369 research outputs found
Scenarios for ultrafast gamma-ray variability in AGN
We analyze three scenarios to address the challenge of ultrafast gamma-ray
variability reported from active galactic nuclei. We focus on the energy
requirements imposed by these scenarios: (i) external cloud in the jet, (ii)
relativistic blob propagating through the jet material, and (iii) production of
high-energy gamma rays in the magnetosphere gaps. We show that while the first
two scenarios are not constrained by the flare luminosity, there is a robust
upper limit on the luminosity of flares generated in the black hole
magnetosphere. This limit depends weakly on the mass of the central black hole
and is determined by the accretion disk magnetization, viewing angle, and the
pair multiplicity. For the most favorable values of these parameters, the
luminosity for 5-minute flares is limited by ,
which excludes a black hole magnetosphere origin of the flare detected from
IC310. In the scopes of scenarios (i) and (ii), the jet power, which is
required to explain the IC310 flare, exceeds the jet power estimated based on
the radio data. To resolve this discrepancy in the framework of the scenario
(ii), it is sufficient to assume that the relativistic blobs are not
distributed isotropically in the jet reference frame. A realization of scenario
(i) demands that the jet power during the flare exceeds by a factor the
power of the radio jet relevant to a timescale of years.Comment: 15 pages, accepted by Ap
Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3
We propose a model to explain the ultra-bright GeV gamma-ray flares observed
from the blazar 3C454.3. The model is based on the concept of a relativistic
jet interacting with compact gas condensations produced when a star (red giant)
crosses the jet close to the central black hole. The study includes an
analytical treatment of the evolution of the envelop lost by the star within
the jet, and calculations of the related high-energy radiation. The model
readily explains the day-long, variable on timescales of hours, GeV gamma-ray
flare from 3C454.3, observed during November 2010 on top of a weeks-long
plateau. In the proposed scenario, the plateau state is caused by a strong wind
generated by the heating of the star atmosphere by nonthermal particles
accelerated at the jet-star interaction region. The flare itself could be
produced by a few clouds of matter lost by the red giant after the initial
impact of the jet. In the framework of the proposed scenario, the observations
constrain the key model parameters of the source, including the mass of the
central black hole: , the total jet power:
, and the Doppler factor of the
gamma-ray emitting clouds, . Whereas we do not specify the
particle acceleration mechanisms, the potential gamma-ray production processes
are discussed and compared in the context of the proposed model. We argue that
synchrotron radiation of protons has certain advantages compared to other
radiation channels of directly accelerated electrons.Comment: 16 pages, 5 figures, submitted to Ap
Effects of Lepton Flavour Violation on Chargino Production at the Linear Collider
We study the effects of lepton flavour violation (LFV) on the production
processes e+e- --> \chi+_i \chi-_j at a linear collider with longitudinal e+
and e- beam polarizations. In the case of LFV the sneutrino mass eigenstates
have no definite flavour, therefore, in the t-channel more than one sneutrino
mass eigenstate can contribute to the chargino production cross sections. Our
framework is the Minimal Supersymmetric Standard Model (MSSM) including LFV
terms. We show that in spite of the restrictions on the LFV parameters due to
the current limits on rare lepton decays, the cross section \sigma(e+e- -->
\chi+_1 \chi-_1) can change by a factor of 2 or more when varying the LFV
mixing angles. We point out that even if the present bound on BR(tau- --> e-
gamma) improves by a factor of thousand the influence of LFV on the chargino
production cross section can be significant. These results could have an
important impact on the strategies for determining the underlying model
parameters at the linear collider.Comment: 11pp; final version for JHE
Rapid TeV variability in Blazars as result of Jet-Star Interaction
We propose a new model for the description of ultra-short flares from TeV
blazars by compact magnetized condensations (blobs), produced when red giant
stars cross the jet close to the central black hole. Our study includes a
simple dynamical model for the evolution of the envelope lost by the star in
the jet, and its high energy nonthermal emission through different leptonic and
hadronic radiation mechanisms. We show that the fragmented envelope of the star
can be accelerated to Lorentz factors up to 100 and radiate effectively the
available energy in gamma-rays predominantly through proton synchrotron
radiation or external inverse Compton scattering of electrons. The model can
readily explain the minute-scale TeV flares on top of longer (typical
time-scales of days) gamma-ray variability as observed from the blazar PKS
2155-304. In the framework of the proposed scenario, the key parameters of the
source are robustly constrained. In the case of proton synchrotron origin of
the emission a mass of the central black hole of , a total jet power of and a Doppler factor, of the gamma-ray emitting blobs, of
are required. Whilst for the external inverse Compton model,
parameters of
, and the are required.Comment: 25 pages, 11 figures, Submitted to Ap
Microwave-induced flow of vortices in long Josephson junctions
We report experimental and numerical study of microwave-induced flow of
vortices in long Josephson junctions at zero dc magnetic field. Our intriguing
observation is that applying an ac-bias of a small frequency and
sufficiently large amplitude changes the current-voltage characteristics
(- curve) of the junction in a way similar to the effect of dc magnetic
field, well known as the flux-flow behavior. The characteristic voltage of
this low voltage branch increases with the power of microwave radiation as
with the index . Experiments
using a low-temperature laser scanning microscope unambiguously indicate the
motion of Josephson vortices driven by microwaves. Numerical simulations agree
with the experimental data and show strongly {\it irregular} vortex motion. We
explain our results by exploiting an analogy between the microwave-induced
vortex flow in long Josephson junctions and incoherent multi-photon absorption
in small Josephson junctions in the presence of large thermal fluctuations. In
the case of long Josephson junctions the spatially-temporal chaos in the vortex
motion mimics the thermal fluctuations. In accordance with this analogy, a
control of the intensity of chaos in a long junction by changing its damping
constant leads to a pronounced change in the shape of the - curve. Our
results provide a possible explanation to previously measured but not yet
understood microwave-driven properties of intrinsic Josephson junctions in
high-temperature superconductors.Comment: 8 pages, 13 figure
A possibility for precise Weinberg angle measurement in centrosymmetric crystals with axis
We demonstrate that parity nonconserving interaction due to the nuclear weak
charge Q_W leads to nonlinear magnetoelectric effect in centrosymmetric
paramagnetic crystals. It is shown that the effect exists only in crystals with
special symmetry axis k. Kinematically, the correlation (correction to energy)
has the form H_PNC ~ Q_W (E,[B,k])(B,k), where B and E are the external
magnetic and electric fields. This gives rise to magnetic induction M_PNC ~ Q_W
{k(B,[k,E]) + [k,E](B,k)}. To be specific we consider rare-earth trifluorides
and, in particular, dysprosium trifluoride which looks the most suitable for
experiment. We estimate the optimal temperature for the experiment to be of a
few kelvin. For the magnetic field B = 1 T and the electric field E = 10 kV/cm,
the expected magnetic induction is 4 \pi M_PNC = 0.5 * 10^-11 G, six orders of
magnitude larger than the best sensitivity currently under discussion.
Dysprosium has several stable isotopes, and so, comparison of the effects for
different isotopes provides possibility for precise measurement of the Weinberg
angle.Comment: 7 pages, 1 figure, 2 tables; version 2 - added discussion of neutron
distribution uncertaint
Hard X-ray Bursts from Collapse of the Super Massive Stars
The very first stars in the Universe can be very massive, up to
. They would leave behind massive black holes that could act as
seeds for growing super massive black holes of active galactic nuclei. Given
the anticipated fast rotation such stars would end their live as super massive
collapsars and drive powerful magnetically-dominated jets. In this paper we
investigate the possibility of observing the bursts of high-energy emission
similar to the Long Gamma Ray Bursts associated with normal collapsars. We show
that during the collapse of supercollapsars, the Blandford-Znajek mechanism can
produce jets as powerful as fewerg/s and release up to
erg of the black hole rotational energy. Due to the higher intrinsic
time scale and higher redshift the initial bright phase of the burst can last
for about seconds whereas the central engine would remain active for
about 10 days. Due to the high redshift the burst spectrum is expected to be
soft, with the spectral energy distribution peaking at around 60keV. The peak
total flux density is relatively low, few,
but not prohibitive. The such events should be rear 0.03 year, the
observations needs long term program and could be done in future.Comment: 4 pages, 2 figures. proceedings of workshop "Many faces of GRB
phenomena - optics vs high energy", SAO Russia, October 12-16, 2009
Hexagons become second if symmetry is broken
Pattern formation on the free surface of a magnetic fluid subjected to a
magnetic field is investigated experimentally. By tilting the magnetic field
the symmetry can be broken in a controllable manner. When increasing the
amplitude of the tilted field, the flat surface gives way to liquid ridges. A
further increase results in a hysteretic transition to a pattern of stretched
hexagons. The instabilities are detected by means of a linear array of magnetic
hall sensors and compared with theoretical predictions.Comment: accepted for publication by Physical Review E/Rapid Communicatio
THE OCCURRENCE OF Pb Cl (OH) AND Pt Sn S COMPOUNDS IN THE MERENSKY REEF, BUSHVELD LAYERED COMPLEX, SOUTH AFRICA
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