1,759 research outputs found
Current-driven filamentation upstream of magnetized relativistic collisionless shocks
The physics of instabilities in the precursor of relativistic collisionless
shocks is of broad importance in high energy astrophysics, because these
instabilities build up the shock, control the particle acceleration process and
generate the magnetic fields in which the accelerated particles radiate. Two
crucial parameters control the micro-physics of these shocks: the magnetization
of the ambient medium and the Lorentz factor of the shock front; as of today,
much of this parameter space remains to be explored. In the present paper, we
report on a new instability upstream of electron-positron relativistic shocks
and we argue that this instability shapes the micro-physics at moderate
magnetization levels and/or large Lorentz factors. This instability is seeded
by the electric current carried by the accelerated particles in the shock
precursor as they gyrate around the background magnetic field. The compensation
current induced in the background plasma leads to an unstable configuration,
with the appearance of charge neutral filaments carrying a current of the same
polarity, oriented along the perpendicular current. This ``current-driven
filamentation'' instability grows faster than any other instability studied so
far upstream of relativistic shocks, with a growth rate comparable to the
plasma frequency. Furthermore, the compensation of the current is associated
with a slow-down of the ambient plasma as it penetrates the shock precursor (as
viewed in the shock rest frame). This slow-down of the plasma implies that the
``current driven filamentation'' instability can grow for any value of the
shock Lorentz factor, provided the magnetization \sigma <~ 10^{-2}. We argue
that this instability explains the results of recent particle-in-cell
simulations in the mildly magnetized regime.Comment: 14 pages, 8 figures; to appear in MNRA
Particle Transport in intense small scale magnetic turbulence with a mean field
Various astrophysical studies have motivated the investigation of the
transport of high energy particles in magnetic turbulence, either in the source
or en route to the observation sites. For strong turbulence and large rigidity,
the pitch-angle scattering rate is governed by a simple law involving a mean
free path that increases proportionally to the square of the particle energy.
In this paper, we show that perpendicular diffusion deviates from this behavior
in the presence of a mean field. We propose an exact theoretical derivation of
the diffusion coefficients and show that a mean field significantly changes the
transverse diffusion even in the presence of a stronger turbulent field. In
particular, the transverse diffusion coefficient is shown to reach a finite
value at large rigidity instead of increasing proportionally to the square of
the particle energy. Our theoretical derivation is corroborated by a dedicated
Monte Carlo simulation. We briefly discuss several possible applications in
astrophysics.Comment: (9 pages, 6 figures, revised version with minor changes
A critical approach to the concept of a polar, low-altitude LARES satellite
According to very recent developments of the LARES mission, which would be
devoted to the measurement of the general relativistic Lense--Thirring effect
in the gravitational field of the Earth with Satellite Laser Ranging, it seems
that the LARES satellite might be finally launched in a polar, low--altitude
orbit by means of a relatively low--cost rocket. The observable would be the
node only. In this letter we critically analyze this scenario.Comment: LaTex2e, 11 pages, 4 figures, 1 table. Accepted for publication in
Classical and Quantum Gravit
Is it possible to test directly General Relativity in the gravitational field of the Moon?
In this paper the possibility of measuring some general relativistic effects
in the gravitational field of the Moon via selenodetic missions, with
particular emphasis to the future Japanese SELENE mission, is investigated. For
a typical selenodetic orbital configuration the post-Newtonian Lense-Thirring
gravitomagnetic and the Einstein's gravitoelectric effects on the satellites
orbits are calculated and compared to the present-day orbit accuracy of lunar
missions. It turns out that for SELENE's Main Orbiter, at present, the
gravitoelectric periselenium shift, which is the largest general relativistic
effect, is 1 or 2 orders of magnitude smaller than the experimental
sensitivity. The systematic error induced by the mismodelled classical
periselenium precession due to the first even zonal harmonic J2 of the Moon's
non-spherical gravitational potential is 3 orders of magnitude larger than the
general relativistic gravitoelectric precession. The estimates of this work
could be used for future lunar missions having as their goals relativistic
measurements as well.Comment: Latex2e, 7 pages, no figures, ets2000.cls and art12.sty used. Major
rewriting in introduction. References adde
FGF-1 and FGF-2 modulate the E-cadherin/catenin system in pancreatic adenocarcinoma cell lines
Fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) have been increasingly recognized to play an important role in the pathobiology of pancreatic malignancy. We have investigated the effects of FGF-1 and FGF-2 on the behaviour and adhesion properties of human pancreatic adenocarcinoma cell lines (BxPc3, T3M4 and HPAF) that were previously characterised for the expression of FGFRs. Here we show that exposure to FGF-1 and FGF-2 leads to significant and dose-dependent increase in E-cadherin-dependent cell-cell adhesion, tubular differentiation, and a reduced capacity to invade collagen gels. FGF stimulation produces phosphorylation of E-cadherin and β-catenin on tyrosine residues, as well as increased E-cadherin localisation to the cytoplasmic membrane and association with FGFR1 demonstrable by coimmunoprecipitation. These results demonstrate that FGF-1 and FGF-2 may be involved in the regulation of cell adhesion, differentiation and invasion of pancreatic cancer. © Cancer Research Campaign http://www.bjcancer.co
Extra-galactic magnetic fields and the second knee in the cosmic-ray spectrum
Recent work suggests that the cosmic ray spectrum may be dominated by
Galactic sources up to ~10^{17.5} eV, and by an extra-Galactic component
beyond, provided this latter cuts off below the transition energy. Here it is
shown that this cut-off could be interpreted in this framework as a signature
of extra-galactic magnetic fields with equivalent average strength B and
coherence length l_c such that B\sqrt{l_c} ~ 2-3.10^{-10} G.Mpc^{1/2}, assuming
l_c < r_L (Larmor radius at 10^{17} eV) and continuously emitting sources with
density 10^{-5}/Mpc^3. The extra-Galactic flux is suppressed below 10^{17} eV
as the diffusive propagation time from the source to the detector becomes
larger than the age of the Universe.Comment: 6 pages, 2 figures; expanded version to appear in Phys.Rev.
LAGEOS-type Satellites in Critical Supplementary Orbit Configuration and the Lense-Thirring Effect Detection
In this paper we analyze quantitatively the concept of LAGEOS--type
satellites in critical supplementary orbit configuration (CSOC) which has
proven capable of yielding various observables for many tests of General
Relativity in the terrestrial gravitational field, with particular emphasis on
the measurement of the Lense--Thirring effect.Comment: LaTex2e, 20 pages, 7 Tables, 6 Figures. Changes in Introduction,
Conclusions, reference added, accepted for publication in Classical and
Quantum Gravit
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