ASCA Software News

[ASCANews Custodian <MUKAI@lheavx.gsfc.nasa.gov>]

ASCA Guest Observers,

This message includes three news items on ASCA software:

	(1) WMAP bug in extractor
	(2) EBOUNDS extension in SIS response matrices
	(3) Roll

Also, a final reminder that the AO-5 proposals are due in 1 week --- Good luck!

					- The ASCA GOF
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(1) WMAP bug in extractor

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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(2) EBOUNDS extension in SIS response matrices

It has come to our attention that v1.0 of sisrmg has been released with
'linear smoothing of the EBOUNDS extension' as the default (both in the
original FTOOLS v3.5 release and in the ASCA-specific update).

The EBOUNDS extension in a response matrix contains the approximate channel
to energy conversion; it is used by XSPEC for plotting only (and not for
fitting).  This conversion, for ASCA SIS, is not completely linear, even
in the PI space: there is a departure from linearity above 10 keV due to
incomplete charge collection, and some nonlinearity around the Si edge.

The linear smoothing option smoothes all these non-linearities out.  However,
since E>10 keV is included in the smoothing, the resultant scale is roughly
3.6 eV per channel compared with the correct average below 10 keV of 3.65 eV
per channel.  This results in a 6.4 keV Fe line apparently plotted at 6.3 keV
instead; a 6.4 keV line model will reproduce a 6.4 keV line in the data, and
both will be plotted at 6.3 keV.  Even though the fit is unaffected, the
appearance of the plot can be confusing, and may lead the user to incorrectly
initialize the line energy (which may cause XSPEC not to converge properly).

We therefore recommend 'no smoothing' as the default when running sisrmg.
One way of achieving this is to change one line in the parameter file
(sisrmg.par in the bin.host directory of FTOOLS) from:

gains,i,h,-1,-1,12,"Amount of Ebounds smoothing"

	to

gains,i,h,0,-1,12,"Amount of Ebounds smoothing"

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(3) Roll

Due to the ~square shape of the CCD chips used in the SIS instruments,
the roll angle of the spacecraft may play an important role in determining
the coverage of an extended source or of a field with multiple point sources.
Guest observers who are preparing proposals for such observations, or planning
the details of already approved observations, may be interested in a new
version of the ASCA-specific utility program, roll.

Roll is available for local installation from the legacy.gsfc.nasa.gov
anonymous ftp account, in the directory asca/software (this contains a
compressed tar file and an installation guide).  Alternatively, users can
remotely login to legacy using the 'xray' account; 'roll' is available at
the 'HEASARC>' prompt.  For further details, see 'roll.hlp' which is inculded
in the tar file.