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Spatial structure in the ACIS OBF contamination

Alexey Vikhlinin, 05/09/04

This memo summarizes the measurements of the spatial and time dependence of the effective contamination optical depth for the L-complex in the ECS spectrum. The developed model results in < 5% uncertainties in the contamination corrections of the effective area at 0.6-0.7 keV and < 3% uncertainties at higher energies across both ACIS-S and ACIS-I arrays.

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Contamination on the ACIS OBF and Changes in the Low Energy QE

This memo is an updated report concerning the reduction in the ACIS low energy QE caused by contamination on the Optical Blocking Filter. Below we summarize the results of our investigation into this problem and provide observers with advice and in their spectral analysis of ACIS data. The results presented here represent the combined efforts of many groups, including, the CXC, the IPI teams at PSU and MIT, and Project Science at MSFC.

Previous version: 11/19/2003


An examination of archived astronomical observations and data acquired from the on-board ACIS calibration source (55Fe) shows that there has been a continuous degradation in the ACIS QE since launch. Our best interpretation is that this is due to molecular contamination building up on the cold optical blocking filter, and/or the CCD chips. This degradation is the most severe at energies below 1 keV. At 1 keV, the degradation is approximately 10% at present; it becomes negligible above 2 keV.

The contamination rate is well constrained by periodic observations of the L-complex in the on-board calibration source. These data indicate the fast build-up soon after launch and a gradual decrease in the deposition rate later in the mission. The chemical composition of the contaminant was established from observations of the continuum sources (mostly Mkn 421 and PKS2155-304) with LETG. These measurements indicate that attenuation is caused mostly by photoelectric absorption on Carbon with a smaller contribution from Oxygen and Florine. A detailed report on the chemical composition measurements is given by Marshall et al. (astro-ph/0308332; see also Herman's page). We have developed a model that adequately describes the time dependence of the contamination and its absorption spectrum at all useful energies. This model is provided to the general user in the CALDB 2.26 release. The residual uncertainties in the contamination modeling in the central region of the ACIS-S array should be of order 10% in the 0.3-0.4 keV range and 5% level or less at other energies.

Caveat - Contamination is not uniform within both ACIS-I and ACIS-S arrays. The current calibration is strictly valid only for the central region of ACIS-S. Additional contamination is observed towards the edges of the arrays. The additional optical depth near 700 eV can be as large as 0.3-0.5 for the observations in 2003. The spatial dependence of the contaminant is being incorporated in the Chandra data analysis system and will be released in the near future.

Measurements of the ACIS contamination

The first indication that there were problems with the low energy ACIS QE was a May 2000 observation of PKS2155-304 with the LETG that showed excess absorption near the C-K edge. A similar feature was also observed in a LETG/ACIS-S observation of 3C273. Since an observation of 3C273 with the LETG/HRC-S did not show this feature, it was concluded that the problem resided with ACIS. Subsequent observations of PKS2155-304 and Mkn421 with LETG/ACIS-S constrained the composition of the molecular contaminant. Figure 1 shows the data for the June 2002 observation of PKS2155-304 along with the best fit broken power-law plus edges at C, N,O, and F. Figure 2 shows the spectral fit after the contamination model was included in the effective area.

When ACIS is in the stowed position, i.e., the HRC-S is at the focal point, it is illuminated by an 55Fe source. Since the discovery that the ACIS CTI significantly increased during the first few weeks of the mission, observations of the 55Fe source have been taken before and after each perigee. Figure 3 shows that the L-complex to Mn-K alpha line ratio has continuously decreased since launch. Since the Mn-K alpha line at 5.9 keV is not affected by absorption, the decrease in the L-complex to Mn-K alpha line ratio must be due to increased opacity at the L-complex at 670 eV. Figure 3 indicates that the opacity at 670 eV is increasing at about 10% per year. To demonstrate that nothing unusual is happening to the 55Fe source, Figure 4 shows that the Mn-K alpha line intensity, produced in the 55Fe decay process, is decreasing at a rate consistent with the half-life of 55Fe.

Figure 1: Fit to the LETG/ACIS-S spectrum of PKS2155-304 taken in June 2002. The 68% uncertainties in the optical depths are 0.015, 0.007, 0.008, and 0.1 for the C,N,O, and F K-edges, respectively.

Figure 2: Overall spectral model for the recent PKS 2155-304 LETG/ACIS observation after correcting the ARF for contamination, showing that edge fits now give small optical depths but that there may be some systematic errors in the 0.3-0.4 keV range.

Abell 1795 was observed with ACIS-S in April 2000 and again in June 2002. A comparison of the spectra extracted from the central 1' minute region in these two observations is shown in Figure 5. Notice that there is essentially no change in the QE above 2 keV. The ratio of the two A1795 spectra is a measure of the increase in the ACIS contamination between the two epochs. This agrees well with the prediction of the contamination model (Figure 6).

Figure 3: L-complex to Mn-K alpha line ratio during the Chandra mission.

Figure 4: The Mn-K alpha line intensity during the Chandra mission.

Figure 5: ACIS-S spectra of A1795 from observations in April 2000 (black), June 2002 (red), and January 2004 (blue).

Figure 6: Ratios of the June 2002 and January 2004 to the April 2000 spectrum of A1795. The solid lines show the prediction for the increased absorption over the corresponding time intervals.

Last modified: 11/15/10

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