**HEAD 2000, November 2000**

*Session 16. Workshops*

Display, Tuesday, November 7, 2000, 8:00am-6:00pm, Bora Bora Ballroom
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## [16.11] GLAST Likelihood Calculations

*P. L. Nolan (Stanford University)*

Although GLAST will detect many more photons than EGRET,
most analyses will be carried out with a small number of
photons. Thus approximate statistical methods, such as
chi-squared tests, will not be applicable. It will be
necessary to use the full Poisson likelihood.

Two factors will make the GLAST analysis more complex than
EGRET's. First, the field of view is very large. The width
of the point spread function varies greatly from the center
to the edge. This is in addition to the well-known energy
dependence. Second, GLAST will operate in a scanning mode,
so that each point source will sweep across the field of
view for 20-40 minutes of each 90-minute orbit. EGRET would
stare at small portions of the sky for 1-3 weeks at a time,
and data from an entire exposure could be added together.
Each GLAST photon will have its own point spread function,
so each will need to be analyzed separately.

Common analyses, such as finding point sources, determining
the brightness of known sources, or spectrum fitting, will
all require calculating likelihood functions. Each
calculation will be a multidimensional integral. These
integrals are displayed in all their glory. For some common
calculations there are techniques which can be used to
simplify the process.

This work was supported by NASA contract NAS5-00147.

If you would like more information about this abstract, please
follow the link to http://giants.stanford.edu/~pln/glastlike-paper.pdf.
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