## 4. The emissionThe emission process is the same as outlined in Paper I, to
which we refer. For the generic "j" collision the emission
, in erg s Here the cooling time, , is defined as the time the plasma sphere takes to become optically thin: with the Thompson cross section,
cm s If all collisions with all possible combinations of parameters take
place, the mean emission in erg s whose order of magnitude and parameter dependence can be derived as However, in practice we shall only observe the contribution of the
collisions which take place in a certain time interval. Hence we can
derive the emission light curve adding at each time The observed mean luminosity of the source is then For a sufficiently large number of collisions
clearly approaches In conclusion the procedure used to derive the emission from the star cluster orbiting around the black hole is as follows. For each set of random chosen parameters, i.e. for each collision, the light curve is computed. From the sum of all these contributions the overall light curve and the mean luminosity are deduced. As the approximate expression (9) for the luminosity shows, the mean luminosity value is directly proportional to and, on the other hand, the light curve shape obviously depends on the collision rate. In Paper I, the authors arrived to the conclusion that, in order to reproduce the observed AGN features, collision rates in the range collisions/yr are required. An easy but qualitative way to arrive to an analogous conclusion is from variability observations, which report intensity variations on time scales in the range (some year days). In the framework in which these intensity variations are due to star collisions occurring at intervals of (), it follows collisions/yr. As an example Fig. 1 shows a light curve with a reasonable value of .
Hence, in the scenario in which AGN luminosity variations are explained by means of stellar collisions, the model must account for collision rate values of the order of ten. However, obtaining collision rate values of the order of ten independently of the AGN physical system, i.e. of the central mass amount, is not obvious. To better explain this point the parameter dependence of can be derived from expression (6) assuming that the star density is constant inside and vanishing outside: So, for different values we can obtain the same value using almost identical values of and (which means different values of the quantity ) or, if is fixed, using different values, suitably chosen, of : Hence, values in the `good' range
can be obtained either changing the system scaled dimensions
or the central star density
or both. So the star cluster core
extension, if defined in terms of ,
will be different in systems with different central masses: we can
suppose that the lowest central masses
(, see Sect. 3.2) are related to
larger ratios in order to avoid too
high star density. For example we obtain the same collision rate
collisions/yr for
,
stars/pc The consequences of the above analysis are twofold: i) high star densities are generally required in order to have suitable values; ii) a change in the density parameters and can also be representative of a change of the central black hole mass. © European Southern Observatory (ESO) 2000 Online publication: June 26, 2000 |