Introduction

The Advanced X-ray Astrophysics Facility (AXAF), to be launched in 1998, will provide revolutionary imaging and spectral capabilities in the energy range 0.1-10 keV. (For an overview of the spacecraft and the instruments, see Weisskopf et al. 1995 and Weisskopf, O'Dell & Van Speybroeck 1996.) At  keV the image quality will be such that more than 80 percent of the photons detected from a point source will fall within 0.5-arcsec radius. The spatial resolution of AXAF is therefore comparable to that of ground-based optical telescopes. The transmission gratings on AXAF can achieve high spectral resolving powers. At low energies, the spectral resolving power exceeds 1000 (see AXAF Proposer's Guide, ASC.TD.402), which is adequate to resolve the K lines of the He- and H-like ions of O (0.57 and 0.65 keV respectively), Mg (1.35 and 1.47 keV), Al (1.60 and 1.73 keV), Si (1.86 and 2.00 keV), S (2.46 and 2.62 keV), Ar (3.14 and 3.32 keV), Ca (3.90 and 4.11 keV) and the L lines of Fe emitted from hot plasmas in various astrophysical environments - e.g., the dense, shock-heated accretion region near the surface of a white dwarf in a magnetic cataclysmic variable (mCV).

The currently best X-ray spectra of mCVs, obtained by the Solid-State Imaging Spectrometer (SIS) on board the Advanced Satellite for Cosmology and Astrophysics (ASCA), show only the major emission lines above 1 keV (see e.g., Fujimoto & Ishida 1997). Theoretical calculations, however, show that many more lines are present (e.g., Cropper, Ramsay & Wu 1998). These lines are useful not only for determining the white-dwarf mass, but also for diagnosing temperature and density structures of the accretion flow, and for deducing the relative chemical abundance of the accreting matter (Ishida 1991, 1997; Fujimoto & Ishida 1997; Wu, Cropper & Ramsay 1998).

We carry out simulation of AXAF grating spectra of accreting magnetic white dwarfs and investigate the diagnostic capability of the grating instruments. In the simulation, we first calculate the structure of the shock-heated emission regions and the raw spectra of its X-ray continuum and line emission. We then fold the raw spectra through a matrix that models the AXAF response. For this simulation we use the High-Energy Transmission Grating (HETG) (see Markert et al. 1994 for a detailed discussion of the HETG). The HETG comprises two grating arrays - the Medium-Energy Grating (MEG) and the High-Energy Grating (HEG). The gratings are mounted in the same structure, and so when the HETG is inserted two independent spectra, one from the MEG and the other from the HEG, ensue. The simulated AXAF response therefore includes the effects of the mirrors, the grating, and the AXAF CCD Imaging Spectrometer (ACIS) detector (Lumb et al. 1993). For a comparison of the spectral resolving capability of AXAF with those of previous instruments, we also carry out a simulation of spectra of the ASCA SIS (Tanaka, Inoue & Holt 1994), which are CCD detectors without gratings.

© Copyright Astronomical Society of Australia 1997