Spectral features in the AXAF HETGS effective area using high-signal-continuum tests

Abstract

In order to probe for small scale spectral features of the High Energy Transmission Grating Spectrometer (HETGS) and the low energy transmission grating, we performed test at the AXAF X-Ray Calibration Facility (XRCF) using a very bright continuum source. THE Electron Impact Point Source (EIPS) was used with the Cu anode and operated at high voltage and low current in order to provide a bright continuum at high energies. The AXAF CCD Imaging Spectrometer Spectroscopy detector (ACIS-S) was used to discriminate orders and to provide high throughput when operate in continuous clocking mode. Many spectral features are observed but most of them are emission lines attributable to the source spectrum. We find that the current models for the HETG efficiency, the LETG efficiency and the AXAF High Resolution Mirror Assembly effective area predict very well the observed fine structure near the Au and Ir M edges where the models are most complex. Edges in the detector filter and quantum efficiency (QE) curves are somewhat more sharply defined in the data than in the current modes. By comparing the positive and negative dispersion regions, we find no significant efficiency asymmetry attributable to the gratings and we can further infer that the QEs of the ACIS-S frontside illuminated (FI) chips are consistent to +/- 10 percent. On the other hand, we derive the ratio of the QE for the backside illuminated (BI) chips relative to that of the FI chips and show that it deviates for the expected ratio. This deviation may result from grade differences due to operation in CC mode while most calibration data are obtained in timed event mode.