6,654 research outputs found
A possible black hole binary in Mkn 501
A simple binary model for Mkn~501 is considered under the assumption that the
TeV and X-ray periodicity of days, observed during the flaring state
in 1997, may be basically interpreted as a doppler-shifted flux modulation due
to the orbital motion of the relativistic jet. For the typical jet properties
inferred from emission models, we find an intrinsic orbital period of
yrs and a centre-of-mass distance of cm, the binary
thus being a very close system. If the binary separation corresponds to that at
which gravitational radiation becomes dominant, one may obtain a maximum
allowed primary mass of and a corresponding maximum
allowed secondary mass in the range of
assuming that gas accretion occurs on around the salpeter time scale.
Interestingly such values agree with the black hole masses expected from merger
scenarios.Comment: 4 pages, 2 figures; updated results of astro-ph/0005478; to appear in
the Proc. of the Heidelberg International Symposium on High Energy Gamma-Ray
Astronomy, Heidelberg, June 26-30, 2000, ed. by H.J. Voelk and F. Aharonian,
AIP Conf. Pro
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Exploring Particle Acceleration in Gamma-Ray Binaries
Binary systems can be powerful sources of non-thermal emission from radio to
gamma rays. When the latter are detected, then these objects are known as
gamma-ray binaries. In this work, we explore, in the context of gamma-ray
binaries, different acceleration processes to estimate their efficiency: Fermi
I, Fermi II, shear acceleration, the converter mechanism, and magnetic
reconnection. We find that Fermi I acceleration in a mildly relativistic shock
can provide, although marginally, the multi-10 TeV particles required to
explain observations. Shear acceleration may be a complementary mechanism,
giving particles the final boost to reach such a high energies. Fermi II
acceleration may be too slow to account for the observed very high energy
photons, but may be suitable to explain extended low-energy emission. The
converter mechanism seems to require rather high Lorentz factors but cannot be
discarded a priori. Standard relativistic shock acceleration requires a highly
turbulent, weakly magnetized downstream medium; magnetic reconnection, by
itself possibly insufficient to reach very high energies, could perhaps
facilitate such a conditions. Further theoretical developments, and a better
source characterization, are needed to pinpoint the dominant acceleration
mechanism, which need not be one and the same in all sources.Comment: 7 pages, 1 figure, proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics Applications
(Villa Olmo, Como 3-7 October 2011
Particle Acceleration in Mildly Relativistic Shearing Flows: the Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets
The origin of the extended X-ray emission in the large-scale jets of active
galactic nuclei (AGNs) poses challenges to conventional models of acceleration
and emission. Although the electron synchrotron radiation is considered the
most feasible radiation mechanism, the formation of the continuous large-scale
X-ray structure remains an open issue. As astrophysical jets are expected to
exhibit some turbulence and shearing motion, we here investigate the potential
of shearing flows to facilitate an extended acceleration of particles and
evaluate its impact on the resultant particle distribution. Our treatment
incorporates systematic shear and stochastic second-order Fermi effects. We
show that for typical parameters applicable to large-scale AGN jets, stochastic
second-order Fermi acceleration, which always accompanies shear particle
acceleration, can play an important role in facilitating the whole process of
particle energization. We study the time-dependent evolution of the resultant
particle distribution in the presence of second-order Fermi acceleration, shear
acceleration, and synchrotron losses using a simple Fokker--Planck approach and
provide illustrations for the possible emergence of a complex (multicomponent)
particle energy distribution with different spectral branches. We present
examples for typical parameters applicable to large-scale AGN jets, indicating
the relevance of the underlying processes for understanding the extended X-ray
emission and the origin of ultrahigh-energy cosmic rays.Comment: 26 pages, 8 figures; to appear in Ap
First results of the CERN Resonant WISP Search (CROWS)
The CERN Resonant WISP Search (CROWS) probes the existence of Weakly
Interacting Sub-eV Particles (WISPs) like axions or hidden sector photons. It
is based on the principle of an optical light shining through the wall
experiment, adapted to microwaves. Critical aspects of the experiment are
electromagnetic shielding, design and operation of low loss cavity resonators
and the detection of weak sinusoidal microwave signals. Lower bounds were set
on the coupling constant GeV for axion like
particles with a mass of eV. For hidden sector photons, lower
bounds were set for the coupling constant at a mass
of eV. For the latter we were probing a previously
unexplored region in the parameter space
Helical motion and the origin of QPO in blazar-type sources
Recent observations and analysis of blazar sources provide strong evidence
for (i) the presence of significant periodicities in their lightcurves and (ii)
the occurrence of helical trajectories in their radio jets. In scenarios, where
the periodicity is caused by differential Doppler boosting effects along a
helical jet path, both of these facts may be naturally tied together. Here we
discuss four possible driving mechanisms for the occurrence of helical
trajectories: orbital motion in a binary system, Newtonian-driven jet
precession, internal jet rotation and motion along a global helical magnetic
field. We point out that for non-ballistic helical motion the observed period
may appear strongly shortened due to classical travel time effects. Finally,
the possible relevance of the above mentioned driving mechanisms is discussed
for Mkn~501, OJ 287 and AO 0235+16.Comment: 6 pages, 1 figure; presented at the 5th Microquasar Workshop,
Beijing, June 2004. Accepted for publication in the Chinese Journal of
Astronomy and Astrophysic
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