OVERVIEW
SPECTRUM-X-GAMMA
The following instruments will fly on SXG: the XSPECT/SODART system, JET-X (Joint European telescope for X-ray astronomy), MOXE (All sky monitor), MART-LIME (High-energy coded aperture telescope), and EUVITA (UV telescope).
Two SODART thin foil X-ray telescopes are included in the XSPECT/SODART system. SXRP, HEPC/LEPC (high and low energy imaging proportional counters), and FRD (position sensitive X-ray spectrometer) sit in the focal plane of one telescope. In the other, there are: SIXA (silicon X-ray array), another HEPC/LEPC pair, and another FRD. Mounted alongside and co-aligned with the SODART telescopes is TAUVEX (UV telescope). OXS (objective grating spectrometer) is mounted in front of one of the telescopes.
The observatory is intended to be operated for a period of 3 years. The satellite will be launched into a deep, highly eccentric orbit with a period of 4 days. This choice of orbit enables long duration (up to about 80 hour) observations to be made. It should be possible to have 10 reorientations of the spacecraft per day.
SXRP
The SXRP exploits the polarization dependence of Bragg reflection from a graphite crystal, and of Thomson scattering from a target of metallic lithium. A "physicist's view" of the instrument is shown in Figure 1. The polarization analyzers for the SXRP consist of a very thin mosaic graphite crystal mounted above a cylindrical lithium scattering target. The graphite crystal and lithium target are centered on the optical axis of the SODART X-ray telescope and are surrounded by four imaging proportional counters (IPCs) that detect the Bragg reflected and Thomson scattered X-rays.
The portion of the SXRP shown in Figure 1 rotates about the optical axis of the telescope with a period of approximately 2 minutes. We have found that a very thin mosaic graphite crystal can be used to efficiently reflect X-rays meeting the Bragg condition, while allowing the X-rays that do not satisfy the Bragg condition to pass through the crystal with only moderate attenuation. This stacked grapite-lithium configuration permits us to obtain data from both polarization elements simultaneously, greatly improving the observing efficiency of the instrument.