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Title:
X-Ray Spectroscopy of Optically Bright Planets with the Chandra Observatory
Authors:
Ford, P. G.; Elsner, R. E.
Affiliation:
AA(Kavli Institute for Astrophysics and Space Research, MIT 37-571, 60 Vassar Street, Cambridge, MA 02139-4307 United States ; ), AB(NASA Marshall Space Flight Center, NSSTC/XD12, 320 Sparkman Drive, Huntsville, AL 35805 United States ; )
Publication:
American Geophysical Union, Spring Meeting 2005, abstract #P44A-06
Publication Date:
05/2005
Origin:
AGU
AGU Keywords:
2494 Instruments and techniques, 5757 Remote sensing, 5794 Instruments and techniques, 7554 X rays, gamma rays, and neutrinos, 7594 Instruments and techniques
Bibliographic Code:
2005AGUSM.P44A..06F

Abstract

Since its launch in July 1999, Chandra's Advanced CCD Imaging Spectrometer (ACIS) has observed several planets (Venus, Mars, Jupiter and Saturn) and 6 comets. At 0.5 arc-second spatial resolution, ACIS detects individual x-ray photons with good quantum efficiency (25% at 0.6 KeV) and energy resolution (20% FWHM at 0.6 KeV). However, the ACIS CCDs are also sensitive to optical and near-infrared light, which is absorbed by optical blocking filters (OBFs) that eliminate optical contamination from all but the brightest extended sources, e.g., planets. Jupiter at opposition subtends ~45 arc-seconds (90 CCD pixels.) Since Chandra is incapable of tracking a moving target, the planet takes 10 - 20 kiloseconds to move across the most sensitive ACIS CCD, after which the observatory must be re-pointed. Meanwhile, the OBF covering that CCD adds an optical signal equivalent to ~110 eV to each pixel that lies within the outline of the Jovian disk. This has three consequences: (1) the observatory must be pointed away from Jupiter while CCD bias maps are constructed; (2) most x-rays from within the optical image will be misidentified as charged-particle background and ignored; and (3) those x-rays that are reported will be assigned anomalously high energies. The same also applies to the other planets, but is less serious since they are either dimmer at optical wavelengths, or they show less apparent motion across the sky, permitting reduced CCD exposure times: the optical contamination from Saturn adds ~15 eV per pixel, and from Mars and Venus ~30 eV. After analyzing a series of short Jupiter observations in December 2000, ACIS parameters were optimized for the February 2003 opposition. CCD bias maps were constructed while Chandra pointed away from Jupiter, and the subsequent observations employed on-board software to ignore any pixel that contained less charge than that expected from optical leakage. In addition, ACIS was commanded to report 5 × 5 arrays of pixel values surrounding each x-ray event, and the outlying values were employed during ground processing to correct for the optical contamination.
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