Add link to IRF theory
@@ -78,7 +78,8 @@ Where :math:`F_{Src}` is the integrated flux of the source model,
:math:`\mathrm{EDISP}` the energy dispersion matrix and
:math:`\mathrm{PSF}` the PSF convolution kernel. The corresponding IRFs are extracted
at the current position of the model component defined by :math:`(l, b)` and assumed
-to be constant across the size of the source.
+to be constant across the size of the source. The detailed expressions to compute the
+predicted number of counts from a source and corresponding IRFs are given in `irf-theory`_.
.. _stack:
