539 research outputs found
Energy spectrum of extragalactic gamma-ray sources
The result of Monte Carlo electron photon cascade calculations for propagation of gamma rays through regions of extragalactic space containing no magnetic field are given. These calculations then provide upper limits to the expected flux from extragalactic sources. Since gamma rays in the 10 to the 14th power eV to 10 to the 17th power eV energy range are of interest, interactions of electrons and photons with the 3 K microwave background radiation are considered. To obtain an upper limit to the expected gamma ray flux from sources, the intergalactic field is assumed to be so low that it can be ignored. Interactions with photons of the near-infrared background radiation are not considered here although these will have important implications for gamma rays below 10 to the 14th power eV if the near infrared background radiation is universal. Interaction lengths of electrons and photons in the microwave background radiation at a temperature of 2.96 K were calculated and are given
Neutrino Emission from HBLs and LBLs
The Synchrotron Proton Blazar model is a promising model to explain high
energy emission from gamma-ray loud BL Lac objects like Mkn 421. In contrast to
leptonic models, the hadronic explanation of gamma-ray emission predicts
ultrahigh energy neutrinos.
The predicted neutrino spectra from a typical High-energy cutoff BL Lac
Object (HBL) and a Low-energy cutoff BL Lac Object (LBL) are presented. We find
that cooling due to muon synchrotron radiation causes a cutoff of the neutrino
spectrum at eV, with the exception of from kaon decay
which may extend to higher energies if meson production takes place in the
secondary resonance region of the cross section.
The impact of the neutrino output from both source populations to the diffuse
neutrino background is discussed.Comment: 4 pages, 3 figures, to appear in: Proc. 27th Int. Cosmic Ray Conf.,
Hamburg/German
The physical parameters of Markarian 501 during flaring activity
We determine the physical parameters (magnetic field and Doppler factor) of
the homogeneous synchrotron self-Compton model allowed by the observed X-ray to
gamma-ray spectra and variability of Markarian~501 during the 15-16 April 1997
flaring activity. We find that magnetic fields between 0.07 G and 0.6 G and
Doppler factors between 12 and 36 could fit (depending on observed variability
time scale) these observations. We take account of photon-photon pair
production interactions of gamma-ray photons occurring both inside the emission
region and during propagation to Earth and find these to be extremely important
in correctly determining the allowed model parameters. Previous estimates of
the allowed parameter space have neglected this effect. Future multi-wavelength
campaigns during strong flaring activity, including observations from optical
to TeV gamma-rays, should enable the physical parameters to be further
constrained.Comment: 18 pages, 5 figures, minor changes, additional reference, accepted
for publication in MNRA
Ultra High Energy Cosmic Rays
Cosmic rays with energies above eV are currently of considerable
interest in astrophysics and are to be further studied in a number of projects
which are either currently under construction or the subject of well-developed
proposals. This paper aims to discuss some of the physics of such particles in
terms of current knowledge and information from particle astrophysics at other
energies.Comment: 44 pages, 17 figures. Invited review, PASA, in pres
Testing the homogeneous synchrotron self Compton model for gamma ray production in Mrk 421
Based on the detected variability time scales of X-ray and TeV gamma-ray
emission, and the observed multiwavelength photon spectrum, of Mrk 421 we place
constraints on the allowed parameter space (magnetic field and Doppler factor
of the emission region) for the homogeneous synchrotron self-Compton model. The
spectra calculated for the allowed parameters are marginally consistent with
the available spectral information above TeV reported by the Whipple
Observatory in the case of a 1 day flare time scale. However, for the recently
reported very short duration flares varying on a time scale of 15 min, the
calculated spectra are significantly steeper, suggesting that the homogeneous
synchrotron self Compton model has problems in describing the relatively flat
observed spectra extending above a few TeV. We determine the maximum ratio of
TeV gamma-ray luminosity to X-ray luminosity during flaring which is allowed by
the homogeneous synchrotron self-Compton model for the case of no significant
photon-photon absorption in the source.Comment: 5 pages, 2 figures, LaTeX uses mn.sty, submitted to MNRA
Radiation force on relativistic jets in active galactic nuclei
Radiative deceleration of relativistic jets in active galactic nuclei as the
result of inverse Compton scattering of soft photons from accretion discs is
discussed. The Klein-Nishina (KN) cross section is used in the calculation of
the radiation force due to inverse Compton scattering. Our result shows that
deceleration due to scattering in the KN regime is important only for jets
starting with a bulk Lorentz factor larger than 1000. When the bulk Lorentz
factor satisfies this condition, particles scattering in the Thomson regime
contribute positively to the radiation force (acceleration), but those
particles scattering in the KN regime are dominant and the overall effect is
deceleration. In the KN limit, the drag due to Compton scattering, though less
severe than in the Thomson limit, strongly constrains the bulk Lorentz factor.
Most of the power from the deceleration goes into radiation and hence the
ability of the jet to transport significant power (in particle kinetic energy)
out of the subparsec region is severely limited. The deceleration efficiency
decreases significantly if the jet contains protons and the proton to electron
number density ratio satisfies the condition where is the minimum Lorentz factor of
relativistic electrons (or positrons) in the jet frame and is the
proton to electron mass ratio.Comment: 10 pages including 8 figures; accepted for publication in MNRA
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