Pulsars and escape: The recipe for LHAASO sources?

Context. Observations of our Galaxy at very-high and ultra-high photon energies have revealed a rich population of sources, many of which have significant angular extension and/or are positionally coincident with energetic pulsars. Aims. We assess whether the properties of these gamma-ray emitters are consistent with the predictions of a model for the non-thermal leptonic emission of a pulsar wind nebula expanding within a supernova remnant, including the possibility of particle escape across the different components of the system up to the ambient medium. Methods. We used a multi-zone model framework that describes the transport and radiation of a population of ultra-relativistic electron-positron pairs. The dynamics of the system was retrieved from numerical hydrodynamics simulations, which allowed us to follow the evolution through advanced stages. Model predictions for typical parameter set-ups are compared to source catalogues issued by the H.E.S.S. and LHAASO collaborations. Results. If particles remain confined in the nebula, 1 − 100 TeV emission is only partially consistent with the properties of observed sources in terms of the surface brightness, angular extent, and photon index. In particular, the systems spend a long time in a bright and soft state that has no equivalent in observations. Conversely, including the possibility of energy-dependent particle escape across the object results in a much better agreement, both in the 1 − 10 TeV range explored with H.E.S.S. or LHAASO-WCDA and in the 20 − 100 TeV range explored by LHAASO-KM2A. Emission components with intermediate to low brightnesses and large to very large sizes result from particles that are trapped in the remnant or that have escaped into the surrounding medium. Conclusions. Particle escape, clearly seen in the form of misaligned jets or halos around middle-aged pulsars, is a very important process in much younger systems with 1 − 100 kyr ages, and shapes their appearance as TeV/PeV sources.