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Background Subtraction

The origin and behaviour of the LAC background is described in Hayashida et al. (1989). The main sources of background include the internal component generated after passage through the Earth's radiation belts, in particular the South Atlantic Anomaly (SAA), the high- and low-energy particles in the Earth's magnetosphere, and the diffuse Cosmic X-ray Background (CXB). The first two sources generate a background which is a strong function of time and energy. Summed over the top- and mid-layer electrodes as well as over the full energy range (1.5-37 keV), this varies between 50 and 100 counts  sec-1. The CXB contributes approximately 18 counts sec-1 to the background, which varies as a function of position in the sky but is constant in time.

The radioactivity induced by passage through the SAA has a 37 day periodicity, which resulted in two background subtractions schemes emerging. The simplest method is to use off-source observations within a few days of the source observation (i.e. with a similar phase in the 37-day orbital period of the Ginga satellite as the source observation) to model the background; this is known as the local method. Section 3.1 describes how to estimate the background using a single local off-source observation, and section 3.2 describes how to extend this procedure when more than one local off-source observation is available.

If an appropriate local background observation is not available, or if it is necessary to use the same background model for observations which are widely spaced in time (e.g. in order to minimise the effects due to variations in the diffuse CXB component), it is necessary to use a background model which explicitly includes the 37 day cycle; this is known as the universal method. The procedure for doing this is briefly described in section 3.3, but it is rather complex and there are a number of traps for the novice. It is not recommended that anyone without pervious experience at Ginga background subtraction attempts this. Because of the complexity in setting up a model which includes the 37-day cycle a set of standard files are available. They provide the quickest and easiest method of Ginga background subtraction. Their use is described in section 3.4.

The local method is recommended for observations performed during the first six months and last three months of operation of Ginga, as the gradual rise and fall of the background made the universal method unreliable. Otherwise the universal method is generally to be perferred.



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