Theoretical grating efficiencies were computed pre-flight, and were measured during XRCF, and reanalyzed by Brad Wargelin and Pete Ratzlaff in 2006. Note that the higher dispersion order counts are explicitly included in the predicted counts at the appropriate bin (i.e., m^th order photons of intrinsic wavelength w are placed in the bin corresponding to first order wavelength m*w) and with the appropriate weight (G(w) = Sum_m g_m(w)).
The extraction efficiencies were estimated using MARX simulations for the standard bow-tie spectral extraction region. Note that in CIAO analysis, the extraction efficiencies are included in the gRMFs when computed with mkgrmf, and are not part of the gARFs made with mkgarf. The new LETGS+HRC-S QE to be released later this year will have the same convention.
The UVIS filter transmission was measured in the lab and has not been modified.
The HRC-S QE is based on lab and XRCF measurements, and has not been modified since flight except for the following cases:
The QEU is derived primarily from flat field lab data, from which an energy-dependent spatial model was built. The spatial model is constrained to be smooth over scales ~ 20x line width.
The 0th-order QE was backed out of the final gARF by dividing out all the other factors and then averaging over the +ve and -ve orders.
The HRC-I gain maps correct for the spatial non-uniformity of the gain across the detector and the gain decline over time. The maps provide a multiplicative correction to transform PHA (or sumamps) to PI (or corrected sumamps): PI=g(x,t)*PHA where g(x,t) is the gain correction value for time t and position x.
At present the time-dependence of the gain correction is implemented by a series of gain maps, each covered a period of approximately 12 months. This approach was chosen for its flexibility -- it is much easier to put a new series of maps in the CALDB then it is to change a functional time-dependent correction hard-coded in hrc_process_events.
The creation of the gain maps can be summarized as follows. To make the initial map, we use use lab flats taken at 6 energies. We bin the lab flats by 128 pixels^2, using the mean PHA (or sumamps) in the 128x128 area as the value of the resulting image pixel. We then average these 6 binned images, then normalize the resulting mean image to the central 9x9 pixels. Finally, we take the reciprocal of the normalized image. (This is done because the gain maps are used as a multiplicative correction in hrc_process_events.)
To create the series of time-stamped maps, we use observations of AR Lac which are taken yearly at the aimpoint and 20 offset locations on the detector. For each set of 21 observations, we first apply the lab gain map. Next, we compare each offset profile to the aimpoint profile, finding a "spatial correction factor" f so that f*offset matches the aimpoint with minimum chi-squared. We use the 21 spatial correction factors to interpolate a minimum curvature surface which we multiply with lab gain map to create a "spatial correction map" for the given year.
Lab data, supplemented along spectroscopic window by in-flight observations of PKS 2155-304 and HZ 43 to determine time dependence. See memo by Brad Wargelin, Pete Ratzlaff, and Mike Juda.
The HRC-I does not have good spectral resolution, but the PHA profile changes based on the incoming photon energy (e.g., see Figure 7.7 of the POG). The profiles are broad, but the mean of the distribution tends to increase with increasing photon energy. Thus, large changes in spectral properties among different sources can be distinguished. In order to quantify this, we have created an RMF based on flight data obtained using the HRC-I/LETG and correcting for gain variations at off-center positions using the gain map.
The data sets used to construct the RMF are Cyg X-2 (Obs ID 87), PKS 2155-304 (Obs IDs 1801, 3716), and HR 1099 (Obs IDs 1388, 1389, 1392, 1393). (And the gain map uses lab data, modified by yearly multi-point AR Lac observations.)
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