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ASCA SIS calibration issues
We have recently become aware of two calibration issues for SIS
that ASCA guest observers and archive users should be aware of.
* Resolution degradation due to non-uniform CTI
Recent analysis of Cas A calibration observations shows that the CTI
effect (charge transfer inefficiency, due to charge traps caused by
cosmic ray protons) are highly non-uniform. (This is separate from
the previously reported RDD effect which primarily affects 4-CCD,
but also 2-CCD, mode data.) As a result, it is estimated that the
FWHM resolution at the Fe K region has degraded from 130 eV at launch to
220-250 eV in Aug 1995 in 1-CCD mode data (72 eV to 110-120 eV at Si K
line; this effect scales roughly with the square root of energy).
* Problem with low-energy calibration
Evidence is mounting that SIS spectra of a wide variety of objects
(AGN, stars) consistently show flux deficit at around 0.5 keV.
This is now believed to be a calibration problem localized to this
energy (but there is also a separate, more general problem resulting
in an overestimate of Nh, by about 3e20, in SIS spectral fits).
Effort is underway to improve the low energy calibration; in the
meantime, SIS data should not be trusted below 0.6 keV.
A full report of the ASCA Calibration Workshop will become available
from the ASCA GOF home page
in the near future, including more details on these issues.
* * *
Please also note that the current (0.9) version of sisrmg (in FTOOLS 3.4)
should not be used for PV data (v0.8 matrices in the ftp area remain valid
for this period). V0.9 matrices correct some (but not all) of the problems
caused by the RDD effect (thus relevant to AO-1 and later data), but this
is treated incorrectly for the earliest mission times.
- ASCA GOF