5th Session of the Sant Cugat Forum on Astrophysics
During the last few years, significant progress on the study of pulsar wind nebuale (PWNe) has been attained both from a theoretical and an observational perspective, focusing on the closest, more energetic, and best studied nebula: the Crab. On the one hand, what was known as the sigma problem (how the nebula magnetization evolves with distance from the pulsar, starting from being Poynting dominated to becoming particle dominated) has been studied in detail (and solved) using precise three dimensional (3D) relativistic MHD simulations. On the other hand, observations of Crab nebula with the Fermi and AGILE satellites have unexpectedly shown the appearance of flares of short duration. The high-energy emission from this source suggests that acceleration of particles up to PeV energies is possible on timescales of ~10 hr, with transient emission briefly dominating the flux, likely linked to reconnection events. The detailed study of the Crab nebula is however far from what is the usual lore in the field.
Now, the number of TeV detected PWNe (~30), mostly contributed by the H.E.S.S. survey of the Galactic plane, is similar to the number of characterized nebulae observed at other frequencies over decades of observations. But in just a few years, the Cherenkov Telescope Array will increase this number to several hundreds, actually providing an essentially complete account of TeV emitting PWNe in the Galaxy. At the other end of the multi-frequency spectrum, the SKA and its pathfinder instruments, will reveal thousands of new pulsars, and map in exquisite detail the radiation surrounding them for several hundreds of nebulae. X-ray missions currently under analysis, like Athena, and others, will also reveal currently unknown nebulae, as well as details of the bright ones in their corresponding energy regimes.
Assuming that the PWN is maintained solely by the pulsar rotational power, the gamma-ray luminosity detected is believed to be the result of Comptonization of soft photon fields by relativistic electrons injected by the pulsar during its lifetime. This scenario can lead to TeV sources without lower energy counterparts, when the synchrotron emission is reduced by the decay of the magnetic field. Also, it can lead to large mismatches in extension between gamma and X-ray energies, when the magnetic field is low enough that electrons emitting keV photons actually cool faster and are more energetic than electrons emitting in the TeV range. The explanation of these basic observational properties of PWNe does not imply that we understand the population detected in detail, nor that our models handle both multi-frequency morphology and radiation, nor that they are versatile enough to quickly apply them to hundreds of sources. Discussing how we achieve these more advanced models will be the aim of the workshop.
This workshop will join together an international group of experts with the aim of assessing the theoretical state of the art in modeling nebulae. It shall do so in view of the current and forthcoming observational data, which will be reviewed, assessing among others, the following questions: What kind of models do we already have and what kinds are needed? Can they be combined? Which are the most promising avenues for unifying model classes? Can they be made versatile enough to interpret observations of hundreds of sources? To what extent are the results from different radiative models comparable? What key features are they missing? Up to what extent 1D models are reliable/useful? Are hybrid hadronic/leptonic models necessary for PWNe in general? What is the best case for a hadronic-dominated PWN? How can we differentiate hadronic from leptonic nebulae at an observational level? What is the impact of hybrid models and how can they be observationally tested? How do we move forward: What features are the models missing to account for the forthcoming data?
More information at the workshop website