# EDM Tue Apr 19 14:45:49 EDT 2011

hcorat.f and hcorat.dat are files for a local XSPEC model for the XIS
contamination.  The parameters are:

C       = carbon column density (N_C) in units of 10^18 cm^-2
HtoC    = N_H / N_C ratio
OtoC    = N_O / N_C ratio

This is a multiplicative model like "varabs".  OtoC depends on time, HtoC
doesn't:

C    = free
OtoC = 0.27602*(1.-exp(-(MJD-53595.34)/341.65))
HtoC = 157.61

where "MJD" is the mean Julian day of the observation.

The on-axis value of C has a specific time dependence, and the off-axis
values include this time dependence as well as a radial dependence that I
don't exactly know.  So I would suggest fixing OtoC and HtoC to their
proper values for a given observation (with MJD deteremined from
"aetimecalc"), and allowing C to vary in the fit.

You will also need to either make new contamination-free ARFs for each
extraction region (with "xissimarfgen" parameter "contamifile=none")
or use the ARFs you've made, and divide the "specresp" column by the
"contami_tranmis" column to get a new contamination-free "specresp".

Let me know if you have problems installing the local model.  You do need
to have HEASOFT installed from source, not from binary, in order for it to
compile and work.