4 - Data reduction and calibration

Summary

The next five chapters cover the practical details of data analysis in X-ray astronomy. In this chapter we will consider what it takes to get the data to the stage of performing scientific analysis, which will itself be described in the following chapter. We will then survey the archives, catalogs, and software available, follow that with a short discussion of statistical issues peculiar to X-ray astronomy, and finish with the special considerations required when analyzing observations of sources which occupy the entire field-of-view (FOV).

We start with a description of the standard files, their contents, and the processing pipeline which produces them, as well as the initial data-reduction steps typically performed by the individual scientist.

The event file

All X-ray detectors measure individual photons. This is in contrast to many instruments for longer wavelengths, which measure integrated flux. The reason for this difference is that X-ray photons have relatively large energies, so single ones can be easily detected, but have relatively low fluxes, so they are easy to count. The basic data structure is thus a list of detected events, each of which has a set of attributes. Current X-ray instrumentation typically measures the position the X-ray arrived on the detector, the time of arrival, and some attribute which relates to the energy of the photon. Polarization is harder to determine and the first sensitive X-ray polarimeter is due to be launched on the GEMS satellite in 2014.