27 research outputs found
Particle flux associated with stochastic processes
Particle flux associated with stochastic processe
Time-Dependent Stochastic Particle Acceleration in Astrophysical Plasmas: Exact Solutions Including Momentum-Dependent Escape
Stochastic acceleration of charged particles due to interactions with
magnetohydrodynamic (MHD) plasma waves is the dominant process leading to the
formation of the high-energy electron and ion distributions in a variety of
astrophysical systems. Collisions with the waves influence both the
energization and the spatial transport of the particles, and therefore it is
important to treat these two aspects of the problem in a self-consistent
manner. We solve the representative Fokker-Planck equation to obtain a new,
closed-form solution for the time-dependent Green's function describing the
acceleration and escape of relativistic ions interacting with Alfven or
fast-mode waves characterized by momentum diffusion coefficient and mean particle escape timescale , where
is the particle momentum and is the power-law index of the MHD wave
spectrum. In particular, we obtain solutions for the momentum distribution of
the ions in the plasma and also for the momentum distribution of the escaping
particles, which may form an energetic outflow. The general features of the
solutions are illustrated via examples based on either a Kolmogorov or
Kraichnan wave spectrum. The new expressions complement the results obtained by
Park and Petrosian, who presented exact solutions for the hard-sphere
scattering case () in addition to other scenarios in which the escape
timescale has a power-law dependence on the momentum. Our results have direct
relevance for models of high-energy radiation and cosmic-ray production in
astrophysical environments such as -ray bursts, active galaxies, and
magnetized coronae around black holes.Comment: Accepted for publication in Ap
High-energy pulses and phase-resolved spectra by inverse Compton emission in the pulsar striped wind - Application to Geminga
(abridged) Although discovered 40 years ago, the emission mechanism
responsible for the observed pulsar radiation remains unclear. However, the
high-energy pulsed emission is usually explained in the framework of either the
polar cap or the outer gap model. The purpose of this work is to study the
pulsed component, that is the light-curves as well as the spectra of the
high-energy emission, above 10 MeV, emanating from the striped wind model.
Gamma rays are produced by scattering off the soft cosmic microwave background
photons on the ultrarelativistic leptons flowing in the current sheets. We
compute the time-dependent inverse Compton emissivity of the wind, in the
Thomson regime, by performing three-dimensional numerical integration in space
over the whole striped wind. The phase-dependent spectral variability is then
calculated as well as the change in pulse shape when going from the lowest to
the highest energies. Several light curves and spectra of inverse Compton
radiation with phase resolved dependence are presented. We apply our model to
the well-known gamma-ray pulsar Geminga. We are able to fit the EGRET spectra
between 10 MeV and 10 GeV as well as the light curve above 100 MeV with good
accuracy.Comment: Accepted by A&
Pulsar acceleration by asymmetric emission of sterile neutrinos
A convincing explanation for the observed pulsar large peculiar velocities is
still missing. We argue that any viable particle physics solution would most
likely involve the resonant production of a non-interacting neutrino of
mass --50 keV. We propose a model where anisotropic magnetic
field configurations strongly bias the resonant spin flavour precession of tau
antineutrinos into . For internal magnetic fields B_{int} \gsim
10^{15} G a - transition magnetic moment of the order of
Bohr magnetons is required. The asymmetric emission of from
the core can produce sizeable natal kicks and account for recoil velocities of
several hundred kilometers per second.Comment: 14 pages, AASTEX, 2 figures (uses epsfig). Minor typos corrected.
Added acknowledgments to the funding institutes BID and Colciencia
Influence of Rotation on Pulsar Radiation Characteristics
We present a relativistic model for pulsar radio emission by including the
effect of rotation on coherent curvature radiation by bunches. We find that
rotation broadens the width of leading component compared to the width of
trailing component. We estimate the component widths in the average pulse
profiles of about 24 pulsars, and find that 19 of them have a broader leading
component. We explain this difference in the component widths by using the
nested cone emission geometry.
We estimate the effect of pulsar spin on the Stokes parameters, and find that
the inclination between the rotation and magnetic axes can introduce an
asymmetry in the circular polarization of the conal components. We analyze the
single pulse polarization data of PSR B0329+54 at 606 MHz, and find that in its
conal components, one sense of circular polarization dominates in the leading
component while the other sense dominates in the trailing component. Our
simulation shows that changing the sign of the impact parameter changes the
sense of circular polarization as well as the swing of polarization angle.Comment: 20 pages, 4 Postscript figures, uses aastex.cls. Accepted for
Publication in ApJ 200
Occupational Therapyâs Role in Addressing College Campus Well-Being
The purpose of this study is to (1) explore occupational therapyâs role in mental health and (2) investigate the effects of Cranial Electrotherapy Stimulation and mindfulness in reducing symptoms of depression, stress or insomnia among college students. This study utilized a pre/posttest design with participants randomized into 2 groups: CES alone (Group A) and mindfulness meditation and CES (Group B). A purposeful sample of college students were recruited, all with a previous diagnosis or self-report of stress/anxiety. This study measured sleep, anxiety, trait mindfulness, and daily function using the Hamilton Anxiety Rating Scale, Freiburg Mindfulness Inventory, Functional Status Questionnaire, and Pittsburgh Sleep Quality Index. Measurements were taken at baseline, weeks 5, 8, and 12. Following intake, students began the 5-week treatment protocol. In week 1, participants completed CES or CES/meditations for 7 days. In weeks 2-5, participants completed 4-5 times/week, 20-40mins each, depending on tolerated dosage. Repeated-measures ANOVA revealed a significant effect of time over 8 weeks for HAM-A [F(1,22) = 19.42, p \u3c 0.05], FMI [F(2,21) = 10.41, p \u3c 0.05], PSQI [F(2,22) = 19.01, p \u3c 0.05], and FSQ Social Role Function [F(2,21) = 5.00, p \u3c 0.05]. There were no significant differences between groups for all four assessments, nor a significant effect of time for the Physical/Psychological FSQ categories. Results reveal that time is the biggest factor in reducing anxiety, and increasing mindfulness, sleep, and daily function, despite differences in groups. This demonstrates that CES and mindfulness might be effective treatment methods for mental health in OT
Evidence for alignment of the rotation and velocity vectors in pulsars
We present strong observational evidence for a relationship between the
direction of a pulsar's motion and its rotation axis. We show carefully
calibrated polarization data for 25 pulsars, 20 of which display linearly
polarized emission from the pulse longitude at closest approach to the magnetic
pole. Such data allow determination of the position angle of the linear
polarisation which in turn reflects the position angle of the rotation axis. Of
these 20 pulsars, 10 show an offset between the velocity vector and the
polarisation position angle which is either less than 10\degr or more than
80\degr, a fraction which is very unlikely by random chance. We believe that
the bimodal nature of the distribution arises from the presence of orthogonal
polarisation modes in the pulsar radio emission. In some cases this orthogonal
ambiguity is resolved by observations at other wavelengths so that we conclude
that the velocity vector and the rotation axis are aligned at birth.
Strengthening the case is the fact that 4 of the 5 pulsars with ages less than
3 Myr show this relationship, including the Vela pulsar. We discuss the
implications of these findings in the context of the Spruit & Phinney
(1998)\nocite{sp98} model of pulsar birth-kicks. We point out that, contrary to
claims in the literature, observations of double neutron star systems do not
rule out aligned kick models and describe a possible observational test
involving the double pulsar system.Comment: MNRAS, In Pres
Current Flow and Pair Creation at Low Altitude in Rotation Powered Pulsars' Force-Free Magnetospheres: Space-Charge Limited Flow
(shortened) We report the results of an investigation of particle
acceleration and electron-positron plasma generation at low altitude in the
polar magnetic flux tubes of Rotation Powered Pulsars, when the stellar surface
is free to emit whatever charges and currents are demanded by the force-free
magnetosphere. We observe novel behavior. a) When the current density is less
than the Goldreich-Julian (GJ) value (0<j/j_{GJ}<1), space charge limited
acceleration of the current carrying beam is mild, with the full GJ charge
density being comprised of the charge density of the beam, co-existing with a
cloud of electrically trapped particles with the same sign of charge as the
beam. The voltage drops are on the order of mc^2/e, and pair creation is
absent. b) When the current density exceeds the GJ value (j/j_{GJ}>1), the
system develops high voltage drops, causing emission of gamma rays and intense
bursts of pair creation. The bursts exhibit limit cycle behavior, with
characteristic time scales somewhat longer than the relativistic fly-by time
over distances comparable to the polar cap diameter (microseconds). c) In
return current regions, where j/j_{GJ}<0, the system develops similar bursts of
pair creation. In cases b) and c), the intermittently generated pairs allow the
system to simultaneously carry the magnetospherically prescribed currents and
adjust the charge density and average electric field to force-free conditions.
We also elucidate the conditions for pair creating beam flow to be steady,
finding that such steady flows can occupy only a small fraction of the current
density parameter space of the force-free magnetospheric model. The generic
polar flow dynamics and pair creation is strongly time dependent. The model has
an essential difference from almost all previous quantitative studies, in that
we sought the accelerating voltage as a function of the applied current.Comment: 35 pages, 29 figures. Accepted for publication in MNRAS. Added new
appendix, several minor changes in the tex
On the escape of cosmic rays from radio galaxy cocoons
(Abridged) A model for the escape of CR particles from radio galaxy cocoons
is presented here. It is assumed that the radio cocoon is poorly magnetically
connected to the environment. An extreme case of this kind is an insulating
boundary layer of magnetic fields, which can efficiently suppress particle
escape. More likely, magnetic field lines are less organised and allow the
transport of CR particles from the source interior to the surface region. For
such a scenario two transport regimes are analysed: diffusion of particles
along inter-phase magnetic flux tubes (leaving the cocoon) and cross field
transport of particles in flux tubes touching the cocoon surface. The cross
field diffusion is likely the dominate escape path, unless a significant
fraction of the surface is magnetically connected to the environment. Major
cluster merger should strongly enhance the particle escape by two complementary
mechanisms. i) The merger shock waves shred radio cocoons into filamentary
structures, allowing the CRs to easily reach the radio cocoon boundary due to
the changed morphology. ii) Also efficient particle losses can be expected for
radio cocoons not compressed in shock waves. There, for a short period after
the sudden injection of large scale turbulence, the (anomalous) cross field
diffusion can be enhanced by several orders of magnitude. This lasts until the
turbulent energy cascade has reached the microscopic scales, which determine
the value of the microscopic diffusion coefficients.Comment: A&A in press, 12 pages, 5 figures, minor language improvement
On the Nature of Pulsar Radio Emission
A theory of pulsar radio emission generation, in which the observed waves are
produced directly by maser-type plasma instabilities operating at the anomalous
cyclotron-Cherenkov resonance and the Cherenkov-drift resonance , is capable of explaining the main
observational characteristics of pulsar radio emission. The instabilities are
due to the interaction of the fast particles from the primary beam and the tail
of the distribution with the normal modes of a strongly magnetized
one-dimensional electron-positron plasma. The waves emitted at these resonances
are vacuum-like, electromagnetic waves that may leave the magnetosphere
directly. In this model, the cyclotron-Cherenkov instability is responsible for
core emission pattern and the Cherenkov-drift instability produces conal
emission. The conditions for the development of the cyclotron-Cherenkov
instability are satisfied for both typical and millisecond pulsars provided
that the streaming energy of the bulk plasma is not very high . In a typical pulsar the cyclotron-Cherenkov and Cherenkov-drift resonances
occur in the outer parts of magnetosphere at . This
theory can account for various aspects of pulsar phenomenology including the
morphology of the pulses, their polarization properties and their spectral
behavior. We propose several observational tests for the theory. The most
prominent prediction are the high altitudes of the emission region and the
linear polarization of conal emission in the plane orthogonal to the local
osculating plane of the magnetic field.Comment: 39 pages, 10 figure