New HRC-S Gain Map and LETG Background Filter

Links

• Filtering Instructions and Software
• CIAO "LETG/HRC-S Grating Spectra" thread
• "A New Gain Map and Pulse-Height Filter for the Chandra LETG/HRC-S Spectrometer" (pdf, 1.9 MB)

Description

A new HRC-S gain map and pulse-height filter for use with LETG data has been developed. As shown in the plots below, over half of Level 2 background events can be removed at most wavelengths with a loss of ~1.25% of X-ray events. Among the differences from the old gain map and PI filter are:

• Use of scaled SUMAMPS (SAMP) instead of PHA, which creates a much more smoothly varying gain map
• Use of SAMP-based PI (SPI)
• A finer spatial grid (1/3-tap versus 1/2-tap)
• Time-dependent gain corrections; the HRC-S gain is now roughly half what it was at the beginning of the mission.

Incorporation into CIAO is expected sometime in 2009, with SAMP and SPI becoming the new PHA and PI. In the meantime, users must reprocess from Level 1.5 and filter their data according to the instructions on this page.

Higher Orders

The pulse-height filter removes only 1.25% of 1st order X-ray events, but an increasing fraction of higher-order events. The mean of the pulse-height distribution increases slowly with photon energy, such that a factor of two difference in energy corresponds to a shift in the mean of ~8%. The mean of 8th order will therefore be about 25% higher than 1st order. As an example, the SPI filter removes 1.25% of 1st order at 160 Å and about 11% of 8th order (λ=20 Å, mλ=160 Å). Extra filtering of higher orders will have negligible effect for most analyses, but should be considered when deliberately studying wavelength ranges with very heavy higher order contamination. Users may instead wish to apply 'Flat Filtering' (see below), which removes a negligible fraction of X-ray events for all orders.

Off-Axis Observations

Great effort was expended to make the new gain map and filter generally applicable to all observations, even those with large offset pointings. As described in the development documentation, however, gain calibration near the HRC-S plate gaps and around the default aimpoint is not quite as good as elsewhere. For observations with large offsets, users may wish to compare results using spifilter versus those obtained using 'Flat Filtering' (see below), although we do not expect any statistically significant differences.

Flat Filtering

In some cases, observers may wish to apply filtering that removes a constant fraction of the background across the entire HRC-S. Examples include analyses of extended or off-axis sources, or spectra with heavy higher-order contamination. After applying the gain map by running addspi as described on the Filtering Instructions and Software page,
        $ dmcopy "evt2_spi.fits[spi=:300]" evt2_filtered opt=all
will remove about 20% of background events with negligible X-ray event loss. (Note that in the current implementation, SPI values can sometimes be negative, so it is necessary to specify spi=:300 and not spi=0:300.)

Because of 'burn in' around the default aimpoint, the gain in that region varies significantly on very small scales and is, for various reasons, difficult to calibrate accurately. Given the usually high S/N for point sources, the small improvement that would be gained by reducing the background, and the potential for nonuniform background removal because of gain calibration uncertainties, SPI filtering is generally not recommended when the objective is to analyze non-grating or 0th-order images near the default aimpoint.

Plots

Comparison of old and new filters, applied to data from 2000 and 2008. Background fractions are relative to Level 2 data that include events with PHA=255. The old PI filter becomes less effective over time because it does not account for decreasing gain. Less background is removed at short wavelengths because the filtering threshold must be higher to avoid excluding X-ray events. The raw BG rate is higher in 2008 than 2000 because of the solar cycle, but relatively more BG is removed in 2008 because the BG pulse-height distribution has relatively more high-channel events.

The 2008 LETG/HRC-S background rate with and without filtering, using the standard 'bowtie' spectral extraction region. The right-hand axis units refer to the ~0.07-Å FWHM resolution of the LETGS.

Last modified: 05/12/10