The structure of the ultrarelativistic prompt emission phase and the properties of the black hole in GRB 180720B

Understand

In analogy with GRB 190114C, we here analyze the ultrarelativistic prompt emission (UPE) of GRB 180720B observed in the rest-frame time interval $t_{\rm rf}=4.84$--$10.89$~s by Fermi-GBM.

• We reveal the UPE hierarchical structure from the time-resolved spectral analysis performed in time sub-intervals: the spectrum in each shorter time interval is always fitted by a composite blackbody plus cutoff power-law model.
• We explain this structure with the \textit{inner engine} of binary-driven hypernova (BdHN) model operating in a quantum electrodynamics (QED) regime.
• In this regime, the electric field induced by the gravitomagnetic interaction of the newborn Kerr BH with the surrounding magnetic field is overcritical, i.e., $|{\bf E}|\geq E_c$, where $E_c=m_e^2 c^3/(e\hbar)$.