Iron lines from Galactic Black Hole Candidates

A crucial difference between GBHCs and AGN are their characteristic time scales. Except for the least massive AGN, the viscous time scales characterizing the inner disk can be many tens of years. Since the viscous time is the timescale on which the mass accretion rate can change appreciably, we see that it is impossible to sample a range of mass accretion rates within a particular AGN on a human timescale. On the other hand, the viscous time scales in GBHCs can range from days to weeks, so individual observations can, and likely do, represent different intrinsic accretion rates. The same is true for dynamical and thermal time scales. A typical X-ray observation lasting several tens of thousands of seconds represents only a few thermal and dynamical time scales in an AGN, but many thousands of such time scales in GBHCs. In fact, a reasonable estimate of the thermal time scale in a stellar mass black hole corresponds to a few seconds, which often also corresponds to the time scale of the peak variability power in the X-ray lightcurve of such sources.

In many ways, because of these differences in characteristic time scales, the study of stellar mass black hole candidates and AGN are complementary. One figure of merit is the square root (since one is typically dealing with Poisson statistics) of the flux received over a characteristic time scale, i.e., $[(L/D^2)(GM/c^3)]^{1/2}$, where $L$ is the source luminosity and $D$ is its distance. For sources with the same fractional Eddington luminosity, the luminosity scales as mass, so this figure of merit scales as $(M/D)$. Thus, although AGN are one thousand or more times further away than stellar mass black holes, they are also up to one hundred million times more luminous and therefore are much more effectively observed over their characteristic dynamical time scales. Stellar mass black holes, however, are much more readily observed over multiple viscous time scales. Ten thousand seconds in the life of a GBHC can be equivalent to as much as a thousand years in the life of an AGN.