What are IRFs?

The instrument response functions (IRFs) provide a mathematical description that links the reconstructed photon arrival direction \(p'\), energy \(E'\) and trigger time \(t'\) of an event to the true incident direction \(p\), energy \(E\) and time \(t\) of a photon. The following figure illustrates this relationship:

\(I(p,E,t)\) is the gamma-ray intensity arriving at Earth as a function of direction \(p\), energy \(E\) and time \(t\). \(e(p',E',t')\) is the expected event rate as function of the reconstructed direction \(p'\), energy \(E'\) and trigger time \(t'\). The expected event rate is obtained by integrating the product of the instrumental response function \(R(p',E',t'|p,E,t)\) and the emitted intensity \(I(p,E,t)\) over the direction, energy and time.