Origin for the Prompt Spectral Evolution Characteristics and High Energy Emission during the X-Ray Flare in GRB 180720B

Abstract

The gamma-ray burst GRB 180720B is very peculiar. On one hand, some interesting features have been found by performing the detailed time-resolved spectral analysis in the prompt phase. First, the `flux-tracking' pattern is exhibited both for the low energy spectral index $\alpha$ and the peak energy $E_{p}$ in the Band function. Second, some parameter relations show strong monotonous positive correlations, include $E_{p}-F$, $\alpha-F$, $E_{p}-\alpha$, and $E_{p}-L_{\gamma,iso}$ for all time-resolved spectra. Lastly, it should be noted that the values of $\alpha$ do not exceed the synchrotron limits (from $-\frac{3}{2}$ to $-\frac{2}{3}$). On the other hand, the photons with the energy of $\gtrsim$ 100 MeV were detected by LAT both in the prompt phase and afterglow. Notably, the 5 GeV photon was observed at 142 s after the GBM trigger. The spectrum of this burst in the LAT range can be described as $F_{\nu}\propto \nu^{-1.3} t^{-1.54\pm0.02}$ in the afterglow phase. And there are six GeV photons during the X-ray flare when the lower energy emission is fading to a weaker level. We try to give reasonable interpretations of the mechanism for prompt emission and the high energy emission (100 MeV to GeV) in the afterglow. The interpretations suggesting that synchrotron origin can account for the prompt emission and synchrotron self-Compton radiation can account for both the spectrum and temporal behavior of the 100 MeV to GeV afterglow emission that have been accepted by us.