# Initial Conditions & Lookup Tables¶

## The Opacity of the Intergalactic Medium¶

Solutions for the evolution of the cosmic X-ray background are greatly accelerated if one tabulates the IGM opacity, $$\tau_{\nu}(z, z^{\prime})$$, ahead of time (see in Appendix C of Haardt & Madau (1996) for some discussion of this technique). ARES automatically looks in $ARES/input/optical_depth for $$\tau_{\nu}(z, z^{\prime})$$ lookup tables. The shape of the lookup table is defined by the redshift range being considered (set by the parameters first_light_redshift and final_redshift), the number of redshift bins used to sample that interval, tau_redshift_bins, the minimum and maximum photon energies (pop_Emin and pop_Emax), and the number of photon energies (determined iteratively from the redshift and energy intervals and the value of tau_redshift_bins). By default, ares generates tables assuming the IGM is fully neutral, but that is not required. To make optical depth tables of your own, see $ARES/input/optical_depth/generate_optical_depth_tables.py. See Section 3 of Mirocha (2014) for more discussion of this technique.

### Tables for mirocha2017-like calculations¶

To generate the table used for the calculations in Mirocha, Furlanetto, & Sun (2017), modify the following lines of \$ARES/input/optical_depth/generate_optical_depth_tables.py:

zf, zi = (5, 50)
Emin = 2e2
Emax = 3e4
Nz = [1e3]
helium = 1


Note

You can run the generate_optical_depth_tables.py script in parallel via, e.g., mpirun -np 4 generate_optical_depth_tables.py, so long as you have MPI and mpi4py installed.

The set of parameters used for these calculations are described in the “Simulations” section of Parameter Bundles.