Quasi-periodic oscillations (QPOs) have been observed in the X-ray luminosity from all types of accreting compact objects, ranging from black holes to high and low magnetic field neutron stars. We are still far from a concensus on the origins of these periodicities and the variety of sources makes it unlikely that one single mechanism is responsible for all that we see. However, models invoking a magnetosphere might succeed in explaining a large fraction of the neutron star QPO's. We are using XTE to rigorously test the applicability of these models to the sub-Hz QPO seen in the bright accreting X-ray pulsars.
The beat and Kepler frequency models were introduced to explain the 15-50 Hz QPOs seen in the brightest Low-Mass X-ray Binaries (LMXBs). The Kepler model presumes that the QPO frequency is the Keplerian frequency, , at the magnetosphere (the location inside of which the flow is dominated by the magnetic field). In the beat frequency model, the QPO frequency is presumed to be the difference, , where is the neutron star spin frequency. This is despite the fact that there are no other signs of either the spin frequency or magnetosphere in the LMXB's.
However, both of these models are readily applicable to accreting X-ray pulsars, where we know that a magnetosphere exists and we have an independent measure of . The torque exerted on the neutron star depends on these same parameters as they tell us whether the accreting matter must lose or gain angular momentum to reach the star. In particular, we expect the neutron star to gain angular momentum (i.e. spin-up) when and vice versa. In this case, the beat frequency should change with the torque and decrease as the star changes from spin-up to spin-down.
BATSE observations have shown that the accreting pulsar Cen X-3 ( , ) alternates between spin-up and spin-down on a monthly timescale, making it the ideal system for studying this phenomena. We are proposing a series of brief pointed observations of Cen X-3 triggered by the BATSE measurement of the torque in order to learn the relation between the QPO frequency and the torque.