arXiv subjects

J. Petri

Publications and source records attributed to J. Petri.

22 records · Page 2Linked to original sources

arXiv2005

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{ź}niak (\cite{Abramowicz2001}). In a first paper (Pétri \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.

astro-ph↗

arXiv2005

A toy model for coupling accretion disk oscillations to the neutron star spin

Lee, Abramowicz & Kluzniak (2004) demonstrated numerically that rotation of neutron star couples with oscillations of its accretion disk, and excites resonances. No specific coupling was assumed, but magnetic field was suggested as the most likely one. Following this idea, we show (Petri 2005) that if the neutron star is non-axially symmetric and rotating, its gravity may provide the coupling and excite resonances. Here, we return to the original suggestion that the coupling is of a magnetic origin, and demonstrate how does it work in terms of a simple, analytic toy-model.

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arXiv2005

An explanation for the kHz-QPO twin peaks separation in slow and fast rotators

In this Letter we further explore the idea, suggested previously by Klu{ź}niak and collaborators, that the high frequency QPOs may be explained as a resonant coupling between the neutron star spin and two epicyclic modes of accretion disk oscillations. We confirm result of Lee, Abramowicz and Klu{ź}niak (\cite{Lee2004}) that the strongest response occurs when the frequency difference of the two modes equals either the spin frequency (for ``slow rotators'') or half of it (for ``fast rotators''). New points discussed in this Letter are: (1) We suggest that the coupling is gravitational, and due to a non-axially symmetric structure of the rotating neutron star. (2) We found that two excited modes may be both connected to vertical oscillations of the disk, and that strong gravity is not needed to excite the modes.

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arXiv2005

Polarization of high-energy pulsar radiation in the striped wind model

The Stokes parameters of the pulsed synchrotron radiation produced in the striped pulsar wind model are computed and compared with optical observations of the Crab pulsar. We assume the main contribution to the wind emissivity comes from a thin transition layer where the dominant toroidal magnetic field reverses its polarity. The radial component of the field is neglected, but a small meridional component is added. The resulting radiation is linearly polarized (Stokes V=0). In the off-pulse region, the electric vector lies in the direction of the projection on the sky of the rotation axis of the pulsar. This property is unique to the wind model and in good agreement with the data. Other properties such as a reduced degree of polarization and a characteristic sweep of the polarization angle within the pulses are also reproduced. These properties are qualitatively unaffected by variations of the wind Lorentz factor, the electron injection power law index and the inclination of the line of sight.

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