this study, we analyzed 15 exponential decaying GRBs observed by both the Burst Alert Telescope(BAT, 15-150 keV) and X-Ray Telescope (XRT, 0.3-10 keV) mounted Swift so that we measure the dependency in the lower energy band. We obtained energy resolved time constants from 17 peaks (pulses) from 16 GRB events. Among them, the energy dependence of time constants of 2 GRBs were well fitted by a single power-law, while 15 GRBs’ were fitted only by broken power-law models. The mean value of the lowenergy index of decay constants was τ ∝ E-(0.54±0.06), which supports synchrotron radiation. Whereas, the high-energy index of decay-time constants was distributed from -0.3 < γ2 < 0.1. This deviation from the dependency of synchrotron radiation means that existence of another radiation component in prompt emission. In the spectral analysis of decaying phase, χ2 in results of the spectral fitting for 9 out of 17 pulses was significantly improved by adding a blackbody model to a non-thermal model (PL, CPL, Band function). However, the spectra in our samples are nonthermal, and the blackbody components derived from the spectral analysis are so weak that it is difficult to explain the high energy index, on the simple blackbody radiation. We conclude, therefore, that synchrotron radiation and “another component” exist in the spectrum fitted with the Band function.
Next, we focused on the mystery of the spectral diversity of peak energies’. Even the most basic question of whether the diversity originates in something intrinsic in the GRB jets or mere observational effects, most notably viewing angles of the observer as suggested in the popular off-axis model has not been answered, yet. We have performed a systematic study of GRBs, which have various Epeak values, observed by Swift, investigating their prompt and X-ray afterglow emissions. We cataloged the long-lasting GRBs observed by the Swift between 2004 December and 2014 February in 3 categories according to the classification by Sakamoto et al. (2008b): X-Ray Flashes (XRFs), X-Ray Rich GRBs (XRRs), and Classical GRBs (C-GRBs). We then derived Eobs peak, as well as Esrc peak if viable, of the Swift spectra of their prompt emission. We also analyzed their X-Ray afterglows and found the trend that the GRB events with a lower Esrc peak, i.e. softer GRBs, are fainter in the 0.3-10 keV X-ray luminosity and decay more slowly than harder GRBs. The intrinsic event rates of the XRFs, XRRs, and C-GRBs were calculated, using the Swift/BAT trigger algorithm. That of either of the XRRs and XRFs is larger than that of the C-GRBs. If we assume that the observational diversity of Epeak is explained with the off-axis model, these results yield the jet half-opening angle of Δθ ≈ 0.16°, jet break time of tjet ～ 30 s and the variance of the observing angles θobs ≲ 0.6°. This implies that the tiny variance of the observing angles of ≲ 0.6° would be responsible for 2 orders of the Epeak diversity observed by Swift/BAT, which is unrealistic. Therefore, we conclude that the Epeak diversity is not explained with the off-axis model, but is likely to originate from some intrinsic properties of the jets of the GRBs.

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