Session C4: Invited Session: Nature's Particle Accelerators

Understanding particle acceleration at supernova shocks

One century after the pioneering discovery of cosmic rays by V. Hess, the present generation of X- and gamma-ray telescopes is finally unravelling the origin of such an extraterrestrial radiation, at least for what concerns particles with energies below $\sim 10^8$ GeV, which are thought to be accelerated at the forward shocks of Galactic supernova remnants (SNRs). I discuss the present theoretical understanding of efficient particle acceleration at non-relativistic, collisionless shocks, addressing with both analytical and numerical (particle-in-cell) techniques the crucial interplay between accelerated ions and magnetic turbulence. In SNRs, in fact, magnetic fields turn out to be a factor of 10-100 larger than in the interstellar medium, because of plasma instabilities triggered by energetic particles. In particular, I show 2D and 3D hybrid (fluid electrons - kinetic ions) simulations of non-relativistic collisionless shocks, pointing out the efficiency of Fermi acceleration and the role of the cosmic-ray-induced filamentation instability in amplifying the magnetic field up to the levels inferred at the blast waves of young Galactic remnants. Finally, I outline the observational counterparts of such a theory of particle acceleration at strong shocks in terms of SNR multi-wavelength emission, with a special attention to Tycho's SNR, arguably the best laboratory where to test hadron acceleration.   

The Crab Pulsar: An Old Friend Full of Surprises

The Crab pulsar and its nebula have been astrophysical standards for several decades. The Crab nebula is the brightest known pulsar wind nebula and the ``crab'' is a widely used flux unit. In 2011 Fermi and AGILE telescopes reported powerful gamma-ray flares from the nebula, with up to 30 fold increase over the quiescent flux. The flares challenge traditional models of acceleration in pulsar wind nebulae, since they seem to violate a fundamental limit on maximum acceleration energy in these models, and their short timescales indicate they originate from regions much less than an arc second in size. New models and simulations suggest that Doppler boosting or shock-driven reconnection at the pulsar wind termination shock may be able to overcome some of these limitations, but there are still questions to be resolved. This same year, the ground-based Air-Cherenkov telescopes MAGIC and VERITAS detected, for the first time, pulsed very high-energy gamma rays up to 400 GeV from the pulsar. No existing models had predicted such emission but new models for inverse Compton radiation from electron-positron pairs are rapidly developing, which imply a high multiplicity of pairs reaching at least these energies.   

Quasars in miniature: new insights into particle acceleration from X-ray binaries

A variety of astronomical objects routinely accelerate particles to high energy, with the maximum possible energy per particle typically limited by the size of the system and magnetic field strength. For that reason, much attention has focused on the massive jets of relativistic plasma ejected from supermassive black holes in Active Galactic Nuclei (AGN), which are at least theoretically capable of producing particles (cosmic rays) up to a whopping 10$^{20\, }$eV. However neither how these jets are formed or function, nor how exactly they accelerate particles, is well understood. While we do not expect the mechanisms for particle acceleration in stellar remnant black holes within X-ray binaries (XRBs) to be particularly different than in other sources, XRBs do offer some unique insights. Primarily, jets very similar to those in AGN come and go on timescales of weeks to months, while often monitored simultaneously across the entire electromagnetic spectrum. Through such observations we have been able to probe the processes by which jets not only build up dynamically, but also at what point the jets begin to accelerate particles, providing hints about the necessary conditions and efficiencies. Because the physics of accretion-driven processes such as jets seems to scale predictably with black hole mass, we can also potentially apply what we are learning in these smaller systems to the same phenomena AGN, giving us a new handle on several longstanding questions. I will review our current understanding of particle acceleration in XRBs, as well as the increasing body of evidence suggesting that XRBs indeed seem to represent scaled-down (and thus handily faster evolving) versions of the much more powerful AGN. I will also touch on how accelerated particles from XRBs may contribute significantly to the low-energy Galactic cosmic ray distribution, with local impact on gas chemistry and star formation.