Gamma-ray bursts and supernovae: from the central engines to the observer
Gamma-ray bursts (GRB) and core collapse supernovae (CCSN) are among the most violent and brightest events in the universe. They have decisive impacts on their environments by injecting energy and heavy elements and are also key probes of the distant universe. A very general scenario has emerged to explain these events, in which many unsettled issues remain unanswered. In CCSN, the collapse of the core of a massive star leads to the formation of a protoneutron star releasing vast amounts of energy in the form of neutrinos. A shock wave is then energized by a process that remains elusive, but probably involves the interplay between neutrinos and the complex multidimensional dynamics driven by (magneto)hydrodynamic instabilities. This shock wave accelerates the ejecta by propagating into the complex structure of the star and circumstellar medium, leading to a variety of light curves and spectra. In GRBs a jet is launched from the central engine, a BH or a magnetar resulting from the collapse of a massive star (in type Ibc supernovae) or from the coalescence of two compact objects. It accelerates to ultra-relativistic velocities as thermal or/and magnetic energy is converted into kinetic energy. A fraction of the jet energy content is then dissipated at the photosphere or beyond via shocks or magnetic reconnection to produce gamma-rays. Eventually the jet is decelerated by the external medium, the dissipated energy being responsible for the afterglow.
The spectacular advent of multi-messenger astronomy and the progress of time-domain astronomy are starting a revolution in our knowledge of these transient events. The most impressive step of this revolution is certainly the ground-breaking direct observation of gravitational waves from the coalescence of binary neutron stars and the detection of an associated GRB and kilonova. These observations confirm the basic theoretical picture but also challenge theoretical models, and many questions remain on the modelling and interpretation of this event.
Already in events observed only in the electromagnetic waves, the complexity and surprising diversity is challenging our understanding of many aspects from the central engine injecting the energy to the processes by which this energy is radiated. For example, the discovery of superluminous supernovae, ultra-long GRBs and the extended emission in some short GRBs has recently triggered intense interest in the model of a millisecond magnetar to power a large diversity of extreme events. The importance of the interaction with circumstellar material coming from outbursts in the latest stages of stellar lifetime has also been highlighted by recent observations and theoretical developments. The discovery last year of a repeating fast radio burst calls for an identification of the associated object, potentially a young magnetar. The interpretation of the large diversity of transients thus remains very uncertain and forces us to reassess the link between various kinds of events. This is even more timely in the perspective of future facilities such as the LSST (large synoptic transient survey), which will observe an unprecedented number of transients. CCSN and GRBs are also prime targets for multimessenger astronomy. Further detections of gravitational waves and neutrinos will bring invaluable probes of these violent events. Recent developments in 3D numerical simulations have emphasized the crucial role of complex multidimensional dynamics for the central engine and will be instrumental in the challenging identification and interpretation of these signatures.