Vela X-1 (4U 0900−40) is an eclipsing high-mass X-ray binary (HMXB) discovered during rocket borne X-ray observations in 1967 (Chodil et al. 1967). It is located at a distance of ∼2.0 kpc (Sadakane et al. 1985; Nagase 1989) in the Vela constellation. Recent estimates using Gaia data infer distance of $2.42^{+0.19}_{-0.17}$ kpc (Bailer-Jones et al. 2018). The system consists of a massive B0.5Ib supergiant HD 77581 (Brucato & Kristian 1972; Hiltner, Werner & Osmer 1972; Jones & Liller 1973; Vidal, Wickramasinghe & Peterson 1973) having mass of about ∼23 $\rm {M_\odot }$ and radius of ∼34 $\rm {R_\odot }$ (Van Paradijs et al. 1976; Joss & Rappaport 1984; Nagase 1989; van Kerkwijk et al. 1995) and a neutron star with mass ∼1.8 $\rm {M_\odot }$ (Van Paradijs et al. 1976; Nagase 1989; Barziv et al. 2001; Rawls et al. 2011). The orbital period of the binary system is about 9 d (Hiltner et al. 1972; Forman et al. 1973; Vidal et al. 1973; Watson & Griffiths 1977; van Kerkwijk et al. 1995). Due to the close proximity of about 1.7 $\rm {R_\star }$ (Conti 1978; Quaintrell et al. 2003) between the neutron star and its companion, the neutron star is immersed in the dense stellar wind of the donor star having typical mass-loss rate of about $\dot{M} \mathrm{\sim 10^{-6} ~{M_\odot } \, yr^{-1}}$ (Hutchings 1974; Dupree et al. 1980; Nagase et al. 1986; Sako et al. 1999). A fraction of the stellar wind is captured and channelled along the strong magnetic field (∼2.7 × 1012 G; Kretschmar et al. 1996; Coburn et al. 2002; Kreykenbohm et al. 2002) of the neutron star on to the magnetic poles, producing regular X-ray pulsations caused by the spin period ∼283 s (Rappaport & McClintock 1975; McClintock et al. 1976) of the neutron star. Although Vela X-1 is known to be a persistent source having luminosity of about 4 × 1036 erg s−1 (McCray et al. 1984; Sadakane et al. 1985; Nagase et al. 1986; Kreykenbohm et al. 2002), it shows a plethora of X-ray variabilities such as sudden flares lasting a few minutes to several hours wherein the luminosity increases by several folds within very short time-scales of a few tens of seconds (Lapshov et al. 1992; Staubert et al. 2004; Kreykenbohm et al. 2008). Occurrence of sudden flares in this system are not so well understood and is believed to be due to enhanced accretion rate due to variabilities in the stellar wind from the companion star (Nagase et al. 1983; Haberl & White 1990) or accretion of clumpy stellar wind (Staubert et al. 2004; Ducci et al. 2009; Fürst et al. 2010; Odaka et al. 2013). Some studies suggest that sudden flares might be related to formation of transient accretion disc (Inoue et al. 1984; Taam & Fryxell 1989; Haberl & White 1990; Kreykenbohm et al. 2008). Another bizarre manifestation seen in Vela X-1 is occurrence of abrupt ‘off-states’ wherein X-ray pulsations cessation (within less than the pulse period) is observed for several tens of minutes at a time (Inoue et al. 1984; Lapshov et al. 1992; Kreykenbohm et al. 1999, 2008; Doroshenko, Santangelo & Suleimanov 2011; Sidoli et al. 2015). These states are poorly understood and might be caused by changes in the accretion rate due to variabilities in the stellar wind (Lapshov et al. 1992; Coburn et al. 2002). Some earlier studies also suggest that ‘off-states’ might be associated with formation of transient accretion discs (Inoue et al. 1984) or the accretion is choked due to the sudden onset of propeller effect (Kreykenbohm et al. 2008). It has also been suggested that the onset of these ‘off-states’ can be caused due to transition from the higher luminosity Compton cooling regime to the lower luminosity radiative cooling regime (Shakura, Postnov & Hjalmarsdotter 2013) or due to unstable hydrodynamic flows in the vicinity of the neutron star (Manousakis & Walter 2015a). Recent numerical studies suggest formation of temporary accretion discs in wind-fed X-ray pulsars (Karino, Nakamura & Taani 2019; El Mellah, Sundqvist & Keppens 2019a; El Mellah et al. 2019b) but conclusive evidence of their existence has been elusive. Interestingly, Liao et al. (2020) infer presence of temporary accretion disc in Vela X-1 during an extended low state lasting at least 30 ks that was accompanied by unusual spin-up event and similar Fe Kα fluxes compared to the preceding flaring period.
Citation: McClintock et al. 1976
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