Composition of the Chandra ACIS contaminant.pdf

a r X i v : a s t r o - p h / 0 3 0 8 3 3 2 v 1 1 9 A u g 2 0 0 3 Composition of the Chandra ACIS contaminant Herman L. Marshalla, Allyn Tennantb, Catherine E. Granta, Adam P. Hitchcockc, Steve O’Dellb, and Paul P. Plucinskyd aCenter for Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA 02139 bNASA Marshall Space Flight Center, Huntsville, AL, USA 35812 cChemistry, McMaster University, Hamilton, ON, Canada L8S-9140 dHarvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA, USA 02138 ABSTRACT The Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory is suffering a gradual loss of low energy sensitivity due to a buildup of a contaminant. High resolution spectra of bright astrophysical sources using the Chandra Low Energy Transmission Grating Spectrometer (LETGS) have been analyzed in order to determine the nature of the contaminant by measuring the absorption edges. The dominant element in the contaminant is carbon. Edges due to oxygen and fluorine are also detectable. Excluding H, we find that C, O, and F comprise 80%, 7%, and 7% of the contaminant by number, respectively. Nitrogen is less than 3% of the contaminant. We will assess various candidates for the contaminating material and investigate the growth of the layer with time. For example, the detailed structure of the C-K absorption edge provides information about the bonding structure of the compound, eliminating aromatic hydrocarbons as the contaminating material. Keywords: Contamination, X-rays, Telescopes, Spectroscopy 1. INTRODUCTION Plucinsky et al. (2003)1 showed that the detection efficiency of the Chandra2 Advanced CCD Imaging Spec- trometer (ACIS)3 declined monotonically in the first three years. The decline was particularly apparent in the strengths of the Mn and Fe L lines relative to the Mn K line from the external calibration source; this ratio dropped by 35%. The decline was attributed to the buildup of a contaminating layer on the A