Notes on SIS low-energy efficiency

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

SIS efficiency and the anomalous measurements of Nh
---------------------------------------------------

We have received many reports of discrepancies between the Nh measured
with SIS data and other instruments (GIS, ROSAT, Galactic Nh from
radio data etc.), above and beyond the previously known problems
(Nh offset of 2-3 times 10^20; calibration problem around 0.5 keV).

This message summarises the current understanding of the problem
and measures that can be taken to reduce the problem.  Updates will
be posted on our 'calibration uncertainties' page

	http://heasarc.gsfc.nasa.gov/docs/asca/cal_probs.html

as they become available.  Please check this page from time to time
for the latest information.

[1] RDD effect

A crucial component of the SIS calibration is the set of grade branching
ratio functions.  Radiation damage, in the form of RDD (Residual Dark
Distribution) effect, is changing these grade branching ratios.  However,
this effect has not been quantified yet, and therefore is not yet implemented
in the current tools.

	Through the tools ascalin/sispi and sisrmg, and associated calibration
	files, we account for the other effects of the radiation damage
	(the secular changes in the PHA to energy conversion and the degrading
	spectral resolution).  However, it is quite possible that some
	assumptions we have made in deriving the calibration files, or
	(for recent data) the fact that we are extrapolating (e.g., the last
	observations used to derive sisph2pi_110397.fits were the 1996
	observations of Cas A) may lead to inaccuracies.  We are in the process
	of incorporating the 1997 calibration observations of Cas A and
	reviewing some of the simplifying assumptions that we have made.

The quantitative calibration of how RDD reduces the SIS quantum efficiency
as a function of energy, is on-going.  Please see

	http://www.astro.isas.ac.jp/~dotani/rdd.html

for details.  The effect is CCD-mode dependent; moreover, there probably is
a temperature dependence in this effect, as a higher temperature increases
the dark current.  The CCD temperatures can be found in the Sn_TEMP columns
(n=0, 1) of the mkf file; the nominal temperature is -61.6 C, but sometimes
can be as high as ~-59C.

[2] Using correctrdd

The average dark current level for a given pixel is believed to be stable
over timescale of ~months.  Thus, by knowing this average (via Frame mode
observations that are now taken regularly), the RDD effect can be largely
removed on an event-by-event basis.  This idea is implemented in the FTOOL
'correctrdd'.

Correctrdd requires the 'RDD map' files, generated from a series of Frame
mode calibration observations.  In the early days of ASCA, Frame mode data
were not taken regularly.  The RDD map files exists for 4-CCD mode data only
in 1993 Apr, 1994 July (except chip 0 on both SIS-0 and SIS-1), 1994 Nov,
and in 1995 Mar.  In 1995 Apr, Frame mode data were taken in 1, 2, and 4 CCD
mode in all chips that are frequently used in the respective clocking modes.
Starting in late 1995, we have started taking Frame mode data regularly.

The RDD map files are available at

	ftp://legacy.gsfc.nasa.gov/asca/data/trend/rev2/rddmap

The file names have the form "rdd9304_s0c0m4_t61_62.fits" encoding the year
and the month, instrument and chip numbers, the clocking mode, and the
temperature range.

Running correctrdd is trivial enough, once the appropriate RDD maps
have been obtained; the tricky issue is the interplay between RDD correction
and DFE correction (the latter is the correction of time-dependent dark
frame error due to, for example, the light leak).  It appears that the
order faintdfe - correctrdd - faint minimizes the errors due to this interplay.
There is beta-test script, 'gordd', available for applying these tools
in appropriate order to unscreened (.unf) files at

	ftp://lheaftp.gsfc.nasa.gov/pub/mukai/gordd

The result of 'gordd' is a (set of) Bright2 mode data that can be reduced
using, e.g., 'ascascreen.'

	http://heasarc.gsfc.nasa.gov/docs/asca/rddrecipe.html

[3] Event Threshold and 2-Pixel Events

We have recently discovered another effect that may lead to spurious
measurement of Nh in SIS data: the current version of sisrmg uses a
simplified model of how 2-pixel events interact with event threshold.
When the event threshold is significantly different from the default,
the efficiency predicted by sisrmg for energies < 2 x <event threshold>
is too high for SIS events of grades 2, 3, or 4.  The same observations
for which RDD is significant are the ones that tend to suffer most from
this effect (since radiation damage also increases the danger of telemetry
saturation in a clocking-mode dependent manner).  Also, the RDD effects
tend to increase the 2-pixel events and make this effect more important
(in early data, the fraction of 2-pixel events is <10% below 1 keV).

The situation is much more benign for grade 0 (single pixel) events;
we recommend fitting a grade 0 spectrum and a grade 234 spectrum
simultaneously, whenever the event threshold is significantly greater
than 0.4 keV.  More details can be found at:

	http://heasarc.gsfc.nasa.gov/docs/asca/2pixel.html