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Updated Effective Area Files were released 15 Jan 2009 along with an updated CALDB and The Proposer's Observatory Guide.
The updates will affect PIMMS, Effective Area Viewer and proposal threads which use this software, as well as all CIAO software which accesses instrument effective areas in the CALDB. Information on the status of this update and about the updated effective area files will be posted prominently on the Proposer Webpages and on the Cycle 11 Effective Area download webpages as well as described in the POG. In most cases proposal and observation planning will not be impacted, so that proposers may start by using the tools released with this Cycle 11 CfP. However we recommend that proposers revisit count rate and other estimates or simulations after we have issued the updates and before they submit their proposals.
The updates involve two issues. The first adjusts the time dependence of the contamination buildup on the ACIS Optical Blocking Filters (OBF) and the second better takes into account a small level of contaminant on the Chandra Telescope (HRMA) that appears to have been present prior to launch.
1) Contaminant Build up on the ACIS Optical Blocking Filter (OBF): The primary impact of the change in ACIS filter modeling is to allow for more accurate extrapolation of throughput to future times, bringing it in-line with current measurements using the ACIS External Calibration Souce (ECS). However, this change will not remove a current uncertainty in the filter throughput below ~500 eV. At the Carbon K-edge, (284 eV) the uncertainty would lead to systematically high, derived column densities at a level ~1e20 cm^{-2}. For proposers, the lack of an accounting for the additional optical depth results in over-estimated count rates at low energies. We provide information and guidance to proposers (POG Section 5.6.1).
2) HRMA Effective Area: The primary impact of the change in the telescope effective area is a lower effective area at low energies which will result in lower continuum-derived temperatures for high-temperature thermal sources such as clusters of galaxies, steeper derived power law slopes (~0.1) and a few percent higher derived fluxes. In terms of estimates for proposals, those made with the current release (i.e. before the January update) will over-estimate the low energy (<2 keV) count rate. Please see POG Section 4.2.2 for details.
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The Chandra X-Ray
Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA. Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2004. All rights reserved. |