35 research outputs found
A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105
(abridged) In this paper we extend the idea suggested previously by Petri
(2005a,b) that the high frequency quasi-periodic oscillations observed in
low-mass X-ray binaries may be explained as a resonant oscillation of the
accretion disk with a rotating asymmetric background (gravitational or
magnetic) field imposed by the compact object. Here, we apply this general idea
to black hole binaries. It is assumed that a test particle experiences a
similar parametric resonance mechanism such as the one described in paper I and
II but now the resonance is induced by the interaction between a spiral density
wave in the accretion disk, excited close to the innermost stable circular
orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry
to deduce the characteristic frequencies of this test particle. The response of
the test particle is maximal when the frequency ratio of the two strongest
resonances is equal to 3:2 as observed in black hole candidates. Finally,
applying our model to the microquasar GRS 1915+105, we reproduce the correct
value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz
features in the power spectrum time analysis is predicted. Moreover, based only
on the two HF-QPO frequencies, our model is able to constrain the mass and angular momentum of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc
Growth rates of the Weibel and tearing mode instabilities in a relativistic pair plasma
We present an algorithm for solving the linear dispersion relation in an
inhomogeneous, magnetised, relativistic plasma. The method is a generalisation
of a previously reported algorithm that was limited to the homogeneous case.
The extension involves projecting the spatial dependence of the perturbations
onto a set of basis functions that satisfy the boundary conditions (spectral
Galerkin method). To test this algorithm in the homogeneous case, we derive an
analytical expression for the growth rate of the Weibel instability for a
relativistic Maxwellian distribution and compare it with the numerical results.
In the inhomogeneous case, we present solutions of the dispersion relation for
the relativistic tearing mode, making no assumption about the thickness of the
current sheet, and check the numerical method against the analytical
expression.Comment: Accepted by PPC
PIC Simulations of the Temperature Anisotropy-Driven Weibel Instability: Analyzing the perpendicular mode
An instability driven by the thermal anisotropy of a single electron species
is investigated in a 2D particle-in-cell (PIC) simulation. This instability is
the one considered by Weibel and it differs from the beam driven filamentation
instability. A comparison of the simulation results with analytic theory
provides similar exponential growth rates of the magnetic field during the
linear growth phase of the instability. We observe in accordance with previous
works the growth of electric fields during the saturation phase of the
instability. Some components of this electric field are not accounted for by
the linearized theory. A single-fluid-based theory is used to determine the
source of this nonlinear electric field. It is demonstrated that the magnetic
stress tensor, which vanishes in a 1D geometry, is more important in this
2-dimensional model used here. The electric field grows to an amplitude, which
yields a force on the electrons that is comparable to the magnetic one. The
peak energy density of each magnetic field component in the simulation plane
agrees with previous estimates. Eddy currents develop, which let the amplitude
of the third magnetic field component grow, which is not observed in a 1D
simulation.Comment: accepted by Plasma Physics and Controlled Fusio
Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds
By means of two- and three-dimensional particle-in-cell simulations, we
investigate the process of driven magnetic reconnection at the termination
shock of relativistic striped flows. In pulsar winds and in magnetar-powered
relativistic jets, the flow consists of stripes of alternating magnetic field
polarity, separated by current sheets of hot plasma. At the wind termination
shock, the flow compresses and the alternating fields annihilate by driven
magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the
wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated
flows), shock-driven reconnection transfers all the magnetic energy of
alternating fields to the particles, whose average Lorentz factor increases by
a factor of sigma with respect to the pre-shock value. In the limit
lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind,
the post-shock particle spectrum approaches a flat power-law tail with slope
around -1.5, populated by particles accelerated by the reconnection electric
field. The presence of a current-aligned "guide" magnetic field suppresses the
acceleration of particles only when the guide field is stronger than the
alternating component. Our findings place important constraints on the models
of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at
https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special
issue of Computational Science and Discovery on selected research from the
22nd International Conference on Numerical Simulation of Plasma
Numerical solution of the linear dispersion relation in a relativistic pair plasma
We describe an algorithm that computes the linear dispersion relation of
waves and instabilities in relativistic plasmas within a Vlasov-Maxwell
description. The method used is fully relativistic and involves explicit
integration of particle orbits along the unperturbed equilibrium trajectories.
We check the algorithm against the dispersion curves for a single component
magnetised plasma and for an unmagnetised plasma with counter-streaming
components in the non-relativistic case. New results on the growth rate of the
Weibel or two-stream instability in a hot unmagnetised pair plasma consisting
of two counter-streaming relativistic Maxwellians are presented. These are
relevant to the physics of the relativistic plasmas found in gamma-ray bursts,
relativistic jets and pulsar winds.Comment: Accepted by Plasma Physics and Controlled Fusio
Forced oscillations in a hydrodynamical accretion disk and QPOs
This is the second of a series of papers aimed to look for an explanation on
the generation of high frequency quasi-periodic oscillations (QPOs) in
accretion disks around neutron star, black hole, and white dwarf binaries. The
model is inspired by the general idea of a resonance mechanism in the accretion
disk oscillations as was already pointed out by Abramowicz & Klu{\'z}niak
(\cite{Abramowicz2001}). In a first paper (P\'etri \cite{Petri2005a}, paper I),
we showed that a rotating misaligned magnetic field of a neutron star gives
rise to some resonances close to the inner edge of the accretion disk. In this
second paper, we suggest that this process does also exist for an asymmetry in
the gravitational potential of the compact object. We prove that the same
physics applies, at least in the linear stage of the response to the
disturbance in the system. This kind of asymmetry is well suited for neutron
stars or white dwarfs possessing an inhomogeneous interior allowing for a
deviation from a perfectly spherically symmetric gravitational field. We show
by a linear analysis that the disk initially in a cylindrically symmetric
stationary state is subject to three kinds of resonances: a corotation
resonance, a Lindblad resonance due to a driven force and a parametric sonance.
The highest kHz QPOs are then interpreted as the orbital frequency of the disk
at locations where the response to the resonances are maximal. It is also found
that strong gravity is not required to excite the resonances.Comment: Accepte
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
Constraining the magnetic field geometry of the millisecond pulsar PSRJ0030+0451 from joint radio, thermal X-ray, and Îł-ray emission
Context. With the advent of multi-wavelength electromagnetic observations of neutron stars - spanning many decades in photon energies - from radio wavelengths up to X-rays and Îł-rays, it has become possible to significantly constrain the geometry and the location of the associated emission regions. Aims. In this work, we use results from the modelling of thermal X-ray observations of PSR J0030+0451 from the Neutron Star Interior Composition Explorer (NICER) mission and phase-aligned radio and Îł-ray pulse profiles to constrain the geometry of an off-centred dipole that is able to reproduce the light curves in these respective bands simultaneously. Methods. To this aim, we deduced a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots. We show that the geometry is compatible with independent constraints from radio and -ray pulsations only, leading to a fixed magnetic obliquity of α â 75° and a line-of-sight inclination angle of ζ â 54°. Results. We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small-scale surface dipole on top of the large-scale off-centred dipole
New constraints on Planck-scale Lorentz Violation in QED from the Crab Nebula
We set constraints on O(E/M) Lorentz Violation in QED in an effective field
theory framework. A major consequence of such assumptions is the modification
of the dispersion relations for electrons/positrons and photons, which in turn
can affect the electromagnetic output of astrophysical objects. We compare the
information provided by multiwavelength observations with a full and
self-consistent computation of the broad-band spectrum of the Crab Nebula. We
cast constraints of order 10^{-5} at 95% confidence level on the lepton Lorentz
Violation parameters.Comment: 23 pages, 9 figures. v2: added comments and references, matches
version accepted by JCA
PIC simulations of the Thermal Anisotropy-Driven Weibel Instability: Field growth and phase space evolution upon saturation
The Weibel instability is investigated with PIC simulations of an initially
unmagnetized and spatially uniform electron plasma. This instability, which is
driven by the thermally anisotropic electron distribution, generates
electromagnetic waves with wave vectors perpendicular to the direction of the
higher temperature. Two simulations are performed: A 2D simulation, with a
simulation plane that includes the direction of higher temperature,
demonstrates that the wave spectrum is initially confined to one dimension. The
electric field components in the simulation plane generated by the instability
equalize at the end of the simulation through a secondary instability. A 1D PIC
simulation with a high resolution, where the simulation box is aligned with the
wave vectors of the growing waves, reveals details of the electron phase space
distribution and permits a comparison of the magnetic and electric fields when
the instability saturates. It is shown that the electrostatic field is driven
by the magnetic pressure gradient and that it and the magnetic field
redistribute the electrons in space.Comment: Plasma Phys Controll Fusion, in press (to appear in june 2009