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Philosophy behind pcabackest

Previous proportional counter experiments in earth orbit have successfully modelled the instrument background as a function of one parameter (e.g. HEAO-1 A2) or several parameters (e.g. Ginga). Some experiments (EXOSAT) have also employed offset detectors. While some early concepts for the PCA included offset or movable detectors, the flight detectors are all fixed on the RXTE spacecraft and are all pointed along the science axis.

The background consists of two components, the diffuse sky background which enters through the collimator as X-rays, and the internal background which arises from interactions between radiation or particles in orbital environment with the detector or spacecraft. While the sky background is assumed constant at any one pointing position, the internal background may vary as the detectors move through different ambient conditions. The primary function of the tool pcabackest is to estimate the variable, internal, component to the background. Pcabackest is also be capable of adding a constant term to approximate the sky background.

It is assumed that the internal backgruond varies on a time scale long comapred to 16 seconds, the natural timescale of the PCA Standard 2 data. The design is that the model can be evaluated once every 16 seconds, and that the equivalent of a standard 2 data file can be created with separate information for each detector and each anode chain.

The intended usage is that each observer will run pcabackest for each data interval of interest. At this point, the user can extract (with the ftools or any other method) both the data and an estimate of the internal background. In the context of the ftools, all of the good time intervals may be applied to the output files from pcabackest exactly as if to the Standard 2 data itself. This allows the observer to exclude certain time intervals from the data collection, and the same intervals are trivially excluded from the background estimate.


next up previous
Next: Running pcabackest Up: PCA Background Estimation Status Previous: PCA Background Estimation Status

Keith Jahoda
Wed Nov 27 15:54:02 EST 1996