Aperture Photometry Values and Errors
In CSC Release 2.0, both values and bounds for aperture photometry quantities are derived from Bayesian background-marginalized posterior probability distribution for the photometry quantity, assuming non-informative priors. These probability distributions are provided in the phot3 data products for each source.
Aperture photometry quantities include such as src_cnts_aper_〈band〉, src_cnts_aper_lolim_〈band〉, src_cnts_aper_hilim_〈band〉, and similarly for other intensity units src_rate_aper, photflux_aper, and flux_aper, and corresponding 〈intensity〉_aper90_〈value,lolim,hilim〉_〈band〉 quantities.
In particular, the value for the photometry quantity is the mode of the distribution and the limits define the smallest, non-negative interval that includes the mode and corresponds to the desired confidence level.
For some sources, both value and lower bound are 0. These represent upper limits, for which the posterior probability distribution is assumed to be an exponential function. For these sources, the hilim quantities represent a 68% confidence upper limit for the source intensity.
Complete details of the aperture photometry algorithms are contained in the documents "Determining X-ray Source Intensity and Confidence Bounds in Crowded Fields" and "Aperture Photometry Specifications for CSC2".