Introduction

X-ray emission provides ∼ 10% of the bolometric luminosity of a typical Seyfert galaxy and as X-ray photons of energy > 2 keV can penetrate column densities of > 1 × 1022 atoms cm−2, such observations provide the best observational probe available (with current instrumentation) of the active nucleus, and its immediate environment.

Variations cannot be observed faster than the light travel time, thus δt ∼ r/c gives an upper limit to the size of the emitting region. A variability timescale may be associated with the dynamical (orbital) timescale of the inner accretion disk. If the X-rays arise from r ∼ 5rs and rs = 2GM/c2 then t ∼ 50M6S (M6 are units of 106 solar mass). This is reasonable if a persistent period were found, but could be misleading otherwise.

Two other parameters of interest are efficiency η (of mass to energy conversion) and compactness ℓ. Efficiency, η ∼ 5 × 10−43dL/dt, if η > 0.1 an exotic mechanism is required (such as relativistic beaming). Compactness, ℓ = Lστ/Rmec3 (Svensson 1986; Guilbert, Fabian & Rees 1983). When ℓ > 10 the source becomes optically thick to γ-rays and pair production becomes important (assuming the spectrum extends to ∼> 1MeV), affecting the X-ray spectrum and the temporal behaviour (e.g., Mosalik & Sikora 1986; Fabian et al. 1986).

Historical

Ariel V and HEAO-1 established long term (days to years) variability as a property of AGN (e.g., Marshall, Warwick & Pounds 1981), with variations in amplitude of factors of ∼ 7 or so being common.