Journal Article

On kHz oscillations and characteristic frequencies of accreting magnetospheres

M. Ali Alpar

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 423, issue 4, pages 3768-3774
Published in print July 2012 | ISSN: 0035-8711
Published online July 2012 | e-ISSN: 1365-2966 | DOI:
On kHz oscillations and characteristic frequencies of accreting magnetospheres

More Like This

Show all results sharing this subject:

  • Astronomy and Astrophysics


Show Summary Details


When an accreting star is close to rotational equilibrium between the dipole component of the stellar magnetic field and the accretion disc, the star’s rotation rate is roughly of the order of the Keplerian rotation rate at the inner boundary of the disc, estimated as the conventional Alfvén radius. A range of frequencies higher than this equilibrium rotation frequency can naturally arise if the accretion flow is channelled by higher multipoles of the star’s magnetic field. The higher multipole components of the magnetic field will balance the material stresses of the accretion flow at radii closer to the star. The Kepler frequencies associated with these generalized Alfvén radii increase with the order of the multipole. Other frequency bands, like the epicyclic frequencies associated with the accretion flow, may in turn be higher than the Kepler frequencies. We present expressions for the spectrum of higher frequencies arising due to these effects. Kilohertz quasi-periodic oscillation (QPO) frequencies that are much higher than the rotation rate of the neutron star, as observed from the recently discovered 11 Hz (P= 90 ms) X-ray pulsar IGR J17480−2446 in the globular cluster Terzan 5, may be due to the modulation of the accretion rate by the excitation of these modes in the accretion flow. The very high QPO frequencies observed from the soft gamma repeaters (SGRs) 1806−20 (P= 5.2 s) and 1900+14 (P= 7.5 s) may also correspond to these characteristic frequencies if SGRs accrete from fallback discs around them.

Keywords: accretion, accretion discs; circumstellar matter; stars: neutron

Journal Article.  5601 words.  Illustrated.

Subjects: Astronomy and Astrophysics

Full text: subscription required

How to subscribe Recommend to my Librarian

Users without a subscription are not able to see the full content. Please, subscribe or login to access all content.