ROSAT Status Report #29

Various checks have been made in order to explain these effects, including a detailed investigation of the in-orbit accuracy of the AMCS star tracker (STC). A relatively simple test is provided by the angular separation between two stars measured simultaneously in the STC field-of-view and a comparison with the known distance. First analyses by GSOC and MBB indicate a variability of the measured distances, partially correlated with the wobble motion. On average the STC errors seem to be a factor of about 3 worse than the ground calibration results. It is also apparent that the effect has been present since the beginning of the mission and in both star trackers.

Up to now there is no straightforward understanding of this phenomenon. We hope to find out soon, whether the effect is of systematic origin or whether it is erratic. In the first case a post-facto correction procedure could be envisaged, which might be able to compensate all or part of the effect.

Spectral Calibration of the PSPC detector

The PSPC is probably the best-calibrated imaging proportional counter ever flown. Every event is transformed through a long chain of correction procedures from the raw telemetry to calibrated time, position and amplitude. In particular we correct for: the ADC nonlinearity, the gain saturation effect, the temporal gain variations, small-scale electronic position variations, spatial gain variations, and large scale window position variations. The detector response matrix contains our best current knowledge of the mirror effective areas, the composition of the PSPC entrance window, and a physical model of the characteristics of the counter gas and its resolution properties.

Nevertheless, there are residual systematic uncertainties on the percent level in almost every element of the above chain. Most of the detailed information originates from extended ground calibrations with monochromatic lines; important elements were also derived or verified in orbit. One of our major problems is that there exists no bona fide continuum calibration source, neither on ground nor in orbit, that has a spectrum known well enough in the 0.1-2 keV range and is suitable for the PSPC (similar to the Crab Nebula for higher energy X-rays). Another concern is the possibility of unexplored temporal variations in the instrument performance. Since fall 1991 the instrument is operated at a gain lower than nominal, which in principle also influences the calibration. However, the shape and energy of the in-orbit aluminum K calibration line (1.5 keV) did not change significantly after the gain switch. To obtain a comparison at other energies we have just taken a series of ground calibration measurements with the PSPC engineering model at lower gain.

Systematic features in the residuals of PSPC spectral fits are a well-known problem, in particular for high signal-to-noise ratio sources. We do not yet have a complete overview of all possible effects; some of the problems seem not to be present in all datasets and some may even be true features of the observed source. Due to the relatively poor energy resolution of the proportional counter the different possibilities can not be unambiguosly disentangled. We are currently undertaking an effort to collect a sample of bright source spectra for a better diagnosis. Problems that we know about are:

While in the next few months we are trying hard to improve our knowledge of the PSPC performance and to push down the uncertainties even further, we advise ROSAT observers to include systematic errors of about 1 percent in their PSPC spectral fits and to be cautious in the interpretation of narrow spectral features, particularly those of small equivalent width. We would also appreciate to receive further information from observers about unexpected spectral features.

EXSAS

Since March '92 the Standard Analysis Processing calculates smoothed background maps by a different method avoiding some of the problems that sometimes came up in images with unusual background features. In short, these maps are produced from normal images by cutting out all the sources detected by the Local Detect Algorithm, then dividing the resulting image by the exposure map, applying a spline fit and finally multiplying the image reconstructed from the spline fit with the exposure map. Contrary to the product delivered before the change, the new background map shows now clearly the support structure of the PSPC window. The construction of background maps by EXSAS is not yet using this new scheme. For consistency reasons users working with source detection in EXSAS should therefore up to the next release of the software (which is planned to contain the new method) take the background maps as produced by the appropriate EXSAS commands.

Status Report #28

Select another report Return to the ROSAT GOF Curator: Michael Arida (ADNET); arida@milkyway.gsfc.nasa.gov
HEASARC Guest Observer Facility

This file was last modified on Monday, 25-Jun-2012 15:16:21 EDT