1,012 research outputs found
The brightness temperature problem in extreme IDV quasars: a model for PKS 0405-385
I re-examine the brightness temperature problem in PKS 0405-385 which is an
extreme intra-day variable radio quasar with an inferred brightness temperature
of K at 5 GHz, well above the Compton catastrophe limit
of K reached when the synchrotron photon energy density exceeds
the energy density of the magnetic field. If one takes into account the
uncertainty in the distance to the ionized clouds responsible for interstellar
scintillation causing rapid intra-day variability in PKS 0405-385 it is
possible that the brightness temperature could be as low as K at
5 GHz, or even lower. The radio spectrum can be fitted by optically thin
emission from mono-energetic electrons, or an electron spectrum with a
low-energy cut-off such that the critical frequency of the lowest energy
electrons is above the radio frequencies of interest. If one observes optically
thin emission along a long narrow emission region, the average energy density
in the emission region can be many orders of magnitude lower than calculated
from the observed intensity if one assumed a spherical emission region. I
discuss the physical conditions in the emission region and find that the
Compton catastrophe can then be avoided using a reasonable Doppler factor. I
also show that MeV to 100 GeV gamma-ray emission at observable flux levels
should be expected from extreme intra-day variable sources such as PKS
0405-385.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
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
A Proton Synchrotron Blazar Model for Flaring in Markarian~501
(abr.) The spectral energy distribution (SED) of blazars typically has a
double-humped appearance usually interpreted in terms of synchrotron
self-Compton models. In proton blazar models, the SED is instead explained in
terms of acceleration of protons and subsequent cascading. We discuss a
variation of the Synchrotron Proton Blazar model, first proposed by M\"ucke &
Protheroe (1999), in which the low energy part of the SED is mainly synchrotron
radiation by electrons co-accelerated with protons which produce the high
energy part of the SED mainly asproton synchrotron radiation. Using a Monte
Carlo/numerical technique to simulate the interactions and subsequent cascading
of the accelerated protons, we are able to fit the observed SED of Markarian
501 during the April 1997 flare. We find that the emerging cascade spectra
initiated by gamma-rays from decay and by from decay
turn out to be relatively featureless. Synchrotron radiation produced by
from decay, and even more importantly by protons, and
subsequent synchrotron-pair cascading, is able to reproduce well the high
energy part of the SED. For this fit we find that synchrotron radiation by
protons dominates the TeV emission, pion photoproduction being less important
with the consequence that we predict a lower neutrino flux than in other proton
blazar models.Comment: 28 pages, 8 Figures, accepted for publication in Astropart.Phy
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
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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