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3.3 X-Ray Variability

Active galactic nuclei vary most conspicuously in hard X-rays (2 - 10 keV). One might hope to use the variability timescale to constrain the size of the X-ray emitting region and hence to estimate the central mass. However, no simple pattern of variability emerges, and defining a meaningful timescale is ambiguous. One approach uses the ``fastest doubling time,'' Deltat, to establish a maximum source size R appeq c Deltat. High-energy photons presumably come from the hot, inner regions of the accretion disk or in an overlying hot corona. For example, if R appeq 5 RS, as deduced in some models, we obtain an upper limit to the mass, MBH ltapprox (c3 / 10G) Deltat ~ 104 Deltat Msun (Deltat in s). Masses estimated in this way are generally consistent with those obtained from other virial arguments, but they are considerably less robust because of uncertainties in associating the variability timescale with a source size. For example, the x-ray intensity variations could originate from localized ``hotspots'' in the accretion flow.

X-ray reverberation mapping may in the future be a more powerful tool. The iron Kalpha line is widely believed to be produced by reprocessing of the hard X-ray continuum by the accretion disk. The strikingly large width and skewness of the line profiles (Figure 1), now routinely detected with ASCA, reflect the plasma bulk motion within 10 - 100 gravitational radii of the center. The temporal response of the line strength and line profile depends on a number of factors that, in principle, can be modeled theoretically; these include the geometry of the X-ray source, the structure of the disk, and the assumed (Schwarzschild or Kerr) metric of the black hole. Time-resolved X-ray spectroscopy should become feasible with the X-ray Multi-Mirror Mission (XMM) in the near future. We can then look forward to constraints both on the masses and the spins of BHs.