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## Region of Interest

The simplest method to find a flux is to sum the counts in the region of interest

`isis> p = region_sum (10, 14.99, 15.05);`

`isis> p.sum;`

0.000126637

Where again the units are Photons s cm. This works best for lines with little continuum or where there are nearby line free regions so that the continuum may be determined and removed from the line emission. The continuum level can be measured using the same method that we used for the line flux.

To be of practical use at least two conditions must be met. The first is that the arf for that region must be fairly constant across the region of interest. This assumption can be checked using the method described in the section on forward folding. The second condition is that the continuum must be fairly well determined. For a weak continuum errors in determining the background level will not affect the line flux much. If the continuum is fairly flat then measuring its contribution off the line and assuming that that is a reasonable estimator for the continuum under the line may be adequate. Finally in some regions the continuum may not be evident, eg. in line rich regions. In this case the continuum may be determined from other methods, for example a theoretical model. In each of these cases the user must determine if this is adequate for the science that they are trying to achieve.