F 011 False source by a hot pixel or by a bad bias value (Fig. 3b)
F 012 False source by a bad column (Fig. 3a)
F 013 False source along the readout direction of a very strong source
F 014 False source by the FEP 0/3 problem (http://cxc.harvard.edu/ciao/caveats)
F 015 Double sources detected by the PSF effect (Fig. 7)
X 021 Visual inspections found it as a spurious source
V 031 Bad pixel/column exists within source extraction radius
V 032 Nearby source exists within the source extraction circle
V 033 Nearby source exists within the background extraction annulus
V 034 Source is found near an extended source
V 035 Background region overlaps with a nearby extended source
V 036 Source near the edge of the chip
V 037 Pile-up (see Chandra POG 2000)
V 038 Uncertain source position by flag = 015 (Fig. 7)
O 051 Source is extended
O 052 Same source in multiple observations
O 053 Target of observation
O 054 X-ray jet
O 055 Variable source