HETG Instrument Information

Diagram of HETG

Extracted HETG spectrum of Vela X-1 (Paerels & Kahn, 2003). Closeup of Si lines region; this region extends from 1.68-2.07 keV. To view gratings spectra of other sources, see TGCat.

The High Energy Transmission Grating (HETG) is used for high resolution spectroscopy of bright sources in the range 0.4-10 keV (31-1.2 Å). The HETG has been used to measure Doppler velocities of orbiting systems, even as low as 50 km/s, and plasma outflow velocities from a few hundred to 10's of thousands of km/s. Because the HETG can clearly resolve lines from O to Fe-K, detailed line diagnostics can be applied.

A complete description of HETG is given in Chapter 8 of the Proposers' Observatory Guide (POG). The HETG is primarily used with ACIS-S. The dispersed spectrum forms a shallow X on the ACIS-S array centered at the zeroth order position, as shown in this image. The table below summarizes common HETG science modes. The optimal mode depends primarily on source intensity.

Common HETG Configurations
Source TypeCount RateACIS ConfigurationFrame Time Wavelength Range Energy Range Resolving Power (E/ΔE)
Extended or Multiple SourcesUp to 10 cps1Full ACIS-S, faint mode3.2 s1.2-31 Å0.4-10 keV1000-65

(200 at 6.5 keV)
Single Point SourceUp to 14 cps1774 row subarray, faint mode2.4 s1.2-31 Å0.4-10 keV
Up to 30 cps1256 row subarray1.1 s1.2-18 Å0.7-10 keV1700-85
>30 cpsFull ACIS-S, CC mode
1.2-31 Å0.4-10 keV1000-65
1. These approximate count rates refer to the zeroth-order HETG count rate, e.g. from PIMMS. We assume a 1.7 photon index power law, absorbed by a column of 3e20.