## PI: Pulse Invariant

The value of PI for an event is given by the equation:

  PI = [(energy/14.6 eV) + 1],


where energy is a column in the event file. The energy is calculated from the event's PHA value, using the appropriate gain table. PI is an integer, not a real and the decimal portion of the computation is discarded; that is, the value is not rounded, but truncated. For example:

energy (eV) (energy/14.6 eV) + 1 PI
992.9 69.01 69
1007.3 69.99 69

The energy and PI values are sufficiently accurate to filter the event data into different wide energy bands.

For spectral fitting, PI values should be used and one must use an RMF to account for the energy resolution of the instrument.