Background Subtraction

This is a complex problem and needs to be thought about carefully before proceeding. The PSPC background consists of a cosmic X-ray background, a particle induced detector background, solar scattered X-rays, afterpulses, long and short term enhancements and auroral X-rays. A full description is given in Snowden et al, 1993.

The particles contribute $\sim2-3\%$ of the background on average ($\sim 5\%$ for an observation with the boron filter), with their major effect being above 1 keV. The needs of background subtraction are as follows:

1. A representative region of background has to be selected from the observation using the Isys GUI or the command driven image processing commands. The background region should satisfy the following criteria:

• Large enough to give good statistics
• Free from sources
• Should not include the ribs or central ring
• As near to source as possible, to avoid ramps in the background

2. The background data need to be corrected for vignetting effects before subtraction. This is energy dependent and is complicated by the fact that the particles in the background have a different radial dependency to the photons (i.e. they are not vignetted).

3. The background has to be normalised for the area of the source and background boxes before subtraction.

xrtsub calculates: $SUB = SRC - BCK \times \frac {A_{s}} {A_{b}} \left[ B_{phot} \frac {V_{s}} {V_{b}} + B_{part} \frac {P_{s}} {P_{b}} \right]$

where A is the area, B_phot the photon fraction of the background, B_part the particle fraction, V is the vignetting function and P is the position dependence of the particles. The subscripts 's' and 'b' refer to source and background respectively.

The program xrtsub caters for points 2 and 3, so the user only has to worry about point 1. The program ignores source or background pixels with bad quality and this feature may be used to create background regions with complicated shapes, which satisfy the requirements of point 1. e.g. an annulus at a large radius with sources and the ribs removed. See the section on subtracting extended sources for a recipe for creating such a background file.

The particles in the background are calculated using a model developed in Snowden et al. 1992 and updated by Plucinsky et al. 1993, which gives the particle flux as a function of energy, time and detector position. Over a single observation this model is entirely dependent on the housekeeping parameter master_veto_rate, which is contained in an array in the eventrate file. The model is well constrained for values between 0 and 170, and this constraint should be applied to the data using xrthk before attempting a difficult subtraction. The model parameters vary on a timescales of months. This effect is taken into account by providing a different set of model parameters depending on the date of the observation.