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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.

** Next:** 3.2.1 Flux of a
**Up:** 3. Feasibility of observations
** Previous:** 3.1 General Assumptions
*rsdc@cfa.harvard.edu *

1999-05-25