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MHD modeling of wind tori and particle acceleration in pulsar wind nebulae

Final Report Summary - PWNTORI (MHD modeling of wind tori and particle acceleration in pulsar wind nebulae)

The primary objective of Dr. Gallant's one-year stay at the Arcetri Observatory was to improve our understanding of pulsar wind nebulae (PWNe) and the accelerated particles they contain, through detailed numerical modelling of young PWNe other than the Crab Nebula, and to clarify the contribution that positrons accelerated in PWNe can make to the recently confirmed rising cosmic-ray positron fraction, through quantification of the expansion and radiative losses they suffer before escaping evolved PWNe. More generally, the aim of this mobility was to bring together Dr. Gallant's familiarity with observational and experimental data on such objects with the theoretical and numerical expertise of the Arcetri group.

The first objective of the project was to perform realistic simulations of PWNe other than the Crab Nebula (which is by far the best-studied such object, but is in many ways not representative of the class). To this end we focused on the young composite supernova remnant G21.5-0.9 as the wealth of observational data available on the pulsar wind termination region makes it uniquely well-suited for such a study: it has been observed deeply and frequently in X-rays by the Chandra Observatory, and the polarisation of the synchrotron emission in the wind termination region has been mapped (in the near-infrared by Dr. Gallant and collaborators). The observed expansion rate of the PWN, combined with the pulsar timing parameters, was used to constrain its dynamics and to provide boundary conditions for detailed, two-dimensional magnetohydrodynamic (MHD) numerical simulations. Such simulations were performed for a range of values of the pulsar wind parameters describing its magnetisation and angular profile. Synthetic synchrotron emission maps were derived from the simulation results and compared with the available observational data. A preliminary conclusion is that the observed synchrotron morphology is best described by a value of the wind magnetisation similar to that which best reproduces the Crab Nebula X-ray morphology within the axisymmetric MHD description. The geometrical inclination angles are compatible with those inferred from modelling of the gamma-ray pulse profile of PSR J1833-1034; a more detailed exploration of the pulsar wind parameter space is nonetheless ongoing. We also tried to model the observed emission with simpler semi-analytical models, in an attempt to capture the main physical effects. The most obvious such model was emission from an equatorial torus region, as previously used in the literature. It was however found that such a model failed to reproduce important features of the observed polarisation map, which by contrast simulated MHD flow solutions could reproduce.
This part of the project has evolved as a collaborative effort involving many members of the Arectri high-energy group, with in particular major contributions from Ph.D. student B. Olmi and Dr. Bandiera, in addition to Dr. Gallant. The results are currently being prepared for publication, and are expected to broaden our knowledge of pulsar wind parameters beyond the Crab case. Given the central role played by polarisation data, our work should help motivate polarisation observations of additional PWNe. In particular, 3C58 would be an excellent candidate for a similar study, but its Northern declination makes it inaccessible to the Very Large Telescope which Dr. Gallant and collaborators have used for previous such observations.

The second objective of the project was to study the energy losses suffered by the electrons and positrons accelerated in evolved PWNe before they are released as cosmic rays into the interstellar medium (ISM). This is of particular interest because of the measurement by the PAMELA, ATIC, and more recently AMS-02 experiments of a rising cosmic-ray positron fraction at energies roughly between 30 GeV and 1 TeV. One possibility is that this excess may constitute the long-sought observational signature of the decay of dark matter particles, but pulsars also constitute a plausible source of these high-energy positrons (and electrons). In the latter scenario, a crucial aspect which has so far been little studied is that the positrons must be accelerated in PWNe to reach the expected energies. They are then confined by the PWN, suffering adiabatic expansion and radiative energy losses before their release into the ISM.
Dr. Gallant's stay in Arcetri resulted in a first quantitative study of these energy losses, examined in a more general way than initially planned by covering the entire evolutionary history of a typical PWN. A simplifying assumption was that the positrons only escape from the PWN once it has reached pressure equilibrium with the ISM, thus providing an upper limit to the losses suffered in a more general scenario where they escape earlier by diffusion. We derived approximate expressions for the expansion and magnetic field in all PWN evolutionary phases: the initial free expansion, compression after reverse-shock contact, subsonic expansion, and bow-shock phases both inside the supernova remnant (SNR) and outside in the surrounding ISM. These expressions were used to compute the adiabatic and radiative losses of the accelerated positrons, and thence to quantify their effect on the contributions from the various PWN evolutionary phases. We conclude that under the above assumptions, radiative losses in the compression phase preclude a significant contribution above roughly 100 GeV from positrons accelerated earlier, and that bow-shock PWNe in the ISM are the most plausible contributors to cosmic-ray positrons at higher energies. Preliminary results of the above study were presented at the COSPAR Scientific Assembly held in Moscow on August 2-10, 2014. A paper detailing this work is about to be submitted, and follow-up more detailed studies are already being planned. These include an investigation of the likely energy-dependent diffusion out of bow-shock PWNe in the ISM, leading to predictions of the emerging positron spectrum. Another subject for future joint research is to quantify the contribution to positrons at lower energy from the pulsar population, taking into account the Sun's position within the spiral structure of the Galaxy and the distributions of the relevant parameters. These studies will ultimately help quantify the expected PWN contribution to cosmic-ray positrons, and predict spectral signatures to distinguish this contribution from alternative ones such as the decay of dark matter particles.

A more general goal of Dr. Gallant's stay in Arcetri was to bring about a synergy between his familiarity with observational and experimental data, in particular in the field of very-high-energy gamma-ray astronomy, with the theoretical expertise of the Arcetri high-energy group on topics of common interest. This led to a joint study of a major outstanding problem in the astrophysics of Galactic cosmic rays, namely their maximum energy in the context of the SNR acceleration scenario. It has been notoriously difficult theoretically to explain the acceleration of protons to energies of a few PeV, and indeed despite recent progress in gamma-ray observations of SNRs, there is no observational evidence that any known SNR is currently accelerating protons to this expected spectrum. A recent suggestion which might resolve this problem is that such high energies are reached only during a short phase after the supernova explosion. We investigated the possible observational consequences of such a "short-lived PeVatron" scenario, and in particular the gamma-ray halos induced by escaped PeV cosmic rays which would surround recent PeVatrons. We demonstrated that such diffuse gamma-ray emission, with energy spectrum extending to hundreds of TeV, could be detectable near young SNRs such as Cassiopeia A with currently planned very-high-energy gamma-ray detectors. We investigated the expected morphology of such "halos", given that, on the time scales involved, the PeV cosmic-ray transport can be nearly ballistic rather than diffusive, depending on parameters.
The future detection of such "PeVatron halos" could provide extremely useful constraints on interstellar cosmic-ray transport, through their extent and shape, and constitute the long-sought observational proof of the acceleration of Galactic cosmic rays up the knee energy in SNRs. Preliminary results of this work were presented at the Texas Symposium on Relativistic Astrophysics, held in Dallas on December 8-13, 2013, and at the workshop "Cosmic Rays Origin - beyond the standard models" held in San Vito di Cadore, Italy, on March 16-22, 2014. A journal paper detailing these results is in preparation, jointly with Dr. Amato from Arcetri and Dr. Lavalle from Montpellier. Such predictions should be of great help to optimise the characteristics of future detectors in order to maximise their science potential. Dissemination of the results to this effect is ensured by the participation in the CTA consortium of both the Arcetri and Montpellier groups. Efforts in this direction also include Dr. Gallant's contribution on science prospects for Galactic gamma-ray astronomy at the Workshop on Air Shower Detection at High Altitude held in Paris on May 26-28, 2014, and his ongoing exchanges with the LHAASO collaboration.