Measurements of the Chandra ACIS Contaminant using the LETG


This page documents the analysis I’ve done using the Chandra Low Energy Transmission Grating Spectrometer (LETGS) with the ACIS detector. This combination has been extremely useful in diagnosing the nature of the contaminant that is building up on ACIS. See the QE degradation web page for public information on the subject. A paper will appear in the proceedings of the SPIE, volume 5165. The presentation is available in either PowerPoint or PDF formats

Contents

Summary
Preliminaries
1. Composition
2. Time Dependence
3. Spatial nonuniformity

Summary

The ACIS contaminant imparts at least three distinct absorption edges on the LETGS data due to C-K, O-K, and F-K. The C-K edge is the deepest and is shown to increase at a rate of about 0.4 optical depths per year. The other components are readily detected in a recent LETGS observation of Mk 421 (from F. Nicastro) and marginally detected in a few previous LETGS calibration observations. The time dependence of the C-K edge is modeled and the other edges are assumed to have the same scaling with time, resulting in a time-dependent spectral model of the contaminant that can be used to correct Chandra grating data for the effect of the contaminant. This model has been implemented using ISIS for users in a standalone procedure.

Preliminaries

There has been a series of CXC internal meetings to analyze the problem and consider a fix. There were many reports and presentations at these meetings, so the objective of of this web page is to provide public access to some of these reports in a coherent way to help users. While this work concentrates on LETG/ACIS data and the conclusions are likely to apply to all ACIS observations, I emphasize that there remain some unresolved issues. The reports are presented in chronological order within several categories due to the development of our understanding of the contamination.

1. Composition

At first, only the C-K edge was clearly detected in the LETG/ACIS data so the first correction file only included a model of the C-K edge. The O-K edge and an F-K edge were found in the very long observation of Mk 421 in October, 2002 (from F. Nicastro). These features are rather weak but Mk 421 was rather bright at the time and it was observed for 100 ks. A model of the absorption due to the contaminant was developed in the following procedure.


The resulting model of the contaminant is available in a file. There are four columns, corresponding to wavelength (Å) and the remaining three correspond to the optical depths due to carbon, oxygen, and fluorine. The fits gave (Henke equivalent) edge optical depths of 2.09 +/- 0.02, 0.07 +/- 0.03, 0.100 +/- 0.007, and 0.066 +/- 0.005 for the C-K, N-K, O-K and F-K edges, respectively.

I obtain the following column densities for C, O, and F: 2.0e+18, 1.75e+17, and 1.45e+17 atoms per sq cm. For N-K, I had a previous upper limit of the edge optical depth of about 5%, giving a column density of 7.1e+16 atoms per sq cm. Carbon is more abundant than the other elements by factors of 11.5, 14, and >30 for F, O, and N, respectively. The uncertainties on these ratios are about +/- 1. Whatever the compound is, the C atoms are mostly bonded to other C atoms.

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2. Time dependence

When it was first reported (originally in July 2000) in the LETGS data, the C-K edge did not show clear variation with time. Time dependence became clear with observations in 2001 and 2002.

Using the contamination model derived above, I fit the LETG/ACIS data for many sources with smooth spectra, allowing the C-K, O-K, and F-K edges to vary independently. The results are given in a text file and given in another web page. Then the C-K edge depth was fit to a simple time dependence model. The shape of the time dependence was assumed to be the same for each of the three detected edges and were merely scaled to observed optical depths (see the previous section) for the date of the Mk 421 observation (2002.86). Because the optical depth values used to create the contamination model were slightly different — 2.10, 0.09, and 0.06 (for C-K, O-K, and F-K, respectively) — the time factors for each component were merely rescaled. The resulting time dependence file is available on-line.

The O-K edge data do not fit the model as well as expected for reasons that are not completely understood.

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3. Spatial nonuniformity

Two observations of 3C 273 gave us the opportunity to see if the contaminant is uniform over the surface of ACIS. The analysis shows that there is a detectable variation in the sense that there is more contaminant at the edge of the detector than in the center. These results are consistent with preliminary results from analysis of data from the ACIS external cal source and with the hypothesis that the thickness depends on the local filter temperature, which is expected to be warmer in the center than at the edge.
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Herman Marshall
hermanm@space.mit.edu
Last updated August 12, 2003