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ASCA Guest Observer Facility

Degradation of Low-Energy Degradation of SIS in 1-CCD mode

This document is a summary of

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

in which full details of the following results can be found.

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.

Fig. 1a: 3C 273 1993/d 160
Fig. 1b: 3C 273 2000/d 9-10
1993 spectra of 3C 273 showing good agreement among 4 instruments 2000 spectra of 3C 273 showing divergence among 4 instruemnts

Time Dependence and Magnitude of the Apparent SIS Degradation

We summarize the results of an investigation into the time-dependence of the SIS degradation. See full report for details.

  • We can very crudely characterize the apparent loss in low-energy efficiency as an excess absorption, or "excess NH."

  • The (model-independent) ratio of SIS-1/SIS-0 spectra shows non-montonic behavior superimposed upon an overall trend in which SIS-1 became significantly worse than SIS-0 over a few months in late 1995/early 1996 - see Fig. 2.

  • We identified a number of observations suitable for quantifying the absolute magnitude of the effect and the results, in terms of the "excess NH" parameterization are shown in Fig. 3. The time-dependence for SIS-0 appears to be consistent with a linear relation but that for SIS-1 is more complicated.

  • Specifically we find that a good approximation for the "excess NH" for SIS0 is

    NH(SIS-0) = 3.635857508e-8(T-3.0174828e7) e20 cm-2

    where T is ASCA time (seconds since 1993, Jan 1 take the average of the start and stop times of your observation). For SIS-1, we suggest that you measure the SIS-1/SIS-0 spectral ratio of the actual data from your observation. If the signal-to-noise of your data is not good enough to do this, then you need not worry about "correcting" your data since statistical errors will domainate.

  • You should be warned that the "excess NH" parameterization should only be used to quantify the degradation of your data and should not be used to apply a spectral correction since the intrepration of spectral features may then be incorrect.

  • It appears that the spectral ratio of SIS-1/SIS-0 is similar for 1-CCD and 2-CCD modes, even though the absolute degradation for 2-CCD mode is worse that that for 1-CCD mode.

    Excess NH history

    Fig. 2: The excess effective column density derived from SIS-1/SIS-0 ratios.

    Parameterized history of ASCA SIS low-energy degradation

    Fig. 3: Derived effective "excess NH" versus ASCA time.

    This file was last modified on Monday, 30-Jul-2001 18:17:25 EDT
    Curator: Michael Arida (SP Sys);
    HEASARC Guest Observer Facility
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    This file was last modified on Monday, 30-Jul-2001 18:17:25 EDT

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