GLAST Likelihood Calculations
My poster paper from the
HEAD meeting in Honolulu,
November 2000
• I wrote an 8-page paper about GLAST
likelihood calculations, a revision of a
document that I worked on about a year
ago. It has lots of equations, lots of
opinions, no graphs, and a few (soft)
numbers.
• It’s available on the net at
httpd://giants.stanford.edu/~pln/glastlikepaper.pdf
Teaser
At a workshop the night before, I got to present a very brief
talk with these points:
• Each photon event is complex. The PSF and energy dispersion vary
with angle and type of event. Thus the instrument response functions
are large, multidimensional objects, perhaps ~100 GB total.
• Most observations will be in zenith scanning mode, so the response to
any point source is dynamic.
• The wide FOV means that there will always be many interesting
sources in view. The wide PSF means that there will be a lot of source
confusion. Any analysis requires fitting multiple sources and stitching
together separate parts of the data stream.
• Chi-squared can’t be used for most sources because there are not many
photons spread over too many bins in the observing space. Poisson
likelihood must be used.