Max-Planck-Institut für extraterrestrische Physik
Revisiting ROSAT PSPC Calibrations
|Deutsche Version||MPE HEG X-Ray Astronomy Wave ROSAT Calibration PSPC status 1996|
Revisiting ROSAT PSPC calibrations
ROSAT Workshop, 1996 November 5
M.J. Freyberg et al., MPE Garching (email: mjf @ mpe . mpg . de )
Recently the REV2 reprocessing of PSPC data was completed. These new data are a major improvement to previous processings. In addition to revised corrections also the raw event information is now available to guest observers.
However, present spectral fits still yield an apparently systematic and significant behaviour of residuals, especially at channels 30-40 and . Additionally observations of some sources where we believe to know the spectral shape (e.g. Mkn421), have shown an unexpected excess at higher energies. For several soft sources (e.g. white dwarfs) a higher PSPC response was reported than expected from present calibration data. This has led to several attempts to improve the PSPC calibration data set, i.e. effective area (EFFAR) and detector response matrix (DRM).
Systematic residuals can also originate from the source model itself, like different abundances for the absorbing ISM or non-LTE models instead of a Raymond-Smith plasma model. Also a clumpy ISM may lead to a soft excess which can not be satisfactorily modelled by standard spectra. Note, that the galactic neutral hydrogen column density, , need not to represent the measured X-ray absorption of the PSPC spectrum using standard cross sections and abundances. In the following we assume that nothing peculiar occurs in the intrinsic source spectra.
Summarizing, each additional improvement alone does not neccessarily lead to an improved significance of the spectral fit. Combining all, however, leads to at least the same with highly reduced systematic residuals at 0.9keV. The low-energy residuals can be reduced but are still significant. Globally, residuals appear more randomly distributed than before (this has not yet been quantified). The changes of the residua of individual sources do strongly depend on the source spectrum as well as on the detector position, but no general trend can be predicted.
New corrections will be distributed in the next EXSAS release (March 1997) and will be available on a patch level before. They concern the spatial gain variation in the detector. PROCESS/CT will apply these corrections to the raw data if desired (see item 2 above). This command can also be used to correct REV2 data that have been reprocessed with a SASS-version before 7.2 (see item 1 of the previous section). Additionally, several tools for determination and correction of the PSPC gain will be included in the new EXSAS release. The additional corrections will also be incorporated in a future SASS reprocessing (REV3). Gain images of the PSPC will be placed in the calibration area. A new calibration data set (EFFAR, DRM) for REV2 data will also be provided as an EXSAS patch as soon as a consistent picture of the remaining residuals is available (items 3 - 5 of previous section).
At the moment you are strongly encouraged to use the present REV2 data (instead of REV0 or REV1) with the present calibration effective area table (EFFAR) and detector response matrix (DRM). In some cases spectral fits may give a slightly worse . There are only a few cases of bright sources (> 20cts/s) which have been observed in the detector center later than May 1992 that show significant changes of the spectral parameters.
Upto the time where new calibration data sets (EFFAR, DRM, gain images) are available you can already make use of the new corrections (PROCESS/CT), e.g., if you have to select your background from a region with quite different effective gain relative to your source position. Then you should correct the background photons as well as the source photons with the corresponding effective gain value, use the corrected background spectrum for the correction of the source spectrum, and use the present calibration set for spectral fits.
The translation was initiated by Ortwin Schwentker on Mo 27 Jan 14:14:40 1997
If you have problems/suggestions please send mail to xray-info @ mpe . mpg . de