CalDB 4.4.2 Public Release Notes

I. INTRODUCTION

CalDB 4.4.2 is an patch to the Chandra CalDB, which includes the following items:

• A correction to the ACIS branch index file to include ACIS CONTAM N0006 listings for ACIS-8 and ACIS-9. This prevents an ARDLIB initialization error for observations taken since 2011-01-01T00:00:00 (UTC).

For the CIAO 4.3 / CalDB 4.4.2 release notes see How CIAO 4.3 and CalDB 4.4.2 Affect Your Analysis

II. SUMMARY OF CHANGES

A. Correction to ACIS Index file for ACIS CONTAM v6

Location:	$CALDB/data/chandra/acis/contam/
Filename:	acisD1999-08-13contamN0006.fits

An error in the CalDB ACIS branch index listing for the above CONTAM N0006 file has been detected, and fixed in this patch of CalDB. The CONTAM file has ten BINTABLE extensions, one for each ACIS chip, and each is selectable by the chip number, or CCD_ID, ranging from zero to 9. The listings for ACIS-8 and -9 were mistakenly omitted from the CalDB 4.4.1 release, due to missing header keywords in the file. This has been rectified.

See the ACIS QE Contamination Why? Page for more information. Technical details concerning the new CalDB data file are given in the section below.

PIPELINES/TOOLS AFFECTED:

CIAO 4.3 (with ACIS-related data only): mkarf, mkwarf, fullgarf, mkinstmap

THREADS AFFECTED:

Only procedures relevant to the chips ACIS-8 (S4) and ACIS-9 (S5) are affected by this correction. The affect is to get ARDLIB to find the contam version N0006 file above for those two chips. The following threads may be relevant:

Using specextract to Extract ACIS Spectra and Response Files for Extended Sources
Using psextract to Extract ACIS Spectra and Response Files for Pointlike Sources
Weighting ARFs and RMFs
Step-by-Step Guide to Creating ACIS Spectra for Pointlike Sources
HETG/ACIS-S Grating ARFs
LETG/ACIS-S Grating ARFs
Object-centered Aspect Solution and Exposure Map

III. TECHNICAL DETAILS

A. Correction to ACIS Index file for ACIS CONTAM v6

The partial listing of the CONTAM N0006 file listing in the index in CalDB 4.4.1 creates a variety of minor issues that are solved by implementing the 4.4.2 patch. There are several avenues of affect that this mistake may have had, most of them harmless, but all will be dealt with here. On the one hand, there are users analyzing ACIS data with CIAO, on the other there are the PIMMS CY13 effective areas and proposal planning Grating ARFs (positive orders only).

Beginning with users, those analyzing ACIS-S/grating data, or ACIS-S4 imaging data for observations taken before the first of the year have nothing to be concerned about. The ARDLIB will select the CONTAM N0006 file for ACIS-4, -5, -6, and -7 automatically, and will select CONTAM N0005 for ACIS-8 and -9 automatically. For observations taken before 2011-01-01T00:00:00, the numerical effect on ARFs or GARFs with ACIS-8 is precisely zero. ACIS-S contamination corrections with CONTAM N0005 and N0006 are identical from launch date until that date.

For users analyzing ACIS-S data taken since the first of 2011, they need to install the CalDB 4.2.2 patch. Without it, an error will occur when computing GARFs or ARFs including ACIS-8 or -9, which will prevent them from completing the ARF calculation. The error that will occur is an ARDLIB initialization error, indicating the CONTAM file cannot be found. This happens because the N0006 listings for ACIS-8 and -9 are absent from the index, and the N0005 CONTAM file has been marked invalid after the first of 2011. Users of the script fullgarf are most likely to see this error.

Now, with respect to the PIMMS effective areas, the problem that occured due to the indexing mistake was more subtle, in that the model OBS_IDs used for ACIS-S/Grating first order effective areas (EAs) are dated before year 2011. So, the obvious error did not occur, and the N0005 CONTAM extensions for ACIS-8 and -9 were selected automatically. However, an additional TIME-delay qualifier is specified in the PIMMS process, to project the contamination correction estimate out into the future, which for CY13 was the date 2012-05-15T00:00:00 (TT=453427266.184 s).

It happens that CONTAM N0005 and N0006 begin to deviate after the precise date 2011-01-13T18:47:30 (TT=411331716.184 s). Furthermore, the prediction from CONTAM N0005 becomes flat after this date, while N0006 properly adds more contaminant correction. The resulting difference in the predicted contamination correction factor is given in Fig. 1 below, by the green trace. The sense of the difference plot is that N0006 predicts deeper and deeper absorption (flux loss) due to the contaminant. Using N0005 would then tend to overestimate the net effective area on ACIS-8 and -9.

Fig. 1: The difference in the CONTAM factor calculated from ACIS-S CONTAM version N0005 and N0006 for three different dates. Note the correction is the same for all the ACIS-S chips, for each file. The red line is the result for an input TIME (TT) which corresponds to (UTC) 2010-12-31T00:00:00. The green is for (UTC) 2012-05-15T00:00:00. The black curve is for (UTC) 2014-06-01T00:00:00, which is near the upper TIME-interpolation limit for CONTAM N0006.

For PIMMS CY13, only the first-order Grating EA's with ACIS-S are affected by the indexing mistake. The resulting error in the EAs is mitigated by the fact that we sum the positive and negative orders. The effect is maximal (from Fig. 1) at the Carbon-K edge (283 eV) which falls off the ends of the ACIS-S/HETG grating patterns, and sits on the ACIS-5 and -8 chips for ACIS-S/LETG. ACIS-5 is a BI chip, and so has a much larger QE at 283 eV than does ACIS-8. This swamps the error at 283 eV to less than 3% of the EA at that energy. For ACIS-S/MEG and ACIS-S/HEG first orders, the error is respectively smaller and smaller, but occurs mostly at the low-end of the EA spectrum, as the C-K edge location recedes further and further off the end of ACIS-9. Figure 2 a-f below gives the overlay plots and the difference plots for the several affected cases.

Fig. 2a: An overlay of the PIMMS ACIS-S/LEG1 summed first order configuration, for the case with the corrected ACIS index (black trace) against the current PIMMS CY13 data (red dash). The x-axis is restricted to give only the energy regions where differences can be detected.	Fig. 2b: The difference plot for Fig. 2a. The maximal difference of 3% occurs at the location of the ACIS-5 and -6 chip gap, where the contam error in ACIS-8 is emphasized, ~400-450 eV.
Fig. 2c: An overlay of the PIMMS ACIS-S/MEG1 summed first orders configuration. Again the black trace is the corrected trace, and the red-dash trace is the released dataset in PIMMS CY13.	Fig. 2d: The difference plot for Fig. 2c. The maximal difference of 2.1% occurs at the overlay of ACIS-4 and the far edge of ACIS-9. Both ACIS-8 and -9 have the contam error in the CY13 trace due to the ACIS indexing mistake; however the error is largest near the C-K edge, which falls of the far end of ACIS-9 in MEG.
Fig. 2e: An overlay of the PIMMS ACIS-S/HEG1 summed first orders configuration. The black trace is the corrected trace, and the red-dash trace is the released dataset in PIMMS CY13.	Fig. 2f: The difference plot for Fig. 2e. Maximal difference value is 0.85%, which corresponds to the extension of the grating pattern across the far edge of ACIS-9, which overlays with ACIS-4 in the pos-neg order sum.

Regarding the Proposal Planning Grating ARFs, published and described here, the orders are not summed as with PIMMS, and hence the postive order files including ACIS-8 and -9 in their configurations will be affected more strongly. Figures 3 a-h below give the overlay and difference plots for the +1 orders only for ACIS-S/LEG, MEG, and HEG, respectively. The higher orders are less affected, again due to the C-K edge receding from the ACIS-S chip array.

Fig. 3a: Overplot of the released (red dash) ACIS-S/LEG+1 GARF file and the corrected version (black).	Fig. 3b: The % difference plot from the traces in Fig. 3a. The black curve shows the difference for ACIS-9, the red for ACIS-8, and the blue for ACIS-7. The differences for the ACIS-7 portion are zero because there was not CONTAM file selection error there. Maximal difference is 11% just at the C-K edge, 283 eV.
Fig. 3c: Overplot of the released (red dash) ACIS-S/MEG+1 GARF file and the corrected version (black).	Fig. 3d: The % difference plot from the traces in Fig. 3c. The two absorption edges seen near 530 eV and 700 eV are O-K and F-K, components of the contamination layer on the OBF. The hitch at 1.35 keV is the edge of the ACIS-7 chip, where the difference is zero.
Fig. 3e: Overplot of the released (red dash) ACIS-S/HEG+1 GARF file and the corrected version (black).	Fig. 3f: The % difference plot from the traces in Fig. 3a. The difference trace is now featureless except for the increasing value as the energies approach the C-K edge at 283 eV, which occurs off the end of ACIS-9 for the HEG+1. ACIS-7 intervenes at 2.7 keV and above in this case.

In view of the relatively restricted regions where the resulting errors are greater than 5%, and the late hour of the proposal cycle at which this problem was discovered, the CXC has elected not to correct the proposal planning GARFs or the PIMMS effective areas due to the CONTAM indexing problem. The small over-estimates of the counting rates or efficiencies measured should not be expected to affect any proposal feasibility study or count rate estimate.

Users analyzing the newest obervations taken since the beginning of year 2011 are strongly encouraged to install or have installed at their site the new CalDB 4.4.2 patch to eliminate this ACIS CONTAM N0006 file selection error in ARDLIB (CIAO 4.3).