Strategy

Our strategy here will be the following.

[1] Examine the ratio of SIS-1/SIS-0 spectra over time. This will give the RELATIVE SIS-1/SIS-0 behaviour and is MODEL-INDEPENDENT.

[2] Find point sources which are non-varying, at least at low energies, AND were observed early in the mission, AND then again later in the mission. This will give the ABSOLUTE SIS-1 and SIS-0 behaviour and is MODEL-INDEPENDENT.

[3] Make use of multi-mission observations to get more data points for the ABSOLUTE calibration. However, this is MODEL-DEPENDENT.

[4] Try to verify the results from the above by forcing agreement of both SIS-1 and SIS-0 (independently) to GIS-2 and GIS-3 by spectral fitting suitable sources. Effectively, extra absorption is added independently to SIS-1 and SIS-0 until a best-fit is obtained in a four-instrument spectral fit. This is meant to verify the ABSOLUTE degradation but is HIGHLY MODEL-DEPENDENT.

For [1] we can use virtually any fairly bright source (more than a few tenths cts/s/SIS), observed at any time, even if it is highly variable.

For [2] , finding suitable sources is a very tall order. The SIS degradation we are investigating became noticable in observations made as early as the begininng of 1994 so we have to find sources which were observed in the PV phase (approxmately March 1993 to the end of 1993) AND then again at least once more at later date. There are several severe problems:

Most observations of point-sources in the PV phase were made in 4-CCD mode.

The most suitable candidates for [2], constant sources such as supernovae and clusters, were invariably observed in 2-CCD or 4-CCD mode whether they were observed in the PV phase or not, because they are generally extended.

Constant sources were almost never observed more than once simply because they are constant (in flux and spectrum) and therefore were very unlikely to be approved for observation by a time-allocation comittee on the basis of scientific merit.

Calibration sources observed throughout the mission are unsuitable for [2]. The SIS cannot observe the Crab; 3C 273 is variable and the Cas A observations which were done in 1-CCD mode had a high lower-discriminator setting, killing the soft spectrum.

The sources for part [2] of the study must have a low-energy spectrum which is well above the background to minimize systematic uncertainties.

We painstakingly searched the entire ASCA public database for suitable sources for part [2] of the study. The enormous problems mentioned above resulted in only THREE suitable sources. These are NGC 4636 (an elliptical galaxy); NGC 1068 (a Seyfert 2 galaxy), and N103 (a supernova).

Anchor Point on 16 December 1993

An observation of 3C 273 on 16 December 1993 was used by the ASCA team to revise the SIS response for the 1994 calibration release by adjusting the SIS quantum efficiency (QE) relative to the ground calibration to force consistency between the four ASCA instruments. However, at the time, it was not realised that the discrepancy which was being corrected was continuously changing with time. Therefore, that calibration was only good for that point in time. This means that when we use that calibration (which is still the current calibration), observations made before 16 December 1993 will appear to have SIS data which TURN UP at low energies and observations made after 16 December 1993 will have SIS data which TURN DOWN at low energies. This down-turn is increasing with time. This anchor point means that effectively the time-dependent SIS low-energy correction we are investigating is zero on 16 December 1993 (because it was forced to be). In terms of an excess absorption parameterization, observations made before 16 December 1993 will be expected to require a NEGATIVE column density.

Next: Degradation of SIS-1 relative to SIS-0
Back: "SIS Low-Energy Degradation" Main Page

This file was last modified on Monday, 30-Jul-2001 18:17:25 EDT
Curator: Michael Arida (SP Sys); arida@milkyway.gsfc.nasa.gov
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This file was last modified on Monday, 30-Jul-2001 18:17:25 EDT