Inductive spikes and gamma-ray flares from the Crab Nebula

The ~400 MeV flaring emission from the Crab Nebula is naturally explained as the result of an abrupt reduction in the mass-loading of the pulsar wind. Very few particles are then available to carry the current required to maintain wave activity, causing them to achieve high Lorentz factors. When they penetrate the Nebula, a tightly beamed, high luminosity burst of hard gamma-rays results, with characteristics similar to the observed flares. This mechanism may operate in other powerful pulsars, such as J0537-6910 (PWN N 157B), B0540-69, B1957+20 and J0205+6449 (3C 58).Comment: Talk presented at the 7th Fermi Symposium, Garmisch-Partenkirchen, October 201

Constraining the properties of the magnetic turbulence in the Geminga region using HAWC γ\gamma-ray data

Observations of extended gamma-ray emission around Galactic cosmic-ray (CR) sources can be used as novel probes of interstellar magnetic fields. Using very-high-energy gamma-ray data from the HAWC Observatory, we place constraints on the properties of the magnetic turbulence within $\approx 25$ pc from Geminga. We inject and propagate individual CR electrons in 3D realizations of turbulent magnetic fields, calculate the resulting gamma-ray emission, and compare with HAWC measurements of this region. We find that HAWC data is compatible with expectations for Kolmogorov or Kraichnan turbulence, and can be well fitted for reasonable coherence lengths and strengths of the turbulence, despite implying a CR diffusion coefficient significantly smaller than those suggested by Galactic CR propagation codes. The best fit is found for a coherence length $L_{\rm c} \approx 1$ pc and a magnetic field strength $B_{\rm rms} \approx 3 \mu$G, and the preferred value for $L_{\rm c}$ increases with $B_{\rm rms}$. Moreover, the apparent lack of strong asymmetries in the observed emission allows us to constrain the coherence length to $L_{\rm c} \lesssim 5$ pc in this region.Comment: submitted to MNRA

Constraints on the properties of the turbulent magnetic field around Geminga using HAWC measurements

We place constraints on the properties of the interstellar turbulence that surrounds Geminga pulsar, using the recent measurements from the HAWC Observatory in this region. We propagate very-high-energy electrons in realizations of 3D isotropic Kolmogorov or Kraichnan turbulence, calculate their gamma-ray emission, and compare with HAWC measurements. We show that the measurements can be well fitted for both models of the turbulence and for reasonable values of its strength, $B_{\rm rms}$, and coherence length, $L_{\rm c}$. Our best fits are obtained for $B_{\rm rms} \simeq 3 \mu$G and $L_{\rm c} \simeq 1$ pc. Furthermore, the absence of strong asymmetries in the observed emission favours $L_{\rm c} \leq 5$ pc.Comment: 6 pages, 3 figures. Submitted to Journal of Physics: Conference Series. Talk presented at the 26th Extended European Cosmic Ray Symposium, Barnaul, July 201

Large-Scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence

We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich-Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape of the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties - notably the power-spectrum - of the interstellar turbulence within a few tens of parsecs from Earth.Comment: 20 pages, 12 figures. Published in The Astrophysical Journa

Inductive spikes in the Crab Nebula - a theory of gamma-ray flares

We show that the mysterious, rapidly variable emission at ~400 MeV observed from the Crab Nebula by the AGILE and Fermi experiments could be the result of a sudden drop in the mass-loading of the pulsar wind. The current required to maintain wave activity in the wind is then carried by very few particles of high Lorentz factor. On impacting the Nebula, these particles produce a tightly beamed, high luminosity burst of hard gamma-rays, similar to those observed. This implies (i) the emission is synchrotron radiation in the toroidal field of the Nebula, and, therefore, linearly polarized and (ii) this mechanism potentially contributes to the gamma-ray emission from other powerful pulsars, such as the Magellanic Cloud objects J0537-6910 and B0540-69.Comment: 5 pages, 2 figures, Supplemental material at https://www.dropbox.com/s/bqx8pn1vb7jzcs5/Supplemental.pdf?dl=

Cosmic Rays in a Galactic Breeze

Motivated by the discovery of the non-thermal Fermi bubble features both below and above the Galactic plane, we investigate a scenario in which these bubbles are formed through Galacto-centric outflow. Cosmic rays (CR) both diffusing and advecting within a Galactic breeze outflow, interacting with the ambient gas present, give rise to gamma-ray emission, providing an approximately flat surface brightness profile of this emission, as observed. Applying the same outflow profile further out within the disk, the resultant effects on the observable CR spectral properties are determined. A hardening in the spectra due to the competition of advective and diffusive propagation within a particular energy range is noted, even in the limiting case of equal CR diffusion coefficients in the disk and halo. It is postulated that this hardening effect may relate to the observed hardening feature in the CR spectrum at a rigidity of $\approx 200$ GV.Comment: 8 pages (2 columns), 5 figures. Published in Physical Review

Inductive acceleration of ions in Poynting-flux dominated outflows

Two-fluid (electron-positron) plasma modelling has shown that inductive acceleration can convert Poynting flux directly into bulk kinetic energy in the relativistic flows driven by rotating magnetized neutron stars and black holes. Here, we generalize this approach by adding an ion fluid. Solutions are presented in which all particles are accelerated as the flow expands, with comparable power channeled into each of the plasma components. In an ion-dominated flow, each species reaches the limiting rigidity, according to Hillas' criterion, in a distance significantly shorter than in a lepton-dominated flow. These solutions support the hypothesis that newly born magnetars and pulsars are potential sources of ultra-high energy cosmic rays. The competing process of Poynting flux dissipation by magnetic reconnection is shown to be ineffective in low-density flows in which the conventionally defined electron multiplicity satisfies $\kappa_{\rm e}\lesssim 10^5\left(4\pi L_{38}/\Omega\right)^{1/4} /\textrm{Max}\left(\eta_{\rm ion}^{1/2},1\right)$, where $L_{38}\times 10^{38}\textrm{erg s}^{-1}$ is the power carried by the flow in a solid angle $\Omega$, and $\eta_{\rm ion}$ is the ratio of the ion to lepton power at launch.Comment: 12 pages, 2 figures. Accepted for publication in the Astrophysical Journa