Analysis and Modeling of Anomalous Scattering in the AXAF HETGS

Tests of the High-Energy Transmission Grating Spectrometer (HETGS) for the Advanced X-ray Astrophysics Facility (AXAF) showed anomalous scattering of monochromatic radiation. The grating resolving power (E/dE) is of order 1000, but tests designed to search for small-angle scattering by the gratings showed events with significant deviations from the dispersed grating orders and concentrated along the dispersion direction.

In this paper, we present a general overview of grating scatter as a result of fluctuations in the grating bar geometry. The grating scatter observed at the AXAF X-Ray Calibration Facility (XRCF) is shown to be consistent with what one expects from scatter due to deviations in the grating bar geometry from perfect bars. For the purposes of modeling, a rectangular bar model is adopted and bar parameters are deduced via fitting the model to the scattering data. The correlations deduced from this model lead to a simple physical picture of grating bar fluctuations where a small fraction of the bars, e.g., 1 in 200 are leaning.