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Time variability studies

 

Time variability of X-ray emission can be studied with the PSPC subject to the following limitations:

1. While each recorded event is time-tagged, the electronics allows only an effective resolution of s. This accuracy applies to the relative arrival times of photons in one observation. The accuracy of absolute timing with respect to UT is expected to be on the order of a few milliseconds.
2. For sources with count rates of more than 100 - 300 counts s the dead time correction will be determined by the source count rate rather than the background count rate.
3. For studies of variability on time scales of minutes the spacecraft wobble and the characteristic time scales of the attitude control should be taken into account (cf. § 4.2.2  and Figure 4.7 ). This wobble exercises a slow drift diagonally to the entrance window wire grids in order to prevent permanent occultations of X-ray sources behind the wires. On the other hand this motion causes all sources to pass behind some thick wire at some unpredictable time and therefore imposes an apparent variability with a typical time scale of 1 minute on an otherwise constant X-ray source. Placing an X-ray source off-axis will increase the apparent size of the source on the detector (cf., Figure 10.19 ) and reduce the effects of shadowing. In case that such off-axis pointings are required the national data centers should be first consulted and the request be made to the respective data center.
4. PSPC observations will be interrupted at least once per orbit and, depending on the satellite-radiation belts geometry, possibly up to three times per orbit. Uninterrupted viewing periods can last, under favorable circumstances, up to 4000 s; however, a far more typical viewing window will be  s.
5. For monitoring X-ray sources on time scales of weeks or longer it should be taken into account that, first, the X-ray detector will be changed from PSPC to HRI and back typically once per week (depending on the observing time requested for each detector) and, second, that sources in the plane of the ecliptic are observable for about one month twice a year.
6. The minimum time scale on which to detect source variability depends on the source count rate (cf., Figure 12.12 ).