6.5 Instrument Specifics - Spectral Properties of the ROSAT PSPC

 

As already mentioned in the overview of this chapter all instrumental dependences are well separated in standardized and extentable modules. Most of them are described in chapter 4. Some of these modules might only be relevant for a specific instrument, some even might be not present or a specific feature is lost. A spectral analysis e.g.cannot be performed if a photon counter has no spectral resolution. But if such a counter has a well defined energy acceptance (e.g.filter), its signal can be used in connection with other similar detectors to combine the whole information to a spectrum.

Spectra taken at the same time with different instruments can be fitted simultaneously. Details can be found under section 6.2

Since the position sensitive proportional counter (PSPC)     -- a major part of the focal plane instrumentation of the ROSAT X-ray telescope (XRT) -- plays a special role in the context of the spectral analysis of EXSAS some of its properties are listed:

• Its energy acceptance in connection with the XRT ranges approximately from .1 keV to 2.4 keV.      
• Its energy resolution behaves as

   

  

  This means, that the detector has an energy resolution (FWHM) of 40 pulse height channels at 1 keV or roughly five to six independent energy bands. This fact always has to be considered when fitting a spectrum. Even if the spectrum contains counts it does not make sense to stay with all pulse height channels of the PSPC. Depending on the statistics in most cases a spectrum of 30-40 bins will be sufficient.

• Its particle background rate is expected to be in the order of

   

  10 counts s arcmin , but will vary with geomagnetic latitude. A rejection efficiency of this background was measured up to 99.5%.

• Its detection efficiency depends very strong on its energy mainly because the incoming X-ray photon has to penetrate the PSPC entrance window.   Up to the carbon edge at .28 keV this efficiency is about 50%, drops after that suddenly to zero and increases then continuously up to 70%.
• Its response function is normalized to 1     and up to now compressed to a matrix with 256 pulse height channels and 729 energy positions, at which the mirror reflectivity is known. But be aware - these dimensions may change with time.
• The current matrix is defined only between pulse height   channel 8 and 250 (inclusive boundaries). Therefore, a PSPC spectrum should be used only between these channels.
• The following linear energy-channel relation is used   (only for plotting purpose):

  

  The energy value of each channel can be taken from the descriptor ENERGY_OF_CHANNEL of the detector response matrix (see section 6.4.2).

• The calibrated amplitudes as given in the photon events files   (label AMPL) are already corrected for possible gain shifts of the detector. That means that all changes in the PSPC detector response matrix up to now are only improvements according to better understanding.
• In the meantime two sets of standard pulse height channel (energy) bands have been defined, the so called ROSAT and SASS bands. The eight ROSAT bands (channels refer always to the calibrated pulse height amplitude according to label AMPL) are used for different instrument maps. From these basic ROSAT bands five SASS bands A, B, C, D, and broad are derived. The exact definition of these bands can be found in Table 6.11.              

  The two hardness ratios HR and HR defined in section 6.6 (see also subsection 4.1.6) are deduced from these SASS bands.

  
Table 6.11: Definition of the PSPC pulse height channel bands