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Last modified: 19 November 2019

URL: https://cxc.cfa.harvard.edu/ciao/bugs/srcflux.html

Bugs: srcflux


Table of Contents

Bugs


Bugs

Error in the model independent NET_FLUX_APER and the NET_PHOTFLUX_APER values (and uncertainties).

There is an error in the computation of the model independent NET_FLUX_APER and the NET_PHOTFLUX_APER values reported in ${root}.flux output file when using psfmethod=file or arfcorr or marx. The fraction of the source flux in the background region is getting incorrectly scaled by the ratio of the area of source-to-background.

Consider the default case where the background area to source area is approximately r=24 (background annulus outer radius is 5 times the source radius) and has typical PSF fractions of a=90% in the source region, and b=9% in the background region. The error in the fluxes will be the ratio of \(\frac{(r*a-b)}{(r*(a-b))} = 1.106\), so a a 10.6% over-estimate of the photon flux. The same error is propagated to the LO and HI confidence interval limits for both NET_FLUX_APER and NET_PHOTFLUX_APER values. The actual values for r,a,b can be obtained from the columns in the ${root}.flux file: r = BG_AREA/AREA, a = PSFFRAC, and b = BG_PSFFRAC.

The offchip flag may be incorrectly set for some observations.

The offchip flag may be incorrectly set for observations with large DY or DZ offsets. This does not affect any of the flux values nor any of the rates.