The HRC-S is Chandra's microchannel plate detector specially designed to work in conjunction with the LETG. The current version of the LETG+HRC-S effective area is based on a combination of existing calibration using the white dwarfs Sirius B and HZ 43 for energies below the C K edge (about 284 eV, 44 Å), and a 2009-2010 cross-calibration with the HETG+ACIS-S combination using blazars PKS 2155-304 and Mkn 421. The revised HRC-S QE resulting from this work was incorporated in CALDB 4.2.0 in Dec 2009.
We no longer maintain pre-computed effective area files for the LETGS. The effective area for an observation depends on the off-axis angle and other specifics of an observation. Effective areas should therefore be calculated on an observation-by-observation basis using CIAO, and in particular the mkgarf and fullgarf tasks (see also the "Analysing LETG Data" page and the LETG+HRC-S science threads).
The LETG+HRC-S effective area depends on the HRMA effective area, the LETG diffraction efficiency, and the HRC-S total quantum efficiency (QE). The latter is a combination of the HRC-S UV/ion shield transmittance and the QE of the microchannel plate photon event detection system.
Prior to in-flight calibration, the on-axis HRC-S QE was calibrated in the laboratory over the energy range 0.28-10.0 keV. QE calibration below 0.28 keV was incomplete, as was the HRC-S QE uniformity, required for computing the LETGS dispersed effective area. However, the accuracy of the ground-based calibration could be improved upon by cross-calibrating with Chandra's other instruments in-fight. The effective area physically covers three microchannel plates (MCPs) and spectrally covers more than two orders of magnitude in energy. Since there is no single source available to cover this broad energy range, the calibration of the LETGS effective area has been achieved in multiple phases.
For the low energy calibration (E < 0.277 keV; wavelengths > 44 Å) observations of the hot white dwarfs Sirius B and HZ 43 were compared with predictions from state-of-the-art pure hydrogen non-LTE white dwarf emission models. For the mid-range calibration (2.0 > E > 0.277 keV; 6 < wavelengths < 44 Å) spectra of the active galactic nuclei PKS 2155-304 and 3C 273 were compared with simple power-law models of their seemingly featureless continua. The residuals of the model comparisons were taken to be true residuals in the HRC-S quantum efficiency (QE) model. Additional in-flight observations of celestial sources with well-understood X-ray spectra have served to verify and fine-tune the calibration. From these studies we have derived corrections to the HRC-S QE over the full practical energy range of the LETGS. The current version of the LETG+HRC-S effective area at energies above the carbon edge is based on an extensive 2009-2010 re-analysis of LETG and HETG observations of the blazars PKS 2155-304 and Mkn 421. This latter analysis was largely tied to cross-calibration using the MEG+ACIS-S combination. A set of observations of PKS2155-304 with the HETG and LETG in 2008 July has proved particularly useful for refining the LETG+HRC-S effective area.
The effective area for the dispersed LETG+HRC-S spectrum depends on a description of the HRC-S QE uniformity - essentially a map of how the QE varies across the detector. The HRC-S QE uniformity map (QEU) was constructed from pre-flight laboratory flat field measurements at discrete energies. This was later refined using in-flight observations by comparison of the observed signals in positive and negative spectral orders. Implementation of the HRC-S QEU with the on-axis QE now allows for the computation of effective area for any reasonable Chandra/LETGS pointing. The absolute accuracy of the LETG+HRC-S effective area is believed to be 15% across the full energy range.
There are some known deficiencies in the current effective area calibration that are under active investigation and analysis:
The white dwarf emission models on which the low energy QE was based have changed since the original calibration was undertaken. The soft X-ray emission also depends strongly on the adopted values for the effective temperature and surface gravity. Refinements in observations and analysis of data obtained at other wavelengths have also lead to small changes in best estimates of these parameters. Consequently, we are currently engaged in a reanalysis of the low E QE using new white dwarf models.
Recent (2011) revisions to the LETG higher order diffraction efficiencies are also expected to have a knock-on effect on the effective area at higher energies at the level of a couple of % or so. Blazar data will be re-analysed to take this into account. At this time, we will likely adopt broken power laws to more accurately represent the observed spectra.
The spectrum extraction efficiencies - the fraction of photons that are collected in the dispersed spectrum compared with photons that pass through the HRMA and LETG combination - are also in the process of being updated. While this analysis will not affect the effective area, the HRC-S QE will need to be revised so as to keep the total area constant once new extraction efficiencies are implemented.
Last modified: 04/15/11
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