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XMM-Newton Science Analysis System


bkgfit (ebkgmap-1.2) [xmmsas_20070708_1801-7.1.0]

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Truncated Poisson fitting:

Essentially this involves masking off all pixels whose values exceed a certain limit. The value of this is that bright non-background regions of an image can be easily excluded. A complication is introduced into the calculation of $L$ because the probability of occurrence of any pixel value now obeys a truncated Poisson distribution, as follows:

$$P_{\rm {trunc}}(\mathbf{a}) = \frac{\{B^{I}(\mathbf{a}) \, e... ...}(\mathbf{a}) \, exp[-B(\mathbf{a})]\}/j!} \rm { for } I<\tau,$$ (5)

where $\tau$ is the truncation level (rounded to the next lowest integer). Otherwise the procedure is unchanged.