HETG Background Rate


Analysis of HETG Background Rate

Here we present a brief synopsis of the analysis of the HETG/ACIS background count rate. For details, see this Technical Note: HETGS Background Count Rate


We have used several HETG observations which were long and of faint or heavily absorbed sources. The latter are useful at long wavelengths since the source spectrum is highly attenuated on the outer array CCDs. We have extracted first order HETG spectra off-source, using small modifications to standard processing, the primary result is shown in Figure 1.

The HETG background rate is less than 1.0 count/ Ms / FWHM / 4 arcsec, with about 10% uncertainty (note: the default cross-dispersion extraction width is 4 arcsec; the size of the extraction cell is about 15 square pixels, or 3.5 square arcsec).

This very low background rate is the result of two effects: the narrow spatial extraction regions possible with Chandra's arcsecond resolution mirrors, and the order-sorting afforded by the energy resolution of the ACIS CCDs. That is, an event is only assigned to a grating spectrum if it is both at the right location with the right CCD PHA value.

HETG Background Rate
Figure 1: The first orders' HETG count rate density, scaled to the FWHM in the dispersion direction and the default cross-dispersion extraction width for HEG (red) and MEG (black). The 1-sigma counting statistics uncertainty level is shown at the bottom of the plot. The approximate locations of each CCD are shown by the alternating color bars for MEG (upper) and HEG (lower), from S0 (leftmost, CCD_ID=4) to S5 (CCD_ID=9).

As if Cosmic Flux ...

Figure 2 shows the background rates as flux, after applying the responses (by dividing the counts by exposure and the count rate for a constant spectrum of 1 photon/cm^2/s/keV). This gives some indication of the flux limit due to the background.

HETG Background Rate as Flux HETG Background Rate as Flux
Figure 2:The background count rate flux-corrected, as if from a cosmic source, vs energy (left) and wavelength (right).

Source Information

Table 1 lists the observations used in this analysis along with other relevant source or observational information.
Source information

This page was last updated Oct 9, 2020 by David P. Huenemoerder. To comment on it or the material presented here, send email to dph@space.mit.edu.
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