2008MNRAS.389..608F

We present the results of a detailed study of the high-mass X-ray binary 2S 0114+650 made with the pointed instruments onboard the Rossi X-ray Timing Explorer. The spectral and temporal behaviour of this source was examined over the pulse, orbital and superorbital time-scales, covering ∼2 cycles of the 30.7 d superorbital modulation. Marginal evidence for variability of the power-law photon index over the pulse period was identified, similar to that observed from other X-ray pulsars. If this variability is real it can be attributed to a varying viewing geometry of the accretion region with the spin of the neutron star. Variability of the neutral hydrogen column density over the orbital period was observed, which we attribute to the line-of-sight motion of the neutron star through the dense circumstellar environment. A reduction in the power-law photon index was observed during the orbital maximum, which we speculate is due to the absorption effects as the neutron star passes behind a heavily absorbing region near the base of the supergiant companion's wind. No significant variability of the column density was observed over the superorbital period, indicating that variable obscuration by a precessing warp in an accretion disc is not the mechanism behind the superorbital modulation. In contrast, a significant increase in the power-law photon index was observed during the superorbital minimum. We conclude that the observed superorbital modulation is tied to variability in the mass accretion rate due to some as yet unidentified mechanism.