3.2 Point Source Sensitivity

The point source sensitivity for an observation, and therefore the required observation time, can be approached in several different ways depending upon what specific question is being asked. In all cases, the required point-source sensitivity is determined by the minimum number of source counts needed to satisfy particular statistical criteria. For example, if the scientific goal is to measure the X-ray flux from a specific object, then the appropriate signal to noise requirement might be 3. On the other hand, the determination of source existence might require a signal to noise ratio of 5 in order to avoid spurious detections due to statistical fluctuations. Three different perspectives for the determination of the required exposure for an observation are considered:

• It is desired to know the flux of a given source. In this context, Gaussian statistics are appropriate as quantitative information about the source is required, rather than a simple detection.

• It is desired to know whether a given source emits X-rays. In this context, Poisson statistics are appropriate as existence, not flux, is at issue.

• It is desired to search a field of an observation for X-ray sources. In this context either Poisson or Gaussian statistics can be used depending on the scientific goals of the observer. However, an additional aspect must be considered: the number of false detections to be expected over the search region.