Caveats about Vignetting sign error bug
This bug has been fixed in CIAO 4.14.
The CXC has identified a sign error when applying the mirror vignetting correction to the effective area in the following tools:
- mkarf: used to create on-axis point source ARFs.
- mkgarf: used to create ARFs for grating dispersed spectra.
- mkinstmap: creates instrument maps, used by mkexpmap to create exposure maps.
The bug causes an incorrect azimuthal angle, Φ, to be used: 180-Φ. This can introduce an error in the effective area up to several percent. The error is both spatially dependent and energy dependent. It affects both ACIS and HRC responses (exposure maps and point-source ARFs) as well as grating ARFs. This bug has always been in CIAO (not something recently introduced).
The variation in the vignetting component of the effective area is relatively small, generally a few percent peak-to-peak, as shown in Figure 1.
Figure 1. Vignetting vs. Azimuthal Angle
and varies with off-axis angle as shown in Figure 2.
Figure 2. Vignetting vs. Energy
Taken together we can see the effect in an example exposure map for ObsID 5532, ACIS-2367. In Figure 3 we see the exposure map at 1.5keV. The aimpoint is on ACIS-7, with two -I array chips turned on. In Figure 4 we see the fractional difference between the exposure before and after the sign correction is applied. We can see that near the aimpoint the fractional difference is very small (less than 0.2%). At large off axis angles, the difference increases up to +/-2%.
Figure 3. Exposure Map for ObsID 5532 at 1.5keV
Figure 4. Fractional Difference in Exposure Map
We can see the energy dependence by computing the ARF at an off-axis location 4 arcmin away from the aimpoint. Note: mkarf should generally only be used for near on-axis point-like sources; we used it here for illustrative purposes. Figure 5 shows the ARFs before and after the vignetting sign correction bug has been fixed. Figure 6 shows the percent difference in the ARFs. As we can see there is slight energy dependence to the percentage difference at this location.
Figure 5. An example ARF before and after bug fixed
Figure 6. Percent difference in ARFs
As we can see from these examples, the magnitude of the error is generally small, especially near the aimpoint.
Furthermore, the CXC Calibration team has done a comprehensive review of existing calibration products and have determined that this bug does not affect them. That is the current calibrations are not already in some how compensating for this bug.