the mass function of the system is determined, allowing an estimate of the mass of theNS.

Bursters are sources that show repeated short duration (minutes) bursts of X-ray flux. These sources are all LMXB with weakly magnetized NS. One has learned to distinguish between two types of bursts: Type I, associated with thermonuclear flashes on the surface of the NS and Type II, first found in addition to Type I bursts in the so called Rapid Burster, which are interpreted as events of intermittent accretion because of disk instabilities.

QPO sources are sources that show so called quasi periodic oscillations (QPOs). In contrast to the coherent modulation of pulsars, this kind of variability reveals itself by broad peaks in the power spectral density function (PSD) obtained in a Fourier analysis of the timing data. Initially detected QPOs in LMXB had frequencies in the range of a few Hertz to a few tens of Hertz. They are usually associated with irregular variability ("noise" components in the PSD) at low and high frequencies. Similar variability properties are also observed in black hole candidates. The QPO sources are all LMXB and tend to exhibit characteristic spectral variations, describable by a smooth movement through a "color/color diagram" (Sect. 15.6). According to the shape of the patterns of the movements, such sources are called Z-sources or Atoll-sources. A more recent discovery, made possible by the Rossi X-ray Timing Explorer (RXTE), are the kHz QPOs, which provide information about the very close vicinity of a spinning, accreting NS.

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