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ACIS QE Contamination and Prior CONTAM Models

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.

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). This page document the most recent changes to the model, prior to the N0010 contamination model.

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.

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.

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 includes 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, is 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 model allows for asymmetry in the ACIS-S CHIPY contamination layer to provide better fits.

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

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

For both ACIS-I and ACIS-S, the new model only has 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.

The N0009/N0008 and prior CONTAM Models

The N0008 and N0009 were released in CALDB 4.5.9 (19 Nov 2013) and in CALDB 4.6.2 (9 Jul 2014) respectively to provide 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 these models. 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. These two models would supersede all prior versions of the contamination model.

The version N0007 upgrade, released in CALDB 4.4.10, includes a ~15% change in the optical depth at 700 eV for observations in 2012 as compared with those in early 2009.

The version N0006 upgrade, released in CALDB 4.4.1, contained improvements to the ACIS-I contamination correction. The ACIS-S portion of the file has been interpolated to match the time range of this ACIS-I model, but is otherwise unchanged. A correction ot the header of the vN0006 QE contamination model was added in CALDB 4.4.2 (01 March 2011). Prior to this update, CIAO would fail when trying to look up the contamination model correction for chips ACIS-8 (S4) and ACIS-9 (S5).

The version N0005 upgrade to the ACIS QE contamination model, released in CALDB 4.2.0 (15 December 2009) contains separate components for the correction on ACIS-S and ACIS-I.

The changes in the vN0005 file will primarily affect low-energy absorption components between C-K edge (0.283 keV) and 1.2 keV. In general, hard spectra will not be affected much by the calibration change.

As the Chandra mission proceeds, the contamination on the ACIS Optical Blocking Filters 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—more so than predicted by the contamination model (version N0007, May 2012, CalDB 4.4.10 compared to the N0008/N0009 models).

There are updates to the time-dependence of the contaminant at the center region of ACIS-I and ACIS-S, and changes to the time-dependence of the spatial component.

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.

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

For both ACIS-I and S, the N0008/N0009 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.

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The version N0007 model, released in CALDB 4.4.10, included a ~15% change in the optical depth at 700 eV for observations in 2012 as compared with those in early 2009.

In particular, the N0007 model updated the time-dependence of the contaminant at the center region of ACIS-I and ACIS-S, and significant changes made to the time-dependence of the spatial component since 2009.

The spatial pattern for ACIS-I was updated from an azimuthally-symmetric pattern to a hyperbolic fit. For ACIS-S, the polynomial fit was replaced by an exponential fit.

Additional technical information is available from:

• The ACIS QE contamination memos
• The ACIS Spatial Contamination Effects memo (PS)
• The Spatial structure in the ACIS OBF contamination memo (PDF)
• Composition of the Chandra ACIS contaminant (PS, 12 pages)
  H. L. Marshall, A. Tennant, C. E. Grant, A. P. Hitchcock, S. O'Dell, P. P. Plucinsky
  astro-ph/0308332