3.3. Temporal Properties

Low-activity galaxies tend to vary on long (months to year) time scales (c.f., M81 in Ishisaki et al. 1996; NGC 4579 in Serlemitsos, Ptak & Yaqoob 1996), but not on short time scales as observed in Seyferts (Ptak et al. 1998; however note that M81 has been observed to vary at the 30% level on day time scales; Pellegrini et al. 2000). Suprisingly, some of the most variable low-activity galaxies have been starbursts. For example, the nuclear source in the starburst NGC 3628 ``shut off'', varying by a factor of ~ 40 (Dahlem, Heckman, & Fabbiano 1995), and M82 has varied considerably in the 2-10 keV bandpass (Ptak & Griffiths 1999; Matsumoto & Tsusru 1999; Gruber et al., these proceedings). This implies that at least some of the contribution to the hard component in starburst galaxies is due to accreting sources. The lack of variability on short times scales is demonstrated in Figure 4. As argued in Ptak et al. (1998), this marked break from the temporal behavior of Seyfert 1s implies a large source extent for the X-ray producing regions in low-activity galaxies. While in some cases this might be due to a multiple sources of X-ray emission, it may also be due to the prevelance of advection-dominated accretion disks, in which the entire disk contributes to the X-ray emission, as opposed to the ``''-disks in Seyferts in which the X-rays are most likely produced by flares. On the other hand, the short-term variability observed in M82 may be our first look at the hard X-ray light curve of an IXO, assuming that the source of the 2-10 keV variability is the off-nuclear point source observed by the ROSAT HRI (Collura et al. 1994).

Figure 4. The trend of ``excess variance'', a measure of short-term variability, with X-ray luminosity in Seyfert 1s and low-activity galaxies, from Ptak et al. (1998).