A detailed study of 2S 0114+650 with theRossi X-ray Timing Explorer

Abstract

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.

High-mass X-ray binaries in the Milky Way

“…Using the optical data Crampton et al (1985) determined also an orbital period of the system P orb ≃ 11.6 days, which was confirmed later in X-rays by Corbet et al (1999a). 1A 0114+650 is the X-ray pulsar with one of the longest known pulse periods P spin ≃ 2.65 h (Farrell et al, 2008), which evolved on the time scale of several years (Wang, 2011). In addition to pulse and orbital variabilities in the system there is a superorbital periodicity with the period of 30.7 days (Farrell et al, 2006).…”

An XMM-Newton and NuSTAR Study of IGR J18214-1318: A Non-pulsating High-mass X-Ray Binary with a Neutron Star

“…The photon indices and cutoff energies of both sources are similar to IGR J18214-1318 (Farrell et al 2008;Reig et al 2009), although 2S 0114+650 is a closer analogue since it has a supergiant donor (Reig et al 1996). Li & van den Heuvel (1999) proposed that the slow spin of the NS in 2S 0114+650 indicates that it was born as a magnetar with B10 14 G, was slowed down efficiently by the propeller effect before its magnetic field significantly decayed to its current expected value of ∼10 12 G. In the case of 4U 2206+54, magnetorotational models, which can account for the NS's spin and spin-down rate, require magnetic fields strengths between 5×10 13 and 3×10 15 G (Ikhsanov & Beskrovnaya 2010); thus, it is possible that 4U 2206+54 currently contains a magnetar and would evolve into a system like 2S 0114+650.…”

Finding a 24-day orbital period for the X-ray binary 1A 1118-616

“…This may also apply to 1A 1118-616 and is consistent with the observed moderate spin-up rate (∼1.1 Hz s −1 ) during its outbursts in January 2009. A discussion of various alternative ways of reaching very long spin periods is given in Farrell et al (2008) in the context of investigating the extremely slow (∼2.7 h) pulsar 2S 0114+650. …”