1,015 research outputs found
Constraining the properties of the magnetic turbulence in the Geminga region using HAWC -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 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 pc and a magnetic field strength G, and the preferred value for increases with
. Moreover, the apparent lack of strong asymmetries in the
observed emission allows us to constrain the coherence length to pc in this region.Comment: submitted to MNRA
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
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, , and coherence length, . Our best fits are obtained for G and pc. Furthermore, the absence of strong asymmetries in the observed
emission favours 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
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=
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
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 GV.Comment: 8 pages (2 columns), 5 figures. Published in Physical Review
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