Our strategy here will be the following.
For  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  , 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:
We painstakingly searched the entire ASCA public database for suitable sources for part  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.
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This file was last modified on Monday, 30-Jul-2001 18:17:25 EDT
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