521 research outputs found
Microlensing Parallax for Observers in Heliocentric Motion
Motivated by the ongoing Spitzer observational campaign, and the forecoming
K2 one, we revisit, working in an heliocentric reference frame, the geometrical
foundation for the analysis of the microlensing parallax, as measured with the
simultaneous observation of the same microlensing event from two observers with
relative distance of order AU. For the case of observers at rest we discuss the
well known fourfold microlensing parallax degeneracy and determine an equation
for the degenerate directions of the lens trajectory. For the case of observers
in motion, we write down an extension of the Gould (1994) relationship between
the microlensing parallax and the observable quantities and, at the same time,
we highlight the functional dependence of these same quantities from the
timescale of the underlying microlensing event. Furthermore, through a series
of examples, we show the importance of taking into account the motion of the
observers to correctly recover the parameters of the underlying microlensing
event. In particular we discuss the cases of the amplitude of the microlensing
parallax and that of the difference of the timescales between the observed
microlensing events, key to understand the breaking of the microlensing
parallax degeneracy. Finally, we consider the case of the simultaneous
observation of the same microlensing event from ground and two satellites, a
case relevant for the expected joint K2 and Spitzer observational programs in
2016.Comment: Accepted for publication in Ap
Generalized Uncertainty Principle from Quantum Geometry
The generalized uncertainty principle of string theory is derived in the
framework of
Quantum Geometry by taking into account the existence of an upper limit on
the acceleration of massive particles.Comment: 9 pages, LATEX file, to appear in Int. Jou. Theor. Phy
Glassy behavior of the site frustrated percolation model
The dynamical properties of the site frustrated percolation model are
investigated and compared with those of glass forming liquids. When the density
of the particles on the lattice becomes high enough, the dynamics of the model
becomes very slow, due to geometrical constraints, and rearrangement on large
scales is needed to allow relaxation. The autocorrelation functions, the
specific volume for different cooling rates, and the mean square displacement
are evaluated, and are found to exhibit glassy behavior.Comment: 8 pages, RevTeX, 11 fig
Effects of Noise in a Cortical Neural Model
Recently Segev et al. (Phys. Rev. E 64,2001, Phys.Rev.Let. 88, 2002) made
long-term observations of spontaneous activity of in-vitro cortical networks,
which differ from predictions of current models in many features. In this paper
we generalize the EI cortical model introduced in a previous paper (S.Scarpetta
et al. Neural Comput. 14, 2002), including intrinsic white noise and analyzing
effects of noise on the spontaneous activity of the nonlinear system, in order
to account for the experimental results of Segev et al.. Analytically we can
distinguish different regimes of activity, depending from the model parameters.
Using analytical results as a guide line, we perform simulations of the
nonlinear stochastic model in two different regimes, B and C. The Power
Spectrum Density (PSD) of the activity and the Inter-Event-Interval (IEI)
distributions are computed, and compared with experimental results. In regime B
the network shows stochastic resonance phenomena and noise induces aperiodic
collective synchronous oscillations that mimic experimental observations at 0.5
mM Ca concentration. In regime C the model shows spontaneous synchronous
periodic activity that mimic activity observed at 1 mM Ca concentration and the
PSD shows two peaks at the 1st and 2nd harmonics in agreement with experiments
at 1 mM Ca. Moreover (due to intrinsic noise and nonlinear activation function
effects) the PSD shows a broad band peak at low frequency. This feature,
observed experimentally, does not find explanation in the previous models.
Besides we identify parametric changes (namely increase of noise or decreasing
of excitatory connections) that reproduces the fading of periodicity found
experimentally at long times, and we identify a way to discriminate between
those two possible effects measuring experimentally the low frequency PSD.Comment: 25 pages, 10 figures, to appear in Phys. Rev.
Neutrino Oscillations in Caianiello's Quantum Geometry Model
Neutrino flavor oscillations are analyzed in the framework of Quantum
Geometry model proposed by Caianiello. In particular, we analyze the
consequences of the model for accelerated neutrino particles which experience
an effective Schwarzschild geometry modified by the existence of an upper limit
on the acceleration, which implies a violation of the equivalence principle. We
find a shift of quantum mechanical phase of neutrino oscillations, which
depends on the energy of neutrinos as E^3. Implications on atmospheric and
solar neutrinos are discussed.Comment: 11 page
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
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