Execution Time

XSTAR is designed to strike a balance between accuracy and speed, but this inevitably involves some disparity between different computing platforms. As a result, many problems of interest require large amounts of time (i.e. hours) on machines which are relatively slow, or which are heavily used for other tasks. As an example, the optically thin ionization balance model shown in chapter 2 required $\sim$15 minutes on a $\simeq$500 MHz alpha machine, but $\geq$ 2 hours on a 200 MHz 680x0 Linux machine. In an effort to avoid wasted time (and CPU cycles) we offer the following suggestions: (i) XSTAR does not attempt to calculate ionization, excitation, etc. for elements whose abundances are specified to be less than 10$^{-15}$ relative to hydrogen. Large reductions in computation time can be achieved by zeroing the abundances of elements which are likely to be unimportant anyway: calcium, argon, and nickel. (ii) For some purposes constant temperature is an adequate approximation, and is often a useful preliminary step in deciding parameter values such as column density, and require a fraction of the execution time of full thermal equilibrium models. (iii) For some purposes a low column density ($\leq$ 10$^{18}$ cm$^{-2}$) will provide sufficient information. Large column densities require significantly more execution time. If large columns are needed, then execution can be speeded up by use of a large value of emult, or a small value of taumax, ans by setting npass to 1.

The parameter files included in the source tree for both xstar and xstar2xspec are set to perform constant temperature models, in order to allow the user to become familiar with xstar without requiring large investments in computer time.