# Parameterization of SIS Efficiency Loss in 1-CCD Mode

*(ASCA GOF Calibration Memo [ASCA-CAL-00-06-01, v1.0 (06/05/00): T. Yaqoob & ASCATEAM])*

Since the launch of

*ASCA*in 1993 the low-energy response of both SIS has been degrading both in effciency and energy resolution. Whilst some of this degradation is corrected by the tool CORRECTRDD a significant problem remains and results in an apparent divergence of the the SIS response compared to the GIS at low energies (see Fig. 1). The problem is likely to be related to the increase of dark current and decrease of charge transfer efficiency (CTE) and is the subject of on-going investiagtion by the

*ASCA*team. Until this problem is fixed, we can try to quantify the apparent loss in SIS low-energy efficiency as a function of time throughout the mission, which will also help to understand and model the degradation. Indeed such a parameterization should help to get the correct form of the time-dependence of the eventual model. Note that the correct model, when it exists, will also correct for the change in energy resolution which is also not accounted for in the current software.

It is found that the efficiency loss can be parameterized
*crudely* with excess absorption.
Although this is very crude, at least it allows us to represent
the degradation with a single number for each SIS at a snapshot in
time. This has already been demsonstrated for a few 2-CCD mode observations in
Hwang
et al 1999 (ApJ 516, 604). The effect on 2-CCD and 4-CCD mode
data is much worse than 1-CCD mode. In this report we consider principally the
1-CCD mode data. The magnitude of the problem is illustrated in Figure 1
below with two observations of 3C 273. In each case a 4-instrument simultaneous
fit was performed with a power law plus Galactic absorption; however
an additional power-law was included for the first observation (1993/day
160) to model a soft-excess and there may be extra intrinsic absorption in the
second observation (2000/day 10-11) which is not included. The main
point, however, is to illustrate the divergence of the four instruments
at low energies, which does not depend on the model.

## Summary and Bottom Line

We have *crudely* parameterized the low-energy degradation of the
*ASCA* SIS-0 and SIS-1 versus time, with the
primary purpose that the form of the
relationship may provide important clues about the physical origin
of the problem. The relationship is summarized in Figure 5.

The spectral characteristics of the low-energy problem appear to be complex but we simply characterized it by a simple number (an effective "excess absorption column density" for each SIS).

We derived a straight line fit to estimate this excess N_{H} parameter
for a given observation time for
SIS-0 (a linear approximation is not good for SIS-1).
The corresponding number for SIS-1
can be derived from the data itself by examining the model-independent
SIS-1/SIS-0 ratio. If the signal-to-noise of the data is insufficient,
then one need not worry abut correcting the data anyway since the
statistics are then worse than the systematics.

Until a physical model of the CCD degradation is available one should
only use this parameterization to estimate the *magnitude* of the
effect. One cannot apply an "excess N_{H} correction"
and derive meaningful
parameters from models soft X-ray features below 2 keV, especially
from 1996 onwards when the degradation became very significant.

Note that no RDD correction was applied to the data in all of this analysis (the current tools have little effect on 1-CCD mode data).

*This file was last modified on Tuesday, 21-Apr-2009 21:28:14 EDT*

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