614 research outputs found
How relevant is the torus activity/geometry for the TeV gamma-rays emitted in the jet of M87 ?
Motivated by unification schemes of active galactic nuclei, we review
evidence for the existence of a small-scale dust torus in M87, a Fanaroff-Riley
Class I radio galaxy. Since there is no direct evidence of any thermal emission
from its torus we consider indirect evidence, such as BLR activity and ageing
arguments to model the cold dust structure of M87. In the context of the jet --
accretion disk -- torus symbiosis we discuss the interactions of GeV and TeV
gamma-rays produced in the jet of M87 with the infrared radiation fields
external to the jet, produced by a less active torus. A thin and cold torus
with less defined outer boundaries could still posses problems to some of the
TeV emission from the jet.Comment: 2 pages, to appear in Proceedings of the conference on Active
Galactic Nuclei: from Central Engine to Host Galaxy, meeting held in Meudon,
France, July 23-27, 2002, Eds.: S. Collin, F. Combes and I. Shlosma
The Directional Dependence of Apertures, Limits and Sensitivity of the Lunar Cherenkov Technique to a UHE Neutrino Flux
We use computer simulations to obtain the directional-dependence of the lunar
Cherenkov technique for ultra-high energy (UHE) neutrino detection. We
calculate the instantaneous effective area of past lunar Cherenkov experiments
as a function of neutrino arrival direction, and hence the
directional-dependence of the combined limit imposed by GLUE and the experiment
at Parkes.
We also determine the directional dependence of the aperture of future
planned experiments with ATCA, ASKAP and the SKA to a UHE neutrino flux, and
calculate the potential annual exposure to astronomical objects as a function
of angular distance from the lunar trajectory through celestial coordinates.Comment: 17 pages, 7 figures; Submitted to Astroparticle Physic
Gamma ray and infrared emission from the M87 jet and torus
The existence of intrinsic obscuration of Fanaroff-Riley I objects is a
controversial topic. M87, the nearest such object, is puzzling in that although
it has very massive central black hole it has a relatively low luminosity,
suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it
is not known with certainty whether or not M87 has a dusty torus.
Infrared observations indicate that if a torus exists in M87 it must have a
rather low infrared luminosity. Using arguments from unification theory of
active galactic nuclei, we have earlier suggested that the inner parsec-scale
region of M87 could still harbour a small torus sufficiently cold such that its
infrared emission is dwarfed by the jet emission. The infrared emission from
even a small cold torus could affect through photon-photon pair production
interactions the escape of 100 GeV to TeV energy gamma rays from the central
parsec of M87.
The TeV gamma-ray flux from the inner jet of M87 has recently been predicted
in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at
least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of
gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported.
We discuss the interactions of gamma-rays produced in the inner jet of M87 with
the weak infrared radiation expected from a possible dusty small-scale torus,
and show that the HEGRA detection shows that the temperature of any torus
surrounding the gamma-ray emission region must be cooler than about 250 K. We
suggest that if no gamma-rays are in future detected during extreme flaring
activity in M87 at other wavelength, this may be expected because of torus
heating.Comment: 7 pages, submitted to Prog. Theor. Phys. Suppl., ps fil
TeV gamma rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object
The unresolved nuclear region of M87 emits strong non-thermal emission from
radio to X-rays. Assuming this emission to originate in the pc scale jet
aligned at to the line of sight, we interpret this
emission in the context of the Synchrotron Proton Blazar (SPB) model. We find
the observed nuclear jet emission to be consistent with M87 being a mis-aligned
BL Lac Object and predict gamma-ray emission extending up to at least 100 GeV
at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS
depending on whether it is high-frequency or low-frequency peaked. Predicted
neutrino emission is below the sensitivity of existing and planned neutrino
telescopes. Ultra-high energy neutrons produced in pion photoproduction
interactions decay into protons after escaping from the host galaxy. Because
energetic protons are deflected by the intergalactic magnetic field, the
protons from the decay of neutrons emitted in all directions, including along
the jet axis where the Doppler factor and hence emitted neutron energies are
higher, can contribute to the observed ultra-high energy cosmic rays. We
consider the propagation of these cosmic ray protons to Earth and conclude that
M87 could account for the observed flux if the extragalactic magnetic field
topology were favourable.Comment: 17 pages, 6 figures. 3 additional references plus minor changes,
acctepted for publication in Astroparticle Physic
Small-Angle Scattering and Diffusion: Application to Relativistic Shock Acceleration
We investigate ways of accurately simulating the propagation of energetic
charged particles over small times where the standard Monte Carlo approximation
to diffusive transport breaks down. We find that a small-angle scattering
procedure with appropriately chosen step-lengths and scattering angles gives
accurate results, and we apply this to the simulation of propagation upstream
in relativistic shock acceleration.Comment: 4 pages, 2 figures, proceedings of World Space Environment Forum
(WSEF2002) to appear in Space Science Reviews, accepte
Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe
We describe a calculation of electromagnetic cascading in radiation and
matter in the early universe initiated by the decay of massive particles or by
some other process. We have used a combination of Monte Carlo and numerical
techniques which enables us to use exact cross sections, where known, for all
the relevant processes. In cascades initiated after the epoch of big bang
nucleosynthesis -rays in the cascades will photodisintegrate He,
producing He and deuterium. Using the observed He and deuterium
abundances we are able to place constraints on the cascade energy deposition as
a function of cosmic time. In the case of the decay of massive primordial
particles, we place limits on the density of massive primordial particles as a
function of their mean decay time, and on the expected intensity of decay
neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with
previous work of the photon spectrum in the cascade and the limits we
calculate for the density of massive particles. The method of calculation of
photon spectra at low energies has been improved. Most figures are revised.
Our conclusions are substantially unchange
Neutrinos Associated With Cosmic Rays of Top-Down Origin
Top-down models of cosmic rays produce more neutrinos than photons and more
photons than protons. In these models, we reevaluate the fluxes of neutrinos
associated with the highest energy cosmic rays in light of mounting evidence
that they are protons and not gamma rays. While proton dominance at EeV
energies can possibly be achieved by efficient absorption of the dominant
high-energy photon flux on universal and galactic photon and magnetic
background fields, we show that the associated neutrino flux is inevitably
increased to a level where it should be within reach of operating experiments
such as AMANDA II, RICE and AGASA. In future neutrino telescopes, tens to a
hundred, rather than a few neutrinos per kilometer squared per year, may be
detected above 1 PeV.Comment: 16 pages, 4 figure
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