Orbits and Super Orbital Periods

The primary indicator to associate a newly detected X-ray source with an accreting NS is evidence for binary motion, which can come from one or more of the following observations: eclipses, smooth periodic modulation, regular absorption dips, regular or semiregular X-ray outbursts, periodic pulsar arrival time modulation, or radial velocity variations. The X-ray emitting compact object could still be a black hole (BH) or a white dwarf (WD). The distinction between NS and BH is not always easy (see chapter 16). If regular pulsations or Type I X-ray bursts are observed, the compact object is a NS. If a very soft spectrum (possibly with a power law type tail) and fast irregular variability is seen, a BH might be suspected. The most reliable signature for a BH, however, is a mass of the compact object in excess of —3M0. Similarly, a WD is identified by masses < 1M0 and by their optical and UV appearance (see chapter 12). To distinguish between high and low mass NS binaries, the spectral type of the optically identified companion is used. In the case that orbital parameters are determined through pulse arrival times, the mass function is known. If neither of the above information is available, one would tentatively classify an object as a LMXB if a soft X-ray spectrum, Type I X-ray bursts, or an orbit of less than 12 h are observed, and as a HMXB if a hard (power law type) X-ray spectrum, X-ray pulsations, or an orbit greater than 12 h are observed.

In several systems modulation of the X-ray flux with periods (or quasiperiods) of the order of a few weeks to several months, much longer than the orbital period, is observed [43]. These super-orbital periods are generally attributed to the accretion disk which, through precessional motion, (semi)regularly blocks the line of sight to the X-ray emitting region.

15.2.3 Accretion Physics

The radiation observed from accreting NS is predominantly in the X-ray range (0.1 keV to beyond 100 keV) and mostly originates from the surface of the NS with contributions from the accretion disk. The radiation mechanism is mostly thermal Bremsstrahlung and black body emission with temperatures in excess of 107 K. Comptonisation by hot thermal electrons is thought to be responsible for spectra characterized by a hard power law and a high energy cut-off. Strongly magnetized NS tend to show one or more Cyclotron resonant scattering lines in their spectra. In the case of LMXB also UV and optical radiation from the accretion disk are observed. In interpreting the observed spectra, secondary effects like absorption and scattering in cold and warm (partially ionized) material, as well as fluorescence and recombination need to be taken into account.

Accreting NS provide a laboratory for accretion physics. The process of accretion, i.e., conversion of gravitational/potential energy into radiation energy, is the most effective energy conversion process (of order 10%, as compared to 0.7% for nuclear fusion). Protons falling down the deep potential well of a NS and being abruptly stopped at the surface of the NS (—10 km radius) reach velocities —0.4 of the speed of light with the corresponding kinetic energy of —200 MeV. The accretion luminosity is given by Lacc = n (— 1037 m17 (M0 (i0b) ergs-1, where M is the mass of the NS, R its radius, n is the energy conversion efficiency, m is the mass accretion rate and m17 the same in units of 1017gs-1, and MQ the mass of the sun. This means that a mass accretion rate of 1017gs-1 (equivalent to 10-9MQyr-1) yields an X-ray luminosity of 1037ergs-1. The thermalized kinetic energy causes high temperatures (106 to beyond 108K), leading to thermonuclear burning in the case of sufficient density and to direct radiation in the X-ray range.

The material accreted from the companion carries angular momentum that needs to be dissipated before the material can fall down onto the NS surface. This happens in an accretion disk where particles circle the NS in quasi Keplerian orbits. Through viscosity, angular momentum is redistributed such that some material can spiral down toward the NS. In the case of low B field, the accretion disk can reach close to the NS surface, for high B field, however, the magnetic field will start to dominate the movement of the plasma at the magnetospheric radius and from there on guide the material along the field lines down to the polar regions of the NS surface (see Fig. 15.2). The interaction between the material and the magnetic field can be substantial, leading to transfer of angular momentum between the accreted material and the NS, causing, depending on the exact conditions, spin-up or spin-down of the NS.

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