Mathematics – Probability
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008head...10.0301c&link_type=abstract
American Astronomical Society, HEAD meeting #10, #3.01
Mathematics
Probability
Scientific paper
How many times have you viewed a strikingly presented astronomical image --- and, in doubt, asked, "Where are the error bars?" Such doubt can come from several places: 1/ The purely statistical uncertainty of the final measurements (e.g. Poisson statistics); 2/ Possible mismatch between one's astrophysics sky-model and "sky-truth" (source model uncertainty); 3/ Related, additional effect of an uncertain background shape on measurements of the source of interest (background model uncertainty); and 4/ Mis-match between one's current best instrument model and "instrument truth" (instrument or calibration uncertainty). Additionally, there can be doubt in the methods used to process the data -- e.g. "How did they choose their stopping or smoothing parameters?"
A number of different groups very generally attack these kinds of problems by embedding very flexible, non- or semi-parametric models (from wavelets to Bayesian Blocks to Markov Random Fields) in full probabilistic frameworks (i.e. including "foward fitting"). These probability frameworks incorporate sky and instrument models and model-mismatch, as well as "fitting" any smoothing or stopping parameters. The CHASC group has used multiscale and MRF models of diffuse sky emission, plus astrophysics-based sky models, together with a hierarchical Bayesian probability structure. We use MCMC to sample the full highly dimensional posterior probability, which explicitly incorporates all the doubts mentioned above. We then choose two (or more) physically-based summary statistics to quantify (and display) the breadth of the uncertainties. But our "credible regions" have an unusual shape: the "error contours" include two spatial dimensions and one intensity dimension, in one example. We have fun analyzing and comparing differing CCGRO/EGRET viewing periods, including those of the 3C273/3C279 region, using our multi-scale model to capture varying and unexpected emission.
The authors acknowledge NASA AISRP and NSF interdisciplinary funding.
CHASC
Chiang James
Connors Alanna
van Dyk David
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