The HETG are optimized for high-resolution spectroscopy over the energy range keV. The single grating assembly holds two kinds of gratings, Medium Energy Gratings (MEG) and High Energy Gratings (HEG), which are optimized for the lower portion and upper portion of the band, respectively. These operate simultaneously, producing spectra which cross at the undispersed image on one of the focal plane imagers. The optimum detector is ACIS, although HRC can be used as a backup or to extend the HETG response to lower energies.
The LETG provide the capability for high spectral resolution observations of point sources in the energy range 0.09 - 3.0 keV (wavelengths 140 - 4 ) with an effective collecting area of 20 . The LETG provide the highest spectral resolution on AXAF at low energies ( 0.05 for 100 ). This resolution will for the first time permit studies of spectral line profiles in the X-ray region.
Figure 5. Top: A simulation of dispersed spectra from the MEG and HEG assemblies of the HETG as seen on the six chips which make up the ACIS spectral array, for a 5-minute integration on the star Capella. Bottom: The histogram projection of the MEG portion of the simulated exposure.
The HETG and LETG achieve their ultimate performance only for point sources. Figure 5 shows an example of dispersed HEG and MEG spectra superimposed on the ACIS grating readout array. The transmission gratings can be useful in studies of moderately extended sources or features as well, although with decreased spectral resolution. We expect that deconvolution algorithms will be developed to exploit such observations as much as possible.
The key performance parameters of the AXAF gratings are shown, together with those of ACIS alone, in Figures 6 and 7. The resolving power () of the gratings changes over their respective ranges from at the higher energy (shorter wavelength) end to at the lower energy (longer wavelength) end. In contrast, ACIS has a resolving power that increases with energy, from to . The effective areas of the gratings (used with their respective prime detectors) range from to cm and are complicated functions of energy, emphasizing the importance of careful pre- and post-launch calibration.
Figure 6. Effective-area curves for ACIS (front-side illuminated chip; solid curve), the HETG as read out with ACIS (dashed curve), and the LETG as read out with the HRC (dotted curve).
Figure 7. Resolving-power () curves for the HEG and MEG gratings of the HETG (solid curves), the LETG (dashed curve), and ACIS alone (dotted curve).
Figure 8. Simulated spectrum of the RS CVn system II Peg for a 14 ks exposure using the HETG read out with ACIS. The observing time was chosen so as to accumulate total counts. The simulation assumes that the HEG and MEG spectra are added together.
As a rough `rule of thumb' for comparing count-rates, ACIS will give times the combined count rate of the two SIS detectors on ASCA, while the HETG will give the ASCA rate. Of course, the higher resolution of the gratings can make detection of emission lines and other spectral features much easier despite its lower sensitivity. To illustrate this, Figure 8 shows a simulated spectrum from a 14 ks observation of the RS CVn star II Peg with the HETG (adding the MEG and HEG spectra).
The high resolving powers of the grating spectro-meters on AXAF will permit us to measure individual emission lines in a wide variety of sources, and will permit us to perform plasma diagnostics of the physical properties in the source, such as temperature and elemental abundance. Absorption features, from absorbing material along the line of sight in the interstellar medium of the Galaxy or in the source itself, are also prime targets. Recall that the first evidence of an X-ray `broad absorption line' in a BL Lac object came from the grating spectrometer on the Einstein Observatory. Although they are not ideal for extended sources, the gratings can, for example, form dispersed images of a supernova remnant in the Large Magellanic Cloud in the light of the strongest lines, just as in a spectroheliogram. Prime candidates for study in our Galaxy are stellar coronae, white dwarf atmospheres, X-ray binaries, and cataclysmic variables. Candidate extragalactic sources are bright AGN and cooling flows in clusters of galaxies.