This page replaces the original web page describing analysis of Chandra Low Energy Transmission Grating Spectrometer (LETGS) with the ACIS detector. The problem of the contamination of the Chandra ACIS detector has clarified considerably over the last year, so there are now better pages to examine which give users a sense for what is happening and how to correct for the contamination. See the QE degradation page for early information on the contaminant from the ACIS team. The acisabs model was implemented in CIAO of the contaminant to provide an interim solution for spectral analysis of ACIS imaging data. For grating spectrometer data, the preferred approach is to use the contamarf implementation of a new model that offers better agreement with high resolution data.

The old page was not revised, in order to preserve the history and context of the original work. Here are some issues and additional information regarding this page that the user might want to consider when viewing the original page.

1) The original portions of the web page were written on July 26, 2000 and updated on August 4, 2000 and finally on Feb. 26, 2001. The page was not publicly released until Sept. 28, 2000 due to data rights issues but the page and the referenced figures were reviewed internally by the CXC Calibration group.

2) Before the original portion of the page was drafted, there were only 3 observations where the C-K edge could be identified and they seemed roughly consistent, given the 5 month time span (December 1999 to April 2000). Only one of these data sets was public: 3C 273. The unnamed GO target was Ton S 180, obs ID 811. I was given permission by the PI to examine these data for calibration purposes but the data were still proprietary, so I did not reveal the target name.

3) The first attempt to provide a correction for the excess absorption was to generate a new transmission function for the ACIS optical blocking filter (OBF), because the only component in the system that had any carbon in it was the OBF. This explanation was abandoned, partly because the additional absorption had a spectral signature that did not match the absorption edge of the OBF. It seemed too large to be due to contamination, which was predicted to be only about 150-250 Å thick, whereas the observed edge indicated over 1500 Å of contaminant (if it was pure C). Thus, there was no physical explanation for the C-K edge absorption even by February 2001. The source of the contaminant is still somewhat unclear but the lubricant on the SIM is the leading candidate. Without a physical explanation, I called this C-K absorption “anomalous”.

4) The two updates indicate that there was no clear trend of the optical depth with time. With improved analysis, it is now apparent that the C-K edge increased substantially in 2000.

5) I noted a slight deviation of the C-K edge structure in the 0.284-0.287 keV region in the data from XTE J1118+480. Because the difference was so small compared to the large change just above 0.287 keV, I attributed the large change to “anomalous absorption” and the smaller difference (13%) to the ACIS OBF. An observation in May 2002 showed that the 0.284-0.287 keV region also varied with time, so the better interpretation of this small difference is that the contaminant has weak NEXAFS structure in this energy range.

6) The filter models still have not been updated but the LETG/HRC effective area was updated using an improved model of the C-K edge, based on my decomposition of the OBF transmission. No other features have been changed in any transmission model.

7) In the update dated 8/4/00, I say that there is a discrepancy between the FI and BI QE data. I provided a detailed memo on the subject on May 13, 2002 and provided a text file with the correction factors. Dick Edgar later verified that there is a discrepancy using ground calibration data, in the sense that the FI QEs look OK while the BI QEs should be increased. The cause of the discrepancy is still not clear.

Herman L. Marshall
hermanm@space.mit.edu
Updated on July 6, 2003