The Chandra Source Catalog includes detected sources whose flux estimates are at least three times their estimated 1 σ uncertainties, typically corresponding to about 10 net source counts on-axis and roughly 20-30 net source counts off-axis.
Master Sources Table:
The source significance in the Master Sources Table represents the highest value of the source flux significance among all science energy bands and individual source observations contributing to the merged source entry.
Source Observations Table:
The detection significance is the signal-to-noise ratio of the source in the source detection region output by wavdetect, an algorithm used to identify source candidates within a single observation. wavdetect computes the source significance of a detection by dividing the net source counts in the source detection region by the Gehrels error of the sum of the estimated background counts (using the background maps supplied to wavdetect) in each pixel of the source detection region.
The flux significance is the flux-based signal-to-noise ratio of the source in the source region. While the detection significance is calculated relative to the background maps, the flux significance uses a local background estimate; it is defined as the photflux_aper value divided by an equivalent Gaussian sigma* of the flux in the annular background region.
*The standard deviation of a 1-D Gaussian distribution that has the same FWHM as the probability density function from which the upper and lower confidence limits are determined.