Accreting NS appear in a large variety of types, persistent and transient, exhibiting various kinds of observational features, both in their spectral and in their temporal behavior. The two primary physical factors determining the properties of individual systems are (1) the mass of the companion and (2) the magnetic field of the NS. Further factors, usually intimately connected to (1) and (2) are (3) the geometry of the binary orbit, (4) the mechanism, geometry, and magnitude of the accretion flow, and (5) the spin of the NS. The mass of the neutron star itself is found in a rather narrow range around 1.4 solar masses. Characteristic observables are the overall luminosity, the shape of the spectrum and the time variability properties.
In this multi-parameter space accreting NS of different appearances occupy preferred regions. The first fundamental property is the mass of the companion. It is, therefore, useful and common to distinguish between High Mass X-ray Binaries (HMXB) and Low Mass X-ray Binaries (LMXB) (both of which include also systems with black hole candidates, see chapter 16). In HMXB the optical companion is a Be star or OB supergiant. They usually have long (days to tens of days) orbital periods and mass transfer is by stellar wind, while in the case of LMXB the companion is of spectral type A or later, the orbital periods are shorter (hours to days) and the mass transfer is by Roche Lobe overflow. High mass systems containing a Be star tend to have orbits with substantial eccentricity and the mass transfer is connected to the circumstellar structure (i.e., an equatorial disk) in these strongly rotating stars.
The second most important factor is the strength of the magnetic field (B) of the NS. In the case of a strong (1012 Gauss) field, the inner boundary of the accretion disk cannot reach close to the surface of the NS but will be truncated at the magne-tospheric radius, defined by equal pressure of the gas and the B field. The accreted material is funneled along the magnetic field lines down to the polar regions of the NS surface where an X-ray emitting hot spot forms (Fig. 15.2). If the magnetic axis is inclined to the rotational axis, a pulsar may be observed. If instead the magnetic field is weak (109 Gauss or less), the accretion can happen over larger parts of the NS surface and the material can accumulate for some time until a nuclear burning flash
Fig. 15.2 Magnetic NS and its magnetosphere may ignite, giving rise to X-ray bursts. Other types of X-ray bursts are generated when clumps of matter fall onto the NS surface within a short time.
There is a whole "zoo" of X-ray sources with respect to their observational appearances, which is of course governed by the physical conditions within the systems. The diagnostic tools for understanding the physics is provided through observations of the spectral and time variability properties of the sources over the widest possible ranges in wavelength and time scales, respectively. In the X-ray range, it has turned out that the study of the timing behavior, including spectral variability, is the tool with the highest diagnostic power. In reaching millisecond time resolution, the natural dynamical time scales in the immediate vicinity of NS are tested.
In the following, a brief introduction is given to some common types of X-ray sources containing accreting NS, describing their spectral and variability properties.
Transients are sources that are not persistent, but rather show up in episodes (days to weeks) of X-ray emission, sometimes recurrent in a regular or irregular way.
Eclipsing X-ray binaries allow a straight forward determination of their binary nature and an easy measurement of the binary period. They are usually found among the HMXB.
X-ray binary pulsars are characterized by a regular (coherent) modulation of the X-ray flux because of the spin of the NS. In these cases the binary nature is confirmed by the observation of Doppler shift of the pulse frequency, thereby allowing to measure the shape and physical parameters of the binary orbit. In this way also
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