New Markov Chain Monte Carlo Methods to Address Pile-up in the Chandra X-ray Observatory

Details New Markov Chain Monte Carlo Methods to Address Pile-up in the Chandra X-ray Observatory New Markov Chain Monte Carlo Methods to Address Pile-up in the Chandra X-ray Observatory

Dec 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...197.1603y&link_type=abstract

American Astronomical Society, 197th AAS Meeting, #16.03; Bulletin of the American Astronomical Society, Vol. 32, p.1431

Statistics

Computation

2

Scientific paper

Pile-up occurs in X-ray detectors when two or more photons arrive in an event detection island during the same frame. Such coincident events are counted as a single higher energy event or lost altogether if the total energy goes above the on-board discriminators. Thus, for bright sources pile-up can seriously distort both the count rate and the energy spectrum. Accounting for pile-up is perhaps the most important outstanding data-analytic challenge for Chandra. In this paper, we describe how Bayesian hierarchical models can be designed to account for such physical processes in X-ray detectors. For example, we can design spectral models with components accounting for instrument response, background, absorption, and pile-up. A Markov Chain Monte Carlo algorithm dramatically simplifies computational complexity by fitting one component at a time. For pile-up, we need to stochastically separate a subset of the observed counts into multiple counts of lower energy based on the current iteration of the particular spectral/spatial model being fit. Because the Bayesian framework allows for the inclusion of other sources of information, event grades (i.e, a description of the likelihood of the degree of pile-up based on the spatial distribution of the charge) can be used to improve the fit. Our method is illustrated with the spectral analysis of 3c273 observed with ACIS-S on board of Chandra X-ray Observatory. The authors gratefully acknowledge funding for this project partially provided by NSF grant DMS-97-05157, the Chandra X-ray Observatory, and by NASA grant NAS8-39073.

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