Pulsar+escape: the recipe for LHAASO sources ?

Abstract

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. We assessed 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. 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 is retrieved from numerical hydrodynamics simulations, which allows to follow the evolution through advanced stages. Model predictions for typical parameter setups are compared to source catalogs issued by the H.E.S.S. and LHAASO collaborations. If particles remain confined in the nebula, 1-100TeV emission is only partially consistent with the properties of observed sources in terms of 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 1-10TeV range explored with H.E.S.S. or LHAASO-WCDA, and in the 20-100TeV range explored by LHAASO-KM2A. Emission components with intermediate to low brightnesses and large to very large sizes result from particles trapped in the remnant or escaped in the surrounding medium. 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 1-100kyr ages, and shapes their appearance as TeV/PeV sources.