6 research outputs found
Ultra-High-Energy Cosmic Rays from the Radio Lobes of AGNs
In the past year, the HiRes and Auger collaborations have reported the
discovery of a high-energy cutoff in the ultra-high energy cosmic-ray (UHECR)
spectrum, and an apparent clustering of the highest energy events towards
nearby active galactic nuclei (AGNs). Consensus is building that such -- eV particles are accelerated within the radio-bright lobes
of these sources, but it is not yet clear how this actually happens. In this
paper, we report (to our knowledge) the first treatment of stochastic particle
acceleration in such environments from first principles, showing that energies
eV are reached in years for protons. However, our
findings reopen the question regarding whether the high-energy cutoff is due
solely to propagation effects, or whether it does in fact represent the maximum
energy permitted by the acceleration process itself.Comment: 8 pages, 7 figures, acccepted for publication in MNRA
Diffusive Cosmic Ray Acceleration at the Galactic Centre
The diffuse TeV emission detected from the inner of the Galaxy
appears to be strongly correlated with the distribution of molecular gas along
the Galactic ridge. Although it is not yet entirely clear whether the origin of
the TeV photons is due to hadronic or leptonic interactions, the tight
correlation of the intensity distribution with the molecular gas strongly
points to a pionic-decay process involving relativistic protons. But the
spectrum of the TeV radiation---a power law with index ---cannot be accommodated easily with the much steeper distribution of
cosmic rays seen at Earth. In earlier work, we examined the possible sources of
these relativistic protons and concluded that neither the supermassive black
hole Sagittarius A* (identified with the HESS source J1745-290), nor several
pulsar wind nebulae dispersed along the Galactic plane, could produce a TeV
emission profile morphologically similar to that seen by HESS. We concluded
from this earlier study that only relativistic protons accelerated throughout
the inter-cloud medium could account for the observed diffuse TeV emission from
this region. In this paper, we develop a model for diffusive proton
acceleration driven by a turbulent Alfv\'enic magnetic field present throughout
the gaseous medium. Though circumstantial, this appears to be the first
evidence that at least some cosmic rays are accelerated diffusively within the
inner pc of the Galaxy.Comment: Accepted for publication in MNRAS letter
Ultra-High Energy Cosmic Ray production in the polar cap regions of black hole magnetospheres
We develop a model of ultra-high energy cosmic ray (UHECR) production via
acceleration in a rotation-induced electric field in vacuum gaps in the
magnetospheres of supermassive black holes (BH). We show that if the poloidal
magnetic field near the BH horizon is misaligned with the BH rotation axis,
charged particles, which initially spiral into the BH hole along the equatorial
plane, penetrate into the regions above the BH "polar caps" and are ejected
with high energies to infinity. We show that in such a model acceleration of
protons near a BH of typical mass 3e8 solar masses is possible only if the
magnetic field is almost aligned with the BH rotation axis. We find that the
power of anisotropic electromagnetic emission from an UHECR source near a
supermassive BH should be at least 10-100 times larger then UHECR power of the
source. This implies that if the number of UHECR sources within the 100 Mpc
sphere is ~100, the power of electromagnetic emission which accompanies proton
acceleration in each source, erg/s, is comparable to the typical
luminosities of active galactic nuclei (AGN) in the local Universe. We also
explore the acceleration of heavy nuclei, for which the constraints on the
electromagnetic luminosity and on the alignment of magnetic field in the gap
are relaxed