Can disk-magnetosphere interaction models and beat frequency models for quasi-periodic oscillation in accreting X-ray pulsars be reconciled?

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00044 Rome, Italy e-mail: bozzo@oa-roma.inaf.it
² Dipartimento di Fisica - Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Rome, Italy
³ Scuola Normale Superiore, Pisa, Italy
⁴ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400 005, India

Received: 23 July 2008
Accepted: 25 October 2008

Abstract

Aims. We review some aspects of the theory of magnetic threaded disks.

Methods. We discuss in particular the equations that determine the position of the inner disk boundary by using different prescriptions for the neutron star-accretion disk interaction. We apply the results to several accretion powered X-ray pulsars that show both quasi-periodic oscillations in their X-ray flux and spin-up/spin-down torque reversals. Under the hypothesis that the beat-frequency model is applicable to the quasi-periodic oscillations, we show that these sources provide an excellent opportunity to test models of the disk-magnetosphere interaction.

Results. A comparison is carried out between the magnetospheric radius obtained with all the prescriptions used in threaded disk models; this shows that none of those prescriptions is able to reproduce the combination of quasi-periodic oscillations and torque behaviour observed for different X-ray luminosity levels in the X-ray pulsars in the present sample.

Conclusions. This suggests that the problem of accretion disk threading by stellar magnetic field still lacks a comprehensive solution. We briefly discuss an outline of possible future developments in this field.