Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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The X-ray power of the superluminal sources exhibits a large variety of quasi-periodic oscillations (QPOs) of high frequency. Of particular interest is the class of fast oscillations with a maximum stable frequency of 67 Hz observed many times in GRS 1915+105, irrespective of the X-ray luminosity of the source (Morgan et al 1997). A QPO with maximum fix frequency of 300 Hz has been observed in GRO J1655-40 (Remillard et al 1999). These stable maximum frequencies are not seen at times of strong radio flares or jet injection. They are believed to be a function of the fundamental properties of the black holes, namely, their mass and spin.

One possible interpretation is that these frequencies correspond to the last stable circular orbit around the black hole. This frequency depends on the black hole's mass and spin, as well as on the rotation direction of the accretion disk, and offers the prospect of inferring the spin of black holes with masses independently determined. Since from optical observations the mass of the hole in GRO J1655-40 is known to be in the range of 4-7 solar masses, one can conclude that GRO J1655-40 contains a Kerr black hole rotating at geq 70% of the maximum spin possible (Zhang et al 1997).

Alternatively, the maximum QPO stable frequency could be related to general relativity disk seismology, more specifically, to the maximum radial epicyclic frequency (Nowak et al 1997), which also depends on the spin of the black hole.

A third interpretation has been proposed in terms of the relativistic dragging of the inertial frame around the spinning black hole (Cui et al 1998). By comparing the computed disk precession frequency with that of the QPO, the spin can be derived if the mass is known. The two sources of sporadic superluminal jets are found to be the black holes that spin at rates close to maximum limit. Obviously, theoretical work to distinguish between these three alternative interpretations will be important to estimate the spin of the black holes with known masses.

X-ray spectroscopy of the two superluminal sources obtained with the satellite ASCA (Ebisawa 1996, Ueda et al 1998) has shown K H and He like iron absorption lines, whereas the observations with SAX have only shown emission features from the relativistic accretion disk around 7 keV, which have been interpreted as iron lines (Matt et al 1998). One expects that with greater sensitivity these lines will show a profile reminiscent of that of the asymmetric iron lines observed in Seyfert galaxies (Tanaka et al 1995). The accretion disks of GRS 1915+105 and GRO J1655-40 are viewed obliquely, and the blueshifted side of the lines should look much stronger due to the Doppler beaming effect. In addition, the center of the line should be redshifted as expected from general relativity effects on radiation escaping from the surroundings of a strongly gravitating object. In the future, perhaps these lines could be used as probes of general relativity effects in the innermost parts of the accretion flows into black holes.

General relativity theory in weak gravitational fields has been successfully tested by observing in the radio the expected decay in the orbit of a binary pulsar, an effect produced by gravitational radiation damping (Taylor & Weisberg 1982). Observations of binary pulsars have also been used to constrain the nature of gravity in the strong-field regime (Taylor et al 1992). Although the interpretation of the maximum stable frequency of the X-ray power spectrum in the superluminal sources is still uncertain, these frequencies are known to originate close to the horizon of the black hole, and perhaps they could be used in the future to test the physics of accretion disks and black holes in the strong field limit.

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