Xray Binaries

Close binaries where a neutron star or a black hole is accreting matter from its companion, usually a main sequence star, will be visible as strong X-ray sources. They are generally classified as high-mass X-ray binaries (HMXB), when the companion has a mass larger than about 10 M0, and low-mass X-ray binaries (LMXB) with a companion mass smaller than 1.2 M0 .InHMXBs the source of of the accreted material is a strong stellar wind. LMXBs are produced by Roche-lobe overflow of the companion star, either because the major axis of the binary decreases due to angular momentum loss from the system, or else because the radius of the companion is increasing as it evolves.

Because of the rapid evolution of the massive component in HMXBs these systems are young and short-lived, 105-107 a. In LMXBs the lifetime is determined by the mass-transfer process, and may be longer, 107-109 a). In many respects they are similar to cataclysmic variables (see Sect. 14.1), and may give rise to analogous phenomena.

Many kinds of variable X-ray sources have been discovered since they were first observed in the 1970's. Among these, the X-ray pulsars and the X-ray bursters can only be neutron stars. In other types of X-ray binaries it can be difficult to determine whether the primary is a neutron star or a black hole.

Neutron stars and black holes are formed in supernova explosions, and in a binary system the explosion would normally be expected to disrupt the binary. An X-ray binary will only form under special conditions. Some examples are shown in Sect. 11.6.

X-ray Pulsars. X-ray pulsars always belong to binary systems and may be either HMXBs or LMXBs. The pulse periods of X-ray pulsars in high-mass systems are significantly longer than those of radio pulsars, from a few seconds to tens of minutes. In contrast to radio pulsars, the period of the pulsed emission of these pulsars decreases with time.

The characteristic properties of X-ray pulsars can be understood from their binary nature. A neutron star formed in a binary system is first seen as a normal radio pulsar. Initially, the strong radiation of the pulsar prevents gas from falling onto it. However, as it slows down, its energy decreases, and eventually the stellar wind from the companion can reach its surface. The incoming gas is channelled to the magnetic polar caps of the neutron star, where it emits strong X-ray radiation as it hits the surface. This produces the observed pulsed emission.

In low-mass systems, the angular momentum of the incoming gas speeds up the rotation of the pulsar. The maximum possible rotation rate of a neutron star before centrifugal forces start to break it up corresponds to a period of about a millisecond. A few millisecond pulsars with periods of this order are known, both in the radio and in the X-ray region. It is thought that these are (or, in the radio case, have once been) members of binary systems.

The emission curve of a typical fast X-ray pulsar, Hercules X1, is shown in Fig. 14.8. The period of the pulses is 1.24 s. This neutron star is part of an eclipsing binary system, known from optical observations as HZ Herculis. The orbital properties of the system can therefore be determined. Thus e. g. the mass of the pulsar is about one solar mass, reasonable for a neutron star.

X-ray Bursters. X-ray bursters are irregular variables, showing sudden brightenings, known as type I X-ray bursts, at random times (Fig. 14.9). The typical interval between outbursts is a few hours or days, but more rapid bursters are also known. The strength of the outburst seems to be related to the recharging time.

Type I X-ray bursts are analogous to the eruptions of classical novae. However, the source of radiation in X-ray bursters cannot be the ignition of hydrogen, since the maximum emission is in the X-ray region. Instead, gas from the companion settles on the surface of the neutron star, where hydrogen burns steadily to helium. Then, when the growing shell of helium reaches a critical temperature, it burns to carbon in a rapid helium flash. Since, in this case, there are no thick damping outer layers, the flash appears as a burst of X-ray radiation.

X-ray Novae. The X-ray pulsars and bursters have to be neutron stars. Other X-ray binaries may be either neutron stars or black holes. All compact X-ray sources are variable to some extent. In the persistent sources the variations are moderate, an the sources always visible. The majority of sources are transient.

If the X-ray bursters correspond to classical novae, the counterparts of dwarf novae are the X-ray novae, also known as soft X-ray transients (SXT). Quantitatively there are large differences between these types of systems. Dwarf novae have outbursts lasting for a few days at intervals of a few months, for SXTs the outbursts happen at decade-long intervals and last for months. A dwarf nova brightens by a factor about 100 during outbursts, a SXT by a factor of 106. The light-curves of neutron-star and black-hole SXTs are compared in Fig. 14.13.

The SXTs are alternating between (at least) two states: During the high state thermal radiation from the accretion disc dominates, whereas in the low state the X-ray have a higher energy, and are produced by Comp-ton scattering by hot electrons in a disc corona or a jet.

Microquasars. One interesting aspect of X-ray binaries is their connection to models of active galactic nuclei (AGN, Sect. 18.7). In both systems a black hole, which in the case of AGN may have a mass in the range 106-109 Me, is surrounded by an accretion disc.

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Fig. 14.13. Light-curves of a neutronstar (Aql X-1) and a black-hole (GRO J1655-40) transient source, as observed by the All Sky Monitor on RXTE. (D.Psaltis 2006, in Compact Stellar X-ray Sources, ed. Lewin, vdKlis, CUP, p. 16, Fig. 1.9)

In an X-ray binary there is similarly an accretion disc surrounding a compact object, a stellar-mass black hole. It will exhibit phenomena in many respects similar to those in AGN. Since the galactic sources are much nearer, and vary on much shorter time-scales, they may allow more detailed observations of these phenomena.

For example, relativistic jets perpendicular to the disc are common in AGN, and they can also be expected in X-ray binaries. A few examples of such microquasars have been discovered, see Fig. 14.14.

Furthermore, in AGN the jet may sometimes be pointing straight at us. Relativistic effects will then lead to a brightening of the source. In a microquasar there might be a similar effect, which would provide one explanation for the ulraluminous X-ray sources (ULX), sources which appear to be too luminous to be produced by ordinary stellar-mass black holes. This is important, because according to an alternative model ULXs contain an intermediate mass black hole with a mass about 103mq. The origin of such intermediate mass black holes, if they exist, is an intriguing problem.

Fig. 14.14. Observed outburst of the microquasar GRS 1915+105 on 9 September, 1997. The disappearance of the internal part of the accretion disc (decrease in the X-ray flux) is followed by an ejection of relativistic plasma clouds (oscillation in the infrared and radio). (S. Chaty, astro-ph/0607668)

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