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ASCA Guest Observer Facility

WMAP Bug in Extractor

1996 Jul 30

We have just discovered a bug in the extractor that may affect a small fraction of ASCA users.

The spectral files written by the extractor contains a weighted map, or WMAP, as the primary array. When a spectrum is extracted from a list of files, the spectrum itself is accumulated correctly from all the files but the WMAP is only accumulated from the last file. (Extracting events, images or light curves works fine.) The resultant WMAP is more or less correct, but does not have the statistical accuracy that it should.

When such a spectral file is then used in ascaarf to create an ancillary response file (ARF) in the extended source mode, this may result in an inaccurate ARF file (particularly if the last file in the list had a much fewer counts than the rest). The point source mode of ascaarf, on the other hand, does not depend on the content of the WMAP image, so is unaffected by this bug.

The bug has been traced and the fix will be included in the next release of FTOOLS. In the mean time, for extended source analysis, it is recommended that you first create a single event file using the 'extract event' command. Spectral files extracted from a single event file are not affected by this extractor bug.

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This file was last modified on Monday, 13-Sep-1999 15:37:09 EDT

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