Gamma-ray flares from PWNe and GRBs

Recent detections of TeV gamma rays from the afterglows of powerful gamma-ray bursts [1] have placed strong constraints on the standard synchrotron self-Compton model [2]. These challenge the external-shock paradigm for production of the radiating particles in the relativistically expanding shell driven by the explosion [3]. Here, we examine the alternative possibility that post-burst activity of the central engine is responsible. In this case, energy may be carried to the shell through a low-density, magnetically dominated bubble, reminiscent of the way a pulsar wind transports energy into its nebula. We identify two candidate mechanisms capable of producing the required high energy particles with very high efficiency: inductive acceleration, as proposed to explain GeV flares from the Crab Nebula [4], and shear acceleration, thought to operate in relativistic jets [5]. The corresponding maximum particle energies and spectra and the implied post-burst luminosities are derived. [1] HESS Collaboration/MAGIC Collaboration Nature 575, 455, 459 \& 464 [2] Zhang, B, 2019 Nature 575, 448 [3] Kirk, JG \& Reville, B, 2010, Astrophysical Journal Letters 710, L16 [4] Kirk, JG \& Giacinti, G, 2017, Physical Review Letters 119, 211101 [5] Rieger, FM \& Duffy, P, 2019, Astrophysical Journal Letters} 86, L26