The HRC-S gain and QE have steadily decreased since the beginning of the mission, by roughly 60% and 10% respectively over the past dozen years. By 2011, event amplitudes at some locations on the HRC-S had dropped enough that a significant fraction of events fell below detection threshold (or did not produce a signal at all), leading to an above-trend decrease in QE of several percent at some wavelengths. Tests at higher operating voltages were conducted on 2012 Jan 9 and the default HRC-S voltage settings were changed in mid-March, restoring performance to what it was circa 2002. A preliminary calibration of the gain is being released in CALDB 4.5.1. A more accurate gain calibration, particularly for wavelengths short of 60 Å, will be released when more data are in hand and temporal trends are better established. The calibration files for the HRC-S QE and LETG/HRC-S effective area are not being revised at this time. Data at λ > 60 Å however, indicate a consistent 5% increase in QE and EA, and we assume that the 5% increase applies at shorter wavelengths as well. QE and EA revisions will be documented when released. Gain changes are summarized below and notes on Data Reprocessing are given at the bottom.
As seen in Figure 1, the HRC-S gain varies both spatially and with time. The region corresponding to LETG wavelengths longward of around +140 Å has especially low gain, and it is at these wavelengths where the excess QE drop is largest (see Figure 2).
|Figure 1: Mean SAMP (PHA) versus wavelength, using background-subtracted HZ43 and PKS 2155 data up to 2008. Gain is particularly low longward of +140 Å and this is where above-trend decreases in QE began to appear around 2011.||Figure 2: CALDB-corrected QE versus time, for data collected prior to raising the HRC-S high voltage. There are excess QE losses at the longest + wavelengths, where event amplitudes are smallest.|
After testing a range of settings, the voltages applied to the top and bottom plates of the HRC-S microchannel plates (MCPs) were each increased by 60 V, which raised the detector gain by ~60% at wavelengths covered by the soft continuum source, HZ43 (see Figure 3). Data at shorter wavelengths (from Mkn 421) show similar increase in gain, but the effects of grating higher orders require more detailed analysis and so for now we simply interpolate gain behavior for λ <: 60 Å from the longer-wavelength HZ43 data. Likewise, until data are collected over a longer time period we conservatively extrapolate the temporal decay seen before the voltage change (see Figure 4) and scale the gain calibration accordingly.
|Figure 3: Mean PI for 0th order and slices of dispersed HZ43 spectra, using the old gain calibration, and normalized to PI values from observations (ObsIDs 1011 and 8274) made at the old voltage settings. Colored curves and the bottom two black curves (solid and dotted) are for short test observations at various voltage settings. Settings for the blue curve (ObsID 62687) were chosen for the new operating voltage. The upper black and gray curves are for 10-ks calibration observations made with the new voltages. (0th order appears high for these two observations because they were made with Yoffset=0.7', which moves 0th order off the low-gain on-axis aimpoint.) The gold curve is the gain correction factor used for post-voltage-change observations (with an additional 6% adjustment to provide a margin of error for filtering).||Figure 4: Mean SAMP (PHA) as a function of time, with data from different wavelengths offset vertically in steps of 10. Smooth curves are from least-squares fits of exponential plus linear decay, and are used to model time dependence of the HRC-S gain map. For the preliminary post-voltage-change gain calibration, these curves will be extrapolated in time but have abrupt jumps at March 2012.|
Given the uncertainties in the short-wavelength gain and in the rate of temporal decline, we have adjusted the gain correction factors by ~6% (relative to what we consider their most likely values) in order to ensure that event PI values err on the low side so that the LETG/HRC-S background filter will not remove more than the expected 1.25% of valid X-ray events (see Figure 5). Background filtering of data collected using the new high voltage will therefore be slightly less effective than with a fully optimized gain map, but is otherwise perfectly "safe." A more accurate gain map will be constructed when adequate data have been accumulated, probably sometime in 2013.
HRC-S data collected between 2012 Mar 17 and 4 July (most of them calibration observations) were sometimes obtained using the old high voltage and sometimes the new. In addition, the gain calibration file for the old high voltage was only designed to work with data up to 2012 Jan 1; a slight revision extended its range. The following list explains which gain file (which is not necessarily the default) should be used to process a particular observation:
As part of Repro IV, all LETG/HRC-S and HRC-S data will be reprocessed using the appropriate gain file and placed in the Archive. In the meantime, some Archive data from observations made between 2012 Mar 17 and 2012 4 July may not have had the correct gain file applied. To check, type
dmkeypar filename.fits GAINCORF echo+and follow the instructions under "How CALDB 4.5.1 Affects Your Analysis" if necessary.
Last modified: 07/24/12
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