HRC-I Gain Maps
HRC-I gain maps are used for hardness ratio or PI spectral analysis studies, and useful for reducing the background contamination for extended sources and weak point sources.
Time-dependent HRC-I gain corrections were first introduced around 2005, using PHA-based analysis; SUMAMPS-based, time-dependent gain corrections were introduced in CALDB 4.2.0 on 15 December 2009. These new gain maps are based on the scaled sums of the three nearest amplifier signals to the event location, providing a more robust PI value without saturation effects. (PHA is the sum of all amplifier signals corresponding to the event in question, and has a much larger small-scale spatial variation than the SUMAMPS metric.)
The corrections introduced by the gain maps compensate for the decline in HRC-I gain, typically ≈5% per year. These corrections allow for the maintainence of a 2% statistical consistency in the average PI. A typical gain map is visualized below, using the gain correction results released in CALDB 4.5.9:
The HRC-I gain maps are generated using a sequence of AR Lac observations at 21 specific locations on the detector, taken annually; and G21.5-0.9 and HZ43 calibration observations, taken occasionally—all for monitoring HRC-I.
The gain maps are updated on an annual basis, with the latest version in CALDB 4.5.9 (19 November 2013). Technical details on the gain maps are described in the memo SUMAMPS-based Gain Maps for the HRC-I.
The change in gain correction can be illustrated with the ratio map between the 2013 and 2012 HRC-I gain maps. The change in gain correction is less than 1% on-axis and roughly 5-10% off-axis. Towards the edge of the microchannel plate, the change is up to 15%.
Users working with recently obtained off-axis HRC-I observations (September 2013 or later) should reprocess the data to apply the updated gain correction by running the chandra_repro script or by following the Create a New Level=2 Event File thread. All others are unaffected.