2. The Solid State Imaging Spectrometers

There are two Solid State Imaging Spectrometers (SIS). Each SIS has four front-illuminated CCD chips. The chips are mounted side by side to form a square. Each chip consists of a 420 x 422 array of pixels. The field of view of each SIS camera is 22 x 22^$\prime$. The energy resolution is 2% FWHM at 5.9 keV.

In normal operation the SIS is operated in one-CCD mode. Observations using all four CCDs of each SIS were only carried out during the performance verification phase of the mission. The mode in which the SIS is read out determines the spatial and timing resolution. In bright mode the CCD is read out once every four seconds, with full spatial information. Unfortunately, Cen X-3 is sub-Nyquist for this sampling frequency so bright mode data is not useful for pulse-phase resolved spectroscopy. In fast mode the CCD is read out every 16 ms. This timing resolution is achieved by sacrificing spatial information. The spatial information in fast mode is reduced to one bit. This bit tells us whether an X-ray event occurred inside or outside a rectangular region of the chip. The source is usually contained within this region which is set by the telescope operator after the first pass over a ground station. Its purpose is to allow observations of bright sources that might saturate the telemetry. If the count rate is too high, only events occurring outside the discrimination region will be transmitted to the ground station. One problem with fast mode is that it does not allow the removal of hot and flickering pixels. Problems caused by these effects should be less than in bright mode, however, because of the shorter integration time.

The XRT's response to incident X-rays depends on the off-axis and azimuthal angles of the source. The only way to determine these for fast mode is by also carrying out bright mode observations of the source. A map of the SIS CCD is made with each point weighted by the number of X-ray counts. This weighted map is used to generate an auxilliary response file (ARF). This ARF file takes into account the XRT response. The energy response of the CCD detectors is contained in a redistribution matrix file (RMF). In generating the RMF, changes in the gain of the detectors are taken into account. These changes occur because of degradation by charged particles. For SIS data taken in bright mode, these gain changes can be applied directly to the pulse-height-analyser (PHA) data to create pulse invariant (PI) data. PI spectral data will all have the same energy-channel relationship and can thus be added.