977 research outputs found
EGMF Constraints from Simultaneous GeV-TeV Observations of Blazars
Attenuation of the TeV gamma-ray flux from distant blazars through pair
production with extragalactic background light leads to the development of
electromagnetic cascades and subsequent, lower energy, GeV secondary gamma-ray
emission. Due to the deflection of VHE cascade electrons by extragalactic
magnetic fields (EGMF), the spectral shape of this arriving cascade gamma-ray
emission is dependent on the strength of the EGMF. Thus, the spectral shape of
the GeV-TeV emission from blazars has the potential to probe the EGMF strength
along the line of sight to the object. We investigate constraints on the EGMF
derived from observations of blazars for which TeV observations simultaneous
with those by the Fermi telescope were reported. We study the dependence of the
EGMF bound on the hidden assumptions it rests upon. We select blazar objects
for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission
have been published. We model the development of electromagnetic cascades along
the gamma-ray beams from these sources using Monte Carlo simulations, including
the calculation of the temporal delay incurred by cascade photons, relative to
the light propagation time of direct gamma-rays from the source. Constraints on
EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB
J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in
the GeV band is lower than the expected cascade component calculated under the
assumption of zero EGMF. Assuming that the reason for the suppression of the
cascade component is the extended nature of the cascade emission, we find that
B>10^{-15} G (assuming EGMF correlation length of ~1 Mpc) is consistent with
the data. Alternatively, the assumption that the suppression of the cascade
emission is caused by the time delay of the cascade photons the data are
consistent with B>10^{-17} G for the same correlation length.Comment: 9 pages, 9 figure
Excitation of MHD waves in magnetized anisotropic cosmologies
The excitation of cosmological perturbations in an anisotropic cosmological
model and in the presence of a homogeneous magnetic field was studied, using
the resistive magnetohydrodynamic (MHD) equations. We have shown that
fast-magnetosonic modes, propagating normal to the magnetic field grow
exponentially and saturated at high values, due to the resistivity. We also
demonstrate that the jeans-like instabilities are enhanced inside a resistive
and the formation of condensations formed within an anisotropic fluid influence
the growing magnetosonic waves.Comment: 12 pages, RevTex, 5 figures ps, accepted for publication to Astronomy
and Astrophysic
Chaotic Motion of Relativistic Electrons Driven by Whistler Waves
Canonical equations governing an electron motion in electromagnetic field of the whistler mode waves propagating along the direction of an ambient magnetic field are derived. The physical processes on which the equations of motion are based .are identified. It is shown that relativistic electrons interacting with these fields demonstrate chaotic motion, which is accompanied by the particle stochastic heating and significant pitch angle diffusion. Evolution of distribution functions is described by the Fokker-Planck-Kolmogorov equations. It is shown that the whistler mode waves could provide a viable mechanism for stochastic energization of electrons with energies up to 50 MeV in the Jovian magnetosphere
Stability of Algorithms in Statistical Modeling
In this paper, we investigate algorithms stability for calculation of multidimensional integrals using the statistical modeling methods. We considered issues of the algorithms optimization and we give sufficient conditions for the stability. We apply our approach to both calculation of integral from the regression function and the moments integral calculation. In all our numerical experiences, we used the mt19937 pseudorandom number generator
A summary of observational records on periodicities above the rotational period in the Jovian magnetosphere
The Jovian magnetosphere is a very dynamic system. The plasma mass-loading from the moon Io and the fast planetary rotation lead to regular release of mass from the Jovian magnetosphere and to a change of the magnetic topology. These regular variations, most commonly on several (2.5–4) days scale, were derived from various data sets obtained by different spacecraft missions and instruments ranging from auroral images to in situ measurements of magnetospheric particles. Specifically, ion measurements from the Galileo spacecraft represent the periodicities, very distinctively, namely the periodic thinning of the plasma sheet and subsequent dipolarization, and explosive mass release occurring mainly during the transition between these two phases. We present a review of these periodicities, particularly concentrating on those observed in energetic particle data. The most distinct periodicities are observed for ions of sulfur and oxygen. The periodic topological change of the Jovian magnetosphere, the associated mass-release process and auroral signatures can be interpreted as a global magnetospheric instability with analogies to the two step concept of terrestrial substorms. Different views on the triggering mechanism of this magnetospheric instability are discussed
A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields
Results are presented of a new VLA-ROSAT study that probes the magnetic field
strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1)
galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity,
and were selected to be free of (unusual) strong radio cluster halos, and
widespread cooling flows. Consistent with these criteria, most clusters show a
relaxed X-ray morphology and little or no evidence for recent merger activity.
Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM
excess out to ~0.5 Mpc from cluster centers. The results, combined with RM
imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the
hot intergalactic gas within these ``normal'' clusters is permeated with a high
filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2}
microGauss, where l is the field correlation length. These results lead to a
global estimate of the total magnetic energy in clusters, and give new insight
into the ultimate energy origin, which is likely gravitational. These results
also shed some light on the cluster evolutionary conditions that existed at the
onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ
Stochastic electron motion driven by space plasma waves
Stochastic motion of relativistic electrons under conditions of the nonlinear
resonance interaction of particles with space plasma waves is studied.
Particular attention is given to the problem of the stability and variability
of the Earth's radiation belts. It is found that the interaction between
whistler-mode waves and radiation-belt electrons is likely to involve the
same mechanism that is responsible for the dynamical balance between the
accelerating process and relativistic electron precipitation events. We have
also considered the efficiency of the mechanism of stochastic surfing
acceleration of cosmic electrons at the supernova remnant shock front, and
the accelerating process driven by a Langmuir wave packet in producing cosmic
ray electrons. The dynamics of cosmic electrons is formulated in terms of a
dissipative map involving the effect of synchrotron emission. We present
analytical and numerical methods for studying Hamiltonian chaos and
dissipative strange attractors, and for determining the heating extent and
energy spectra
Detection of microgauss coherent magnetic fields in a galaxy five billion years ago
Magnetic fields play a pivotal role in the physics of interstellar medium in
galaxies, but there are few observational constraints on how they evolve across
cosmic time. Spatially resolved synchrotron polarization maps at radio
wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies
that are believed to grow from a seed field via a dynamo effect. To directly
test and characterize this theory requires magnetic field strength and geometry
measurements in cosmologically distant galaxies, which are challenging to
obtain due to the limited sensitivity and angular resolution of current radio
telescopes. Here, we report the cleanest measurements yet of magnetic fields in
a galaxy beyond the local volume, free of the systematics traditional
techniques would encounter. By exploiting the scenario where the polarized
radio emission from a background source is gravitationally lensed by a
foreground galaxy at z = 0.439 using broadband radio polarization data, we
detected coherent G magnetic fields in the lensing disk galaxy as seen 4.6
Gyrs ago, with similar strength and geometry to local volume galaxies. This is
the highest redshift galaxy whose observed coherent magnetic field property is
compatible with a mean-field dynamo origin.Comment: 29 pages, 5 figures (including Supplementary Information). Published
in Nature Astronomy on August 28, 201
Oxygen and hydrogen ion abundance in the near-Earth magnetosphere: Statistical results on the response to the geomagnetic and solar wind activity conditions
The composition of ions plays a crucial role for the fundamental plasma
properties in the terrestrial magnetosphere. We investigate the
oxygen-to-hydrogen ratio in the near-Earth magnetosphere from -10 RE<XGSE}< 10
RE. The results are based on seven years of ion flux measurements in the energy
range ~10 keV to ~955 keV from the RAPID and CIS instruments on board the
Cluster satellites. We find that (1) hydrogen ions at ~10 keV show only a
slight correlation with the geomagnetic conditions and interplanetary magnetic
field changes. They are best correlated with the solar wind dynamic pressure
and density, which is an expected effect of the magnetospheric compression; (2)
~10 keV O+ ion intensities are more strongly affected during disturbed phase of
a geomagnetic storm or substorm than >274 keV O+ ion intensities, relative to
the corresponding hydrogen intensities; (3) In contrast to ~10 keV ions, the
>274 keV O+ ions show the strongest acceleration during growth phase and not
during the expansion phase itself. This suggests a connection between the
energy input to the magnetosphere and the effective energization of energetic
ions during growth phase; (4) The ratio between quiet and disturbed times for
the intensities of ion ionospheric outflow is similar to those observed in the
near-Earth magnetosphere at >274 keV. Therefore, the increase of the energetic
ion intensity during disturbed time is more likely due to the intensification
than to the more effective acceleration of the ionospheric source. In
conclusion, the energization process in the near-Earth magnetosphere is mass
dependent and it is more effective for the heavier ions
- …