Old pulsar wind nebulae and the role of the thermal filaments

Abstract

Old pulsar wind nebulae are among the foremost galactic high-energy gamma-ray sources. However we still lack a robust approach to their modeling, especially in the light of forthcoming high-energy observatories like Astri or CTAO. Part of the problem is due to the complex interaction that characterizes these systems. Understanding this complexity has then become mandatory for further advancements. We aim to develop a new approach to investigate the role the thermal thick layer of massive filaments, seen in objects like the Crab nebula and 3C 58, but likely present in all pulsar wind nebulae, can exert on the dynamics of the late reverberation phase, and compare results with standard approaches that neglect such a layer. A new formulation of the one-zone thin-shell plus Lagrangian formalism, developed in a series of previous papers, is here extended to the case of a thick layer of filamentary ejecta, complementing our former work, mostly focused on the initial free-expansion phase. We compare the dynamics of reverberation with and without the presence of filaments, and show that in the former case reverberation may be substantially anticipated and the following compression takes much longer. The total compression of the system does not seem to change much, and the qualitative behavior is preserved. Our results suggest that the presence or absence of an extended filamentary layer might affect the duration of both the free-expansion phase (shortening it) and the following compression phase during reverberation (lengthening it), but it does not change much the overall compression of the nebula. While this changes the relative number of systems in these two phases, and their contribution to high-energy emission, some peculiar radiative effects associated with the level of compression in old systems, like the "super-efficiency", might not be much affected.