Classical X-ray binaries

Most stars in the Universe have at least one or more stellar companions - they exist in binaries, triples and multiples [2, 87, 351]. In a fraction of binary systems, the separation of the components is small enough that they gravitationally interact and can exchange matter [159, 299, 158]. If one of the components is a compact object such as a neutron star, stellar-mass black hole or a white dwarf, accretion onto the compact object can occur and the system becomes a so-called X-ray binary, shining brightly in the X-ray energy band.

X-ray binaries can be broadly separated into two classes: high mass X-ray binaries and low mass X-ray binaries. The former consist of a compact object and a high mass star with a strong wind. The accretion onto the compact object then occurs mostly via wind capture [424]. The second class consists of a compact object and a lower mass star, which fills the Roche lobe of the system [89]. The accretion of material onto the compact object then occurs through the first Lagrangian point between the two components of the binary.

X-ray binaries show different behaviour based on the nature of the compact object in the system.

Black hole X-ray binaries

Black hole X-ray binaries [334], particularly those in low mass X-ray binaries show strongly transient behaviour. Normally, they spend prolonged periods of time in a quiescent state during which very little X-ray emission is seen [237, 275, 288, 241]. Then they enter into short outbursts with lengths of 10s-100s of days [95]. During these outbursts, the binary is known to follow a ‘Q’ shaped hardness-intensity diagram (Fig 1.6). The outburst begins in a hard state, when the total luminosity is low (L . 0.01 × LE), the X-ray spectrum is very hard and dominated by emission from an

optically thin corona [390] which might be the base of a jet [234], and a stable and compact radio jet is present [126]. The accretion disc might be truncated at ∼100 RG in

the hard state and the emission shows rich time variability behaviour [82]. Sometimes the binary can enter into a bright hard state when the X-ray luminosity is at or above 10% of its Eddington luminosity [84].

Fig. 1.6 The hardness-intensity (Q) diagram of black hole X-ray binary outbursts. The outburst starts in a hard state (A, B). The binary can then transition into a soft state (D, E) through an intermediate state (C), followed by a transition back into a hard state (F) and return into quiescence. This figure is adapted from Fender and Belloni [108].

The X-ray binary can then transition into a soft state [387] or alternatively it can drop back down into quiescence after a so-called ‘failed’ outburst [156]. In the soft state, the hard powerlaw emission from the corona is much weaker. Instead, accretion disc multicolour thermal emission dominates its X-ray spectrum [219]. During the soft state, the accretion disc is known to extend down to the ISCO, and the luminosity of the system is around ∼ 0.1 × LE or even higher. The time variability is also weaker

and a compact jet is usually replaced with discrete jetted ejecta [109, 56]. Last but not least, accretion disc winds are observed in some black hole X-ray binaries in the soft state.

1.2 Accretion and ejection of matter in different systems 17

Ionised disc winds were first discovered in GRO J1655-40 [403] and GRS 1915+105 [200], but have since been detected in about 10 black hole binaries, exclusively in their soft states [78]. The signatures of these disc winds are highly ionised absorption lines, usually of iron, in the hard X-ray band (around 7 keV), however transitions of other ions such as silicon and sulphur have also been detected. The observed outflow velocities measured from the line blueshifts are in the range between 300 and 3000 km/s. The inferred mass outflow rates are large and in the range between 1% and 100% of the mass accretion rate, so disc winds can be an important component of the accretion flow in X-ray binaries. They are most likely driven by Compton heating [280, 79] or by magnetic fields [259, 261, 122]. X-ray luminosities of binaries are too low for isotropic radiation driving and their spectra are too hard for radiation line driving.

Such winds might be present in all soft state black hole binaries [298] but their structure is likely not spherically symmetric and thus not always observable in absorp- tion. Instead, they are probably concentrated towards the accretion disc as all the wind detections were made in high inclination systems, and conversely low inclination binaries do not appear to show signatures of disc winds [Fig. 1.7, 310]. In some cases, since the apparent outflow velocities are quite low, these winds could be ‘failed’ outflows, i.e. they could be bound flows [283].

Neutron star X-ray binaries

Accreting neutron star binaries can be broadly split into two categories based on the strength of the surface magnetic field of the neutron star.

Weakly magnetised accreting neutron stars, with ∼ 109 G surface magnetic fields are usually observed with very small rotational periods in the range of milliseconds [428, 407]. They can be classified into two categories based on their behaviour in the colour-colour diagram, as Atoll sources (following elongated curved branches sometimes resembling a geographical ‘atoll’ in the colour-colour diagram) and Z sources (following three branches which resemble a letter Z) [270, 149, 357]. These systems show similar hard/soft state behaviour to black hole X-ray binaries [227]. In their soft states, particularly the brightest Z sources show luminosities approaching their Eddington luminosity (2 × 1038 erg/s for a 1.4 M⊙ neutron star).

Accretion disc winds have been discovered in multiple neutron star X-ray binaries including GX 13+1 [402, 365, 404] and Cir X-1 [40]. Their properties are very similar to that of disc winds observed in black hole X-ray binaries, suggesting the same driving mechanism [79]. The wind driving can therefore occur either through Compton heating

Fig. 1.7 Ionised wind detections (coloured circles) and non-detections (coloured triangles) of high inclination (left plot) and low inclination (right plot) X-ray binaries placed on the hardness-luminosity diagram (HLD). Disc winds are only detected in high inclination binaries in the soft state, suggesting an equatorial wind structure. This figure is taken from Ponti et al. [310].

or with magnetic fields, possibly with a contribution from the radiation pressure in the luminous Z sources. At the same time, there is a class of lower luminosity neutron star binaries which show static ionised absorption from atmospheres above their accretion discs [78]. These objects also show the so-called accretion disc coronae of highly ionised material, revealing themselves through rich X-ray emission line spectra [170, 324].

Highly magnetised accreting neutron stars are also called the X-ray pulsars [427, 272, 27]. The most famous objects of this class are Centaurus X-3 and Hercules X-1, both discovered in the early 1970s with the first X-ray satellite Uhuru [137, 138, 387, 136]. The surface magnetic field of the neutron star, which can have a strength of ∼ 1012 G [measured via the cyclotron scattering feature, 398, 426] dominates the inner accretion flow in these systems [206, 135]. It creates a magnetosphere [244] truncating the accretion disc to as far as 1000 RG, inside which the accreting matter is channelled

along the magnetic field lines towards the neutron star. As the matter approaches the neutron star’s magnetic poles, it forms an accretion column co-rotating with the neutron star [65]. The accretion column emits anisotropic radiation, hence a fraction of the pulsar’s radiation is pulsed at the rotation frequency of the neutron star (typically ∼ 1 s). Beyond the magnetosphere, there is a standard accretion disc, however if it is

1.2 Accretion and ejection of matter in different systems 19

truncated at ∼1000 RG, its temperature is low (< 0.1 keV) and it is not observed in

the X-rays.

Until recently, there has been very little evidence for disc winds from accreting pulsars. In Chapter 7 of this thesis, I present the first discovery of an ionised outflow in the famous X-ray pulsar Hercules X-1.