Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B

Abstract

The ejecta composition is an open question in gamma-ray bursts (GRB) physics 1 . Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis 2 , suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function 3-5 , consistent with a synchrotron radiation origin 5,6 and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well 7,8 . There are also bursts showing a subdominant thermal component and a dominant synchrotron component 9 , suggesting a likely hybrid jet composition 10 . Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being ∼ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L p,iso ∼ 4 × 10 53 erg/s) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.

Constraining Anisotropic Lorentz Violation via the Spectral-lag Transition of GRB 160625B

“…In principle, stronger constraints on coefficients for Lorentz violation could be derived by including the higherenergy photons detected by the Fermi Large Area Telescope (LAT). However, the LAT data are much sparser in terms of photon number, and there are only ten > 1 GeV LAT photons during the second sub-burst of GRB 160625B (Zhang et al 2016). This makes it challenging to extract the LAT-band light curve with high temporal resolution and requires additional assumptions in the error analysis beyond those adopted here.…”

“…Its redshift is z = 1.41 (Xu et al 2016). This gamma-ray burst is special because its gamma-ray light curve consists of three dramatically different isolated sub-bursts with very high photon statistics (Burns 2016;Zhang et al 2016). Since the second sub-burst of GRB 160625B is extremely bright, its light curve in different energy bands can be readily extracted.…”

Extremely Bright GRB 160625B with Multiple Emission Episodes: Evidence for Long-term Ejecta Evolution

“…Interestingly, the Fermi/LAT was also triggered by this burst at T 0 + 188.54 seconds (Dirirsa et al 2016), and more than 300 photons with energy above 100 MeV were detected. The highest photon energy is about 15 GeV (Dirirsa et al 2016;Zhang et al 2016b). This GRB triggered GBM again at T 0 + 660 seconds.…”

“…Its prompt γ-ray lightcurve is composed of three episodes: a short precursor, a very bright main emission episode, and a weak emission episode. The three episodes emission are separated by two long quiescent intervals (Zhang et al 2016b). Interestingly, Zhang et al (2016b) found that a pure thermal spectral component and non-thermal spectrum (known as Band function; Band et al 1993) are existed in the precursor and main emission episodes, respectively.…”

“…The three episodes emission are separated by two long quiescent intervals (Zhang et al 2016b). Interestingly, Zhang et al (2016b) found that a pure thermal spectral component and non-thermal spectrum (known as Band function; Band et al 1993) are existed in the precursor and main emission episodes, respectively. They suggested that the thermal component is from the photosphere emission of a fireball, and non-thermal component is from a Poynting-flux-dominated outflow (see also Fraija et al 2017).…”