On kHz oscillations and characteristic frequencies of accreting magnetospheres

Official URL: http://dx.doi.org/10.1111/j.1365-2966.2012.21172.x

When an accreting star is close to rotational equilibrium between the dipole component of the stellar magnetic field and the accretion disc, the stars rotation rate is roughly of the order of the Keplerian rotation rate at the inner boundary of the disc, estimated as the conventional Alfven 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 stars 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 Alfven 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.