Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

2.2. Variability of the X-Ray Emission of Seyfert Galaxies

Here we mention only the key points and most recent results because an excellent review is given by (Mushotzky et al 1993). The X-ray emission of Seyfert galaxies consists of several components, including a power law in the medium energy X-ray range(1-10 keV; 0.9, where f ^-), a soft excess usually below 1 keV, and a reflection hump in the 10- to 30-keV range. Superimposed on this continuum is a prominent Fe line and often (50%) absorption edges from highly ionized oxygen (Fabian 1996). In the hard X-ray range (> 50 keV), the few OSSE data available indicate that the power law steepens, possibly with a cutoff around 100 keV (Johnson et al 1994).

Both the soft excess and the medium X-ray power-law component are variable, albeit differently; the soft excess is more strongly variable and is often but not always correlated with the medium energy X-rays. There are very few cases where the soft and medium X-ray variations appear uncorrelated. The best example is NGC 5548, observed 25 times at approximately daily intervals in December 1992 to January 1993.The soft excess component varied (factor 10) independently of the hard X-ray flux (factor 3), most noticeably in a soft X-ray flare lasting 8 days which had no medium X-ray counterpart (Done et al 1995).

The medium energy power-law component in general varies in intensity with very little change of the spectral index.In some well-observed AGN, the flux variations are accompanied by a softening of the spectrum with increasing intensity (e.g. Perola et al 1986, Mushotzky et al 1993 and references therein, Grandi et al 1992, Guainazzi et al 1994, Leighly et al 1996, Molendi et al 1993).

The most extreme soft X-ray variability occurs in Narrow-LineSeyfert 1 galaxies (NLS1), a subset of AGN with very steep softX-ray spectra (1 4 in the range 0.1-2.4 keV), narrow optical emission lines with full width at half maximum (FWHM) 2000 km s^-1, and prominent opticalFeII emission (Osterbrock & Pogge 1985, Boller & al 1996). To explain the absence of broad lines in NLS1, it has been proposed that the intense soft X-rays could blow away the inner broad-line region (BLR) or ionize it to states currently undetectable (Pounds et al 1995, Guilbert & Rees 1988). The steep soft X-ray spectrum of NLS1 may indicate a high accretion rate or a small black hole mass.In broad-line AGN, the medium-hard X-ray power law generally has luminosity significantly higher than the soft excess (Pounds & Brandt 1996), but in the NLS1 RE 1034+39, for example, the soft excess exceeds the luminosity of the medium-hard component, and moreover, the source has an exceptionally steep medium X-ray spectrum - characteristics shared with Galactic black hole candidates. This suggests that in NLS1, or at least in some of them, the accretion rate is close to the Eddington limit and the soft X-rays represent viscous heating of the accretion disk (Pounds et al 1995). Alternatively, a small black hole with an accretion rate of ~0.1 Eddington accretion rate could also emit a very hot spectrum with such an intense soft X-ray component.

Although the NLS1 display the most extreme soft X-ray variations, their range of variability merges with that of "classical" broad-line AGN. Here we summarize a few of the most spectacular examples of variability.The largest variation observed in one year was by a factor of 70 in RE J 1237+264; this object has remained weak and has the very same steep slope as in the high state (Brandt et al 1996). An optical spectrum taken a few months after the soft X-ray outburst shows emission lines of [FeX] and H that are approximately 10 times brighter than those observed before or well after the burst (Pounds & Brandt 1996, Brandt et al 1996).

Strong variations are also seen in the FeII strong AGN PHL 1092; if the radiation is isotropic, the rapid variability requires that mass be transformed into energy with an efficiency of at least 0.13, exceeding the theoretical maximum for a nonrotating black hole (Forster & Halpern 1996). Somewhat in contrast, a drastic spectral change from ultrasoft to typical soft X-ray Seyfert spectrum in RX J0134-42 occurred without change in count rate (Mannheim et al 1996; see also Pounds et al 1995).

An exceptional case is the persistent giant and rapid soft X-ray flux variability of the radio-quiet, ultrasoft, strong FeII, narrow-line Seyfert 1 galaxy IRAS 13224-3809 (Boller, Brandt, Fabian & Fink 1997). In the first systematic monitoring of an ultrasoft NLS1, a 30-day Roentgen Satellite (ROSAT) High Resolution Imager observation revealed at least five giant amplitude variations, with the maximum observed amplitude of about a factor of 60. A variation by about a factor of 57 was detected in just two days. Variations by a factor of about 30 were also seen to occur during a 1994 observation with the Advanced Satellite for Cosmology and Astrophysics (ASCA; Otani et al 1996). Relativistic boosting effects provide the most plausible explanation of the X-ray data and may be relevant to understanding the strong X-ray variability of some steep spectrum Seyferts more generally. The variability is probably nonlinear in character, which suggests that flares and spots in the accretion disk interact nonlinearly or are affected by nonlinear flux amplification.

X-ray variability by a factor of about 50 has also been observed in one broad-line AGN, E1615+061 (Piro et al 1997). The high-state spectrum was very steep ( ~ 3), whereas the low-state slope was near normal ( ~ 1). There is only an upper limit to the variation time scale (16 years) and the soft X-ray activity of this broad-line AGN may not be related to the extremely fast soft X-ray variability of the NLS1 class. Finally, another extraordinary extragalactic X-ray transient is the NLS1 WPVS007, which decreased by a factor of more than 400 between the ROSAT All Sky Survey and the ROSAT pointed observations three years later (Grupe 1996).