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ACIS QE Contamination

Introduction

The effective low-energy ACIS QE is lower now than it was at launch. This problem is thought to be associated with the deposition of one or more materials on the ACIS detectors or optical blocking filters. Since the depth of these contaminants is growing with time, the effective low-energy QE is becoming lower as time passes. A correction for this contamination is incorporated when creating ACIS response files.

The ACIS QE contamination model also accounts for spatial variations in the contamination on the ACIS optical blocking filters. The contamination is expressed as a function of time, energy, and ACIS chip coordinates. For imaging analysis of extended sources or point sources far off-axis, there is a significant change in instrument and exposure maps when the calibration is applied.

The response tools are designed to incorporate corrections for ACIS contamination via ARDLIB and a CALDB contamination file. The necessary calibration files have been available since CALDB 2.26 (2 February 2004) and most recently updated in CALDB 4.7.9 (28 June 2018) and CALDB 4.7.3 (15 December 2016).

The versions N0011 and N0010 models are the result of recent ACIS monitoring and modeling and allows for improved fits to standard extended source spectra with stable photoabsorption and other fitted parameters of their systematic models, but the degree of such a change on the results for various source models is hard to estimate without any particulars. The N0011 model is an improvement over the N0010 for the ACIS-I chips, for observations since the start of 2016, but otherwise, the two models are identical for the ACIS-S chips. The models are intended to replace all previous versions (including N0009) of the contamination model.

The N0010 model overestimates the absorption affect on ACIS-I, particularly starting in early 2018. While the ACIS-S N0010 model also diverges from the observed contamination effects during the same time period, development of a modified ACIS-S contamination model is ongoing.

To learn about the previous versions of the contamination model, particularly the version N0008/N0009 models, can be found.

The overall affect of the contaminant build-up is best illustrated using the convolved spectra from Abell 1795—a stable source observed annually for calibration purposes since the start of the mission—demonstrating the loss of ACIS-S effective area over time.

ACIS-S Time-Evolution

ACIS-S Time-Evolution

The result of a simultaneous fit spectra, extracted from nine observations of A1759 in 2000, 2002, 2004, 2005, 2009, 2010, 2011,2012 and 2013.

Technical Details

As the Chandra mission proceeds, the contamination on the ACIS Optical Blocking Filters (OBF) for ACIS-I and ACIS-S continues to evolve, with a marked increase in the rate of accumulation since the middle of 2009. While the cause of the increased accumulation is not well-understood, the deposition curve has shown near-linear, monotonically increasing behavior with time.

Time-dependence (center of chips)

Time-dependence (center of chips)

The time-evolution of the optical depth of the contaminant layer on the optical blocking filter in the center of both ACIS-I (square) and ACIS-S (diamond) from A1795 measurements, and external calibration source L/K measurements (triangle).

The rapid increase in the contaminant indicates that ACIS-S is not ideal for very soft sources in the future, particularly below the 283 eV carbon edge.

The contamination model was developed in a systemmatic procedure, using observations of standard sources A1795 and Mkn421, on ACIS-I and ACIS-S separately. The version N0010 model—and propagated into the N0011 model—included new time-dependent and spatial variation of several model components (C, O, and F) known to dominate the chemical composition of the contaminating layer on the ACIS OBF. The spatial model, in particular, was much improved over prior models, giving more consistent results at off-axis pointings on both ACIS-I and ACIS-S. The issues with the lack of knowledge on the contamination layer optical depth at high CHIPY locations on ACIS-S3 caused problems in prior models, but the new spatial model allows for asymmetry in the ACIS-S CHIPY contamination layer to provide better fits.

ACIS-S/LEG-1 Effective Areas

ACIS-S/LEG-1 Effective Areas

To illustrate the spatial variations in the model, first-order ACIS-S/LETG effective areas derived from various ObsIDs with the same Z-offset (-8 mm) and Y-offset (1.5 arcmin). ObsIDs from four different years are chosen as shown in the key. The solid curves are done with N0010 model; the dashed curves with N0009 model.

The subsequent change in ACIS-I effective areas (EAs) between the N0009 and N0010 is illustrated below.

ACIS-I Effective Areas

ACIS-I Effective Areas

The ACIS-I aimpoint effective areas versus energy for mid-year times as indicated in the key. The solid curves were calculated with the new N0010 file; the dashed curves of the corresponding color were with N0009.

Similarly, the change in ACIS-S EAs are illustrated below.

ACIS-S Effective Areas

ACIS-S Effective Areas

The ACIS-S aimpoint effective areas versus energy for mid-year times as indicated in the key. The solid curves were calculated with the new N0010 file; the dashed curves of the corresponding color were with N0009.

For both ACIS-I and ACIS-S, the new models only have a minor affect on the pre-2005 EAs; and while the contamination is negligible above 2.0 keV, but near the O-K absorption edge at 0.535 keV, the difference between the N0009 and N0010 models is not insignificant, so the new contamination model will affect low-energy source modeling results for observations after 2005, especially analysis where a significant portion of the considered spectrum is below 1.0 keV.

However, starting in late-2017, calibration observations started to show that the predictive component of the N0010 contamination model was over-estimating the rate of accumulation on the OBF and the affects of contamination, considerably reducing the effective area of data products generated with the model across the 0.06-3.0 keV energy range compared to the true effective area. This would require adjustments to both the ACIS-I and ACIS-S time-dependent contamination models. The N0011 contamination model is a modification of only the N0010 ACIS-I model, with no changes to the ACIS-S time and spatial dependence, requiring more time to refine and validate before being released in a subsequent contamination model.

ACIS-I3 Effective Areas

ACIS-I3 Effective Areas

The effective areas from the N0011 (dashed) compared with those from the N0010 (solid) contamination models for the ACIS-I3 aimpoint. The red curves are mid-cycle from the year 2000, the green curves from 2010, blue curves from 2016, and cyan curves from 2018.

The effective area traces for 2000, 2010, and 2016 show only small apparent changes between the N0010 and N0011 contamination models, but for the year 2018, there is considerably less contamination affect in this soft band for the newer model, reflecting source spectra observed early in 2018. It appears that an under-correction for energies above the F-K edge, at ~0.7 keV, since 2015 caused the optical depth after 2015 to gradually rise quicker in the model than which is seen in observed data over time.

Additional technical information is available from:

• The Update to the ACIS Contamination Model memo (8 January 2010)
• The Contamination on the ACIS OBF and Changes in the Low Energy QE page
• The ACIS Spatial Contamination Effects memo (19 January 2005) (PDF)
• The Spatial structure in the ACIS OBF contamination memo (20 April 2004) (PDF)
• The Composition of the Chandra ACIS contaminant paper (August 2003)
  H. L. Marshall, A. Tennant, C. E. Grant, A. P. Hitchcock, S. O'Dell, P. P. Plucinsky

Applying the Correction

The following CIAO response tools automatically take the contamination into account:

• mkarf
• mkgarf
• mkwarf
• mkinstmap

As well as the scripts which use them:

• specextract (calls mkwarf and mkarf)
• fullgarf (calls mkgarf)
• fluximage (calls mkinstmap)
• merge_all (tool withdrawn)
• merge_obs and flux_obs (calls mkinstmap)

Each of the tools contains an ardlibparfile parameter with the value"ardlib.par." The location of the calibration file is specified in the ardlib.par file by a set of 10 parameters (one per CCD):

unix% plist ardlib | grep CONTAM
AXAF_ACIS0_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS1_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS2_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS3_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS4_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS5_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS6_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS7_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS8_CONTAM_FILE = CALDB            Enter ACIS Contamination File
AXAF_ACIS9_CONTAM_FILE = CALDB            Enter ACIS Contamination File

If anything other than "CALDB" is returned, issue the following command so that the tool will be able to find the correct file:

unix% foreach d ( 0 1 2 3 4 5 6 7 8 9 )
foreach? pset ardlib AXAF_ACIS${d}_CONTAM_FILE="CALDB"
foreach? end

You may also use "punlearn ardlib" to reset all the ardlib parameters to the default values. This will also clear out any other information that has been set, however, such as bad pixel filenames.

Turning Off the Correction

It is possible to "turn off" the contamination correction, e.g. if you would like to compare results with and without it applied. To do so, the ARDLIB qualifier "CONTAM=NO" must be specified in the appropriate parameter, as given in the following table:

There are examples in the help files on how to use the qualifier with each tool. For example, when running mkarf on an ACIS-S3 observation:

unix% pset mkarf detsubsys="ACIS-S3;CONTAM=NO"

Examining the Effects of the Correction

Comparing ARFs using Old vs. New Contamination Model

It is useful to compare ARF responses created using an older (4.7.2 or earlier) versus the newest contamination model (4.7.3) to examine the effects of the correction. One could do this by following the procedure below, which uses ACIS-S imaging observation 11800, taken in July 2010, as an example. The CIAO tool mkwarf is used to create the old and new on-axis ARF responses.

1) Find out how 'old.arf' was created using dmhistory, which will show that it was either mkarf or mkwarf (mkwarf in this example).

% dmhistory infile=old.arf tool=

# dmhistory (CIAO 4.5): WARNING: Found "pixlib" library parameters

# dmhistory (CIAO 4.5): WARNING: Found "ardlib" library parameters

 TOOL  :mkwarf infile="11800_tdet.fits[wmap]" outfile="old.arf" weightfile="11800.wfef" spectrumfile="" egridspec="0.3:11.0:0.01" pbkfile="pbk0.fits" threshold="0" feffile="CALDB" mskfile="msk1.fits" asolfile="" mirror="HRMA" detsubsysmod="" dafile="CALDB" ardlibpar="ardlib" geompar="geom" clobber="no" verbose="1" 

2) Re-run dmhistory, but this time, updating the mkwarf parameter file with the parameter settings returned in step 1. Check that the mkwarf parameter file was properly set by using the 'plist' command.

% dmhistory infile=old.arf tool=mkwarf action=pset
# dmhistory (CIAO 4.9): WARNING: Found "pixlib" library parameters

# dmhistory (CIAO 4.9): WARNING: Found "ardlib" library parameters

% plist mkwarf

Parameters for /home/user/cxcds_param4/mkwarf.par

        infile = 11800_tdet.fits[wmap] Input detector WMAP
       outfile = old.arf         Output weighted ARF file
    weightfile = 11800.wfef       Output FEF weights
  spectrumfile =                  Input Spectral weighting file (<filename>|NONE)
     egridspec = 0.3:11.0:0.01    Output energy grid [kev]
       pbkfile = pbk0.fits        Parameter block file
    (threshold = 0)               Percent threshold cut for FEF regions
      (feffile = CALDB)           FEF file
      (mskfile = msk1.fits)       Mask file
     (asolfile = )                Stack of aspect solution files
       (mirror = HRMA)            ARDLIB Mirror specification
 (detsubsysmod = )                Detector sybsystem modifier
       (dafile = CALDB)           Dead area file
    (ardlibpar = ardlib)          Parameter file for ARDLIB files
      (geompar = geom)            Parameter file for Pixlib Geometry files
      (clobber = no)              Clobber existing outputs
      (verbose = 1)               Tool chatter level
         (mode = ql)              

3) After updating the CALDB to the latest version with the most recent contamination model, re-run mkwarf with these parameter settings, except for changing the outfile name to 'new.arf'. (Note that you may need to change directories or adjust the file paths depending on how things were run to create old.arf.)

% mkwarf outfile=new.arf

4) Compare the old and new ARFs by plotting them together. For this dataset, the old and new ARFs are shown below:

Comparison of old and new models

Comparison of old and new models

An on-axis ARF response created using older CALDB version 4.7.2 CONTAM N0009 (black curve) for an ACIS-S observation taken in July 2010, compared to that produced with the updated contamination model included in CALDB 4.5.9 CONTAM N0010 (red curve).

The N0007 and prior CONTAM Models

The version N0008 model was released in CALDB 4.5.9, provides a more realistic model of the contaminant—without use of an artificial "fluffium" component as in previous models—resulting in a more accurate representation and prediction of current and future effective ACIS QE. Subsequently, there is a significant loss of effective area for present and future observations using the model as compared to previous models; however, early- and mid-mission effective areas are not much affected by the new model.

The version N0009 upgrade accounts for the rapid build-up of contaminant on the optical blocking filter since early-mid 2013; but is otherwise identical to the N0008 contamination model.

Time-dependence (center of chips)

Time-dependence (center of chips)

Updated time-evolution of the optical depth of the contaminant layer on the optical blocking filter in the center of both ACIS-I (red) and ACIS-S (blue) from A1795 measurements, and external calibration source L/K measurements (black). The solid line is the fit in the latest N0008 model.

Time-dependence of the contaminant spatial pattern

Time-dependence of the contaminant spatial pattern

The same keys apply as in the previous figure. A1795 and ECS measurements show the change of the optical depth of the contaminant layer on the optical blocking filter at 700 eV between the edge and center of the detector. Note the rapid spatial variation of the contaminant with time since 2009.

At high CHIPY locations on ACIS-S3 for imaging spectroscopy, there is uncertainty in the depth of the contaminant layer found in the N0008 model. Fitting results may be compromised at these locations.

The subsequent change in ACIS-I effective areas (EAs) between the N0007 and N0008 is illustrated below.

ACIS-I Effective Areas

ACIS-I Effective Areas

The ACIS-I aimpoint effective areas versus energy for January 01 of 2000 (green), 2004 (blue), 2009 (cyan), and projected for 2015 (red). The solid curves are the EAs from the N0007 contamination model and the dashed curves are from the N0008 model.

Similarly, the change in ACIS-S EAs are illustrated below.

ACIS-S Effective Areas

ACIS-S Effective Areas

The ACIS-S aimpoint effective areas versus energy for January 01 of 2000 (green), 2004 (blue), 2009 (cyan), and projected for 2015 (red). The solid curves are the EAs from the N0007 contamination model and the dashed curves are from the N0008 model.

For both ACIS-I and S, the new model only has a minor affect on the early- and mid-mission EAs, except for very near the C-K, O-K, and F-K absorption edges, which do not affect fitting results significantly near the edges for mid-mission observations.

The rapid increase in the contaminant indicates that ACIS-S is not ideal for very soft sources in the future, particularly below the 283 eV carbon edge.

Information on releases prior to the version N0007 model can be found in "Prior ACIS QE Contamination and CONTAM Models".