Arcetri High Energy Group

Pulsar Wind Nebulae

Rotation powered pulsars lose most of their energy to the production of a relativistic magnetized wind mostly made of electrons and positrons. PWNe are bubbles of relativistic plasma that arise from the confinement of such a wind by the surrounding medium (either the remnant of the parent supernova or the ISM). These sources generally show a broad non-thermal emission spectrum, shining from radio to γ-ray frequencies. The emission is interpreted as mostly due to synchrotron and Inverse Compton radiation from relativistic particles interacting with the ambient magnetic and photon field. The highly relativistic outflow emanating from the pulsar magnetosphere is slowed down at a termination shock where particles are accelerated with extreme efficiency. A number of properties put PWNe among the most intriguing and interesting sources in the sky: they are relativistic sources, likely hosting the most relativistic outflows in Nature (the pulsar winds, with Lorentz factors possibly reaching 107-108); they are the most efficient accelerators in the Galaxy, with efficiencies reaching 20-30%; the only firmly detected Galactic PeVatrons; and the only well established reservoirs of leptonic anti-matter in the Galaxy. In summary, PWNe are sources of chief interest for High Energy Astrophysics, being the nearest and brightest relativistic sources, excellent cosmic accelerators and potentially the primary contributors of cosmic ray positrons, at least above some energy.

Our group has been deeply involved in the study of PWNe, touching virtually all aspects of their physics. We were one of the first two groups to perform 2D relativistic MHD simulations of a pulsar wind interacting with the surrounding SNR. These simulations allowed us to show how the anisotropic energy flux of the pulsar wind, higher at the pulsar rotational equator than at the poles, could explain the findings by Chandra, whose high-resolution X-ray observations had just highlighted the existence of an axisymmetric ``jet-torus’’ morphology in the Crab Nebula and then several other young PWNe. The assumed wind structure was that predicted by theoretical modeling of the wind launching by the pulsar magnetosphere, so that our results also provided independent confirmation of the correct description of the pulsar wind energy flow, including vanishing magnetization in the equatorial region. The overall magnetization of the wind is found instead to be larger than estimated based on 1D modeling: only sufficiently magnetized outflows can lead to substantial hoop stresses downstream of the termination shock so as to divert the post-shock flow towards the axis and give rise to jets.

Our further studies including radiation diagnostics opened then the way to some other puzzles: within axisymmetric MHD simulations, the morphology and emission spectrum of the Crab Nebula, the class prototype and one of the astrophysical objects for which data are most abundant, could not be simultaneously reproduced with a given value of the wind magnetization. More specifically the magnetization that could best reproduce the X-ray emission morphology would give origin to a nebular magnetic field well below the value inferred from multi-wavelength spectral modeling. A possible solution to this puzzle, that is currently being investigated, is that the discrepancy be due to the reduced dimensionality of our simulations. Axisymmetry suppresses the development of kink instabilities that could instead be very important in these systems hosting mainly toroidal fields: if kinks can tangle the field lines without substantial magnetic dissipation then a larger magnetic field strength can be accommodated in the nebula without increasing hoop stresses beyond the level that best fits the morphology. This idea is currently being investigated with the help of 3-D relativistic MHD simulations, whose preliminary results are very promising.

At the same time more refined diagnostics for the evolution of the accelerated particles is being implemented, and a special attention is planned to be devoted, as already in the past, to time-variability in the inner nebula. Our final purpose is that of using all the information available from observations to clarify the most fascinating aspects of PWN physics: the composition, in terms of particles and fields, of the wind that emanates from the pulsar magnetosphere and what the particle acceleration mechanism is at the pulsar wind termination shock, a shock that should not be accelerating particles at all according to our standard theories and is proven to be instead one of the best accelerators in the Universe.

Aside from explaining how particles are accelerated in these nebulae, another increasingly hot topic has to do with their release in the ISM. The excess of cosmic ray positrons measured by PAMELA and AMS02 has prompted to consider pulsars as possible contributing sources. Since alternative scenarios either include a radical change in our view of Cosmic Ray origin or propagation, or interpret the excess as a signature of Dark Matter related processes, assessing the pulsar contribution to this excess is particularly topical. Our group has developed pulsar based models for the origin of the positron excess that assume that the particles we see are accelerated at PWN shocks, with the typical flat spectrum that we deduce from observations of these objects in the radio band, and are only released in the ISM when the pulsar exits the parent supernova remnant. Pulsars are a population with high proper velocities and when they leave their parent SNR often find themselves moving in the ISM with supersonic speed: this gives rise to the so-called Pulsar Bow Shock Nebulae observed in radio, X-ray and Hα emission.

Following up on our early work on these objects, we are currently performing new 2 and 3D relativistic MHD simulations aimed at clarifying the impact of instabilities and the physics of particle escape from these systems. Our final goal is that of assessing more reliably the PWN contribution to the measured leptonic antimatter in Cosmic Rays and their relevance for the production of high energy gamma-ray haloes such as those recently observed by HAWC. This is again very important in view of the advent of CTA.