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Fluxes and energies are read from a text file, with name given by the `spectrum_file' keyword. The format of the file is as follows: the first line must contain the integer number of (energy, flux) pairs; each of the the remaining lines contains one (energy, flux) pair, with energy in eV and flux in energy units (overall normalization is arbitrary). These values will be interpolated onto the energy grid used internally by XSTAR using logarithmic interpolation. An example of a file which results in a $\varepsilon^{-1}$ spectrum power law spectrum between 0.1 Ry and 1000 Ry (and zero elsewhere) is as follows:

1.e-3  1.e-10
1.359 1.e-10
1.3598 1.e+11
1.3598e+5 1.e+6
1.360e+5 1.e-10
2.e+5  1.e-10

As already states, be aware that small numbers (less than 1.e-30, say), may result in undesired results owing to the limitations of many machines in the range of exponents. And remember that it is not necessary to use actual physical units in specifying the input spectrum (except to distinguish between photon and energy fluxes) since the entire spectrum is renormalized to conform to the luminosity specified.

Tim Kallman 2018-12-14