Last Updated: 2020-jun-16

Chandra/HRC-I Quantum Efficiency vN0012

Vinay Kashyap & Pete Ratzlaff

We have updated the QE files for the HRC-I, following on from the 2018 version (v11) which introduced a time-dependence into the HRC-I QE. In this version, we extend the time dependence to include the latest epoch of observation.

Details

During recent calibration observations, a steady decline has been seen in the observed count rates and estimated fluxes of the soft source whute dwarf system HZ43 (Figure 1) and the hard source SNR G21.5-00.9 (Figure 2).

We continue to tie the HRC-I low energy (E<626 eV) QE to the shape of the HRC-S QE (in this case v14), while being normalized to match the zeroth-order aimpoint count rates of HZ43. The HZ43 rates are loess-smoothed with a 2nd-degree polynomial over ±5 nearest observations, in order to avoid overfitting the QE corrections to the measured rates. This implies a retroactive change even at older epochs of ~0.5%, while newly predicted rates match the observed rates with an rms of 1% (Figure 3).

Additionally, we also impose a grey decline of 1.5% percent/year in the QE post-2016 before applying the HRC-S shape correction in order to account for the QE drop visible at high energies. This has the effect of correcting the drop in flux for G21.5-00.9 (Figure 4).

The update (Figure 5) applies to all previous epochs, and as before, QE files are provided for each epoch when a HZ 43 calibration observation is made.

Figure 1: Comparing observed and predicted HZ43 count rates prior to QE correction. Upper panel: Absolute count rates as measured during each observation (red x) and statistical 1-sigma error bars on them (vertical bars), along with the count rate predicted by QE v11 (diamonds). The loess-smoothed curve used as the target for QE correction is also shown (cyan curve). Lower panel: As the upper panel, but normalized by the measured counts averaged between 2002 and 2010 (green solid horizontal line; 1-sigma errors on the average are shown as green dashed horizontal lines). Note that the predicted QE shows a deficit of >10% in 2020.
Figure 2: Measured Count rates and estimated fluxes of G21.5-00.9. Upper panel: Count rates relative to the measurement in 2008, for the shell (blue curve with 1-sigma error bars) and the plerion (red curve with 1-sigma error bars). The most recent observations show a trend of decreasing count rates. Lower panel: Estimated flux assuming an absorbed power-law spectrum, calculated using QE v11. Since the QE does not incorporate a temporal change at high-energy, the estimated flux also shows a decreasing trend, of approximately 1.5% per year.
Figure 3: HZ43 count rates after QE correction. Upper panel: As in Figure 1 (top), shows the measured count rate (magenta x's, with 1-sigma error bars), the loess-smoothed target (cyan curve), and the count rates predicted using QE v12 (diamonds). Lower panel: The ratio of the observed count rate to the predicted rate. The horizontal cyan line indicates unity. The caption indicates both the statistical (average of the 1-sigma errors) and the systematic (1-sigma scatter in the residuals).
Figure 4: G21.5 fluxes after HRC-I QE correction. The upper panel shows the estimated fluxes for the plerion observed with the HRC-S as calculated using QE versions 14 (blue) and 15 (orange). The lower panel shows the plerion fluxes estimated for HRC-I observations using QE versions 11 (blue) and 12 (orange). Note that the HRC-I QE v12 succeeds in correcting the flux decline, though there is an as yet unresolved difference in the estimated fluxes between the HRC-S and the HRC-I.
Figure 5: Magnitude of HRC-I QE corrections. Upper panel: Plots showing the QE curves adopted for each epoch. Lower panel: The corrections applied relative to epoch 2000-Feb. The colors change from blue to red, with bue being the oldest and red the newest.