High Resolution Spectroscopy with HETG

The first Chandra grating observation was a calibration observation of the star Capella. The HETG was inserted into the optical path behind Chandra's mirror and X-rays were detected on the ACIS-S detector. About two hours of exposure produced the spectra shown here and demonstrate high resolution spectroscopy with Chandra.

What Chandra Sees

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The center of this image shows the star Capella as a bright point of light. Because the HETG is in place, "X-ray rainbows" in a shallow "X" pattern are created extending from Capella. Each position along the "X" pattern corresponds to a different X-ray "color" (wavelength or energy.) The color coding in this figure roughly matches the ACIS energy plot in the next figure. Note that lower energies (colored red) are dispersed further from the central image.

ACIS Spectrum

The ACIS detector itself can measure the energy of X-rays and is able to show the presence of some elements by the clear bumps produced in the ACIS spectrum. Here there are two clear features in the 1 keV to 2 keV range corresponding to lines from Magnesium and Silicon. The ACIS spectrum shown here is illustrative and has not been optimized.

ACIS is the spectrometer of choice in the case of faint or very extended sources.

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HETGS Spectra

The grating spectrum is formed by looking at the number of X-rays detected along the "X-ray rainbow". The grating spectrum is able to reveal individual X-ray lines emitted by a variety of elements and their ions, e.g., lines from Iron (Fe) and Neon (Ne) are seen in the figure at right. This energy region corresponds to the region indicated by the next-to-left-most white box in the ACIS spectrum above. The green spectrum is from the MEG gratings and the red spectrum is from the HEG gratings.

To examine the grating spectra associated with other regions, click on the previous figure and then click on the desired box.

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"Plasma Diagnostics"

The X-rays seen here are created by atoms heated to temperatures of millions of degees. At these high temperatures most of the atom's outer electrons are stripped away; the atom is a positively charged ion and is part of a high temperature plasma. Measuring the intensity, flux, of the different lines emitted gives information on the state of the plasma. The ratios of line fluxes from the same ion depend on the temperature and density, while those of different ions give information about the ionization state. One can further investigate chemical composition and abundance by using lines and line blends from different elements.

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Revised 8/28/99;
(relocated on 8/9/2004.)