EVIDENCE OF ACCRETION-GENERATED X-RAYS IN THE YOUNG ...

one set of ACIS CCDs) and later retracted during imaging observations. The HETG
(Fig. 11) is composed of two sets of gratings, with each set employing a di↵erent
grating spacing. One set comprises the Medium-Energy Gratings (MEG), which are
used in conjunction with the outer two pairs of mirrors and deal with the lower-
energy (0.4–5.0 keV) X-ray photons, while the other set comprises the High-Energy
Gratings (HEG), which are used in conjunction with the inner two mirror sets that
focus the higher-energy (0.8–10.0 keV) X-rays. The MEG and HEG systems are
composed of 336 individual gratings (144 HEG and 192 MEG) arranged on the HETG
Support Structure (HESS), which places them in proper position behind the HRMA.
The gratings themselves are composed of gold bars electroplated onto a polyimide
substrate, with the bars and corresponding spacings roughly 0.1µm for the HEG
and 0.2µm fortheMEG.Therulingsofthetwosetsofgratingsareangled⇠10
degrees to one another so that the medium- and high-energy spectra are separated
from one another. This results in the dispersed X-ray spectrum appearing as an
“X”. Due to multiple gratings needing to focus dispersed spectra onto a single focal
plane, the gratings are arranged in several planes coinciding with the Rowland circle
geometry while the detectors are placed on the opposite portion of the Rowland circle
to intercept the focused spectra.
The Low-Energy Transmission Grating (LETG) is mounted on another swing-arm
and operates in a similar manner to the HETG. The LETG is used primarily with the
HRC-S array (energy range is 0.07 keV–10.0 keV), though it can be used with ACIS-S
(energy range is 0.2–10 keV) with reduced quantum e ciency. Much like the HETG,
the LETG gratings are mounted on a circular support structure, in this case known
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