Analyzing extended sources with ASCA data

["Charles Day, ASCA GOF, NASA GSFC" <DAY@lheavx.gsfc.nasa.gov>]

Dear ASCA PIs and ASCA archival investigators,

As you may know, the point spread function (PSF) of ASCA's X-ray
telescopes (XRT) has a relatively sharp core (FWHM of 50 arcsec) but
broad wings (half-power diameter of 3 arcmin). In addition, the GIS has
its own PSF which is comparable in width to that of the XRT but of a
different shape (a Gaussian with a sigma of 30 arcsec at ~6 keV and a
1/sqrt(E) dependence).

For analyzing point sources, the practical consequence of the broad PSF
is that a generous extraction radius should be used to capture most of
the counts (e.g., 4 arcmin for the SIS, 6 arcmin for the GIS).

For extended sources, however, the astrophysical implications of the
broad PSF are more complicated and harder to deal with. If you are
analyzing an extended source, then please note the following effects
which, if not taken into account, could lead to spurious scientific
results. It is important to realise that these effects are not
calibration uncertainties or instrumental defects, but features
intrinsic to the design of the XRT which emphasizes effective area as
much as angular resolution.


   1. General redistribution of counts.

   The XRT PSF has a sharp core and broad wings. If your extended source
   has a brightness distribution which is peaked on the same scale as
   the PSF, then the outer parts of the image will contain a significant
   proportion of counts from the core. For example, the moderate
   redshift cluster A2218 has a core radius of about 1 arcmin. If the
   image is divided into annuli (3 arcmin wide centered on the core)
   then the second annulus (between 3 and 6 arcmin) will contain more
   emission from the cluster core than from the parts of the cluster
   which actually lie within the annulus.


   2. XRT vignetting

   The effect described above will be compounded by telescope vignetting
   if the core of your object is close the optical axis of the XRT.


   3. Energy dependence of the XRT PSF

   The XRT scatters high-energy X-rays more than low-energy X-rays. In
   other words, the PSF broadens with increasing energy. One consequence
   of this effect is to introduce spurious (outwardly increasing)
   temperature gradients in isothermal distributions. And if there is a
   negative temperature gradient present, its size will appear reduced.
   
   
   4. Off-axis ARF (Auxiliary Response File)

   If an off-axis region is specified and an ARF constructed for it then
   if this region really contains counts from closer to the optical axis
   then the ARF will not be correct and the difference will be in the
   sense of increasing the measured temperature.


   5. Energy dependence of the GIS PSF

   Unlike the SIS, the GIS has an intrinsic PSF which is of comparable
   size to that of the XRT. The energy dependence of the GIS PSF is a
   further complication.


In view of these effects, extracting spatially resolved spectral
information from ASCA observations is not trivial, especially in the
case of sources which are of order 10 arcminutes or less in size. It is
likely, therefore, that extensions to present analysis techniques are
needed to address the issue.

The ASCA instrument teams are calibrating the PSF as accurately and
completely as practicable. To this end, the ASCA Team recently conducted
a series of supplementary calibration observations of the bright X-ray
binary Cygnus X-1. Images extracted from these observations, which
placed the target at various positions in the field of view, are
available for several energy bands in the directories:

   caldb/data/asca/gis/bcf/psf  (unsmoothed datasets)
   caldb/data/asca/xrt/bcf/psf  (smoothed, i.e., GIS-compensated)

They are accessible via anonymous FTP at legacy.gsfc.nasa.gov. The
ray-tracing PSFs which have been available since March 1993 have been
moved to:

   caldb/data/asca/xrt/bcf/psf/ray_trace

In general, dealing with the effects of the broad PSF will involve
deconvolution, the complexity of which will depend on the particular
case in question. Modifications are being made to XSPEC that will
provide the potential for dealing with this problem by allowing
models to read information from the headers of PHA files. The GOF
staff will be available to advise users in generating the appropriate
deconvolution software for other, more complex cases.

The ASCA Team is also producing a PSF generating FTOOL called MAKE_PSF
which will output a PSF for arbitrary energy and position. MAKE_PSF will
be included in the next-but-one FTOOLS release in Spring or early
Summer.


If you have questions or comments about any aspect of this message,
please send email to:

   ascahelp@athena.gsfc.nasa.gov

Charles Day and the ASCA GOF