Authors: Daniel B. Reisenfeld (Los Alamos National Laboratory), David A. Osthus (LANL), Brian P. Weaver (LANL), and Paul H. Janzen (University of Montana)

The IBEX-Hi sensor on board the Interstellar Boundary Explorer (IBEX) has been imaging the heliosphere in energetic neutral atoms (ENAs) over the last solar cycle.  Processing the ENA measurements made by IBEX-Hi is challenging, as the signal rate is extremely low (~1 signal count every 10 seconds) and the signal-to-background ratio is of order 1-to-1.  Data in general have been released in the form of sky maps with pixels of 6° longitude by 6° latitude, a pixel size similar in scale to the sensor’s angular resolution. Despite these sensitivity limitations, exciting discovery science has been derived from IBEX sky maps, such as the discovery of a narrow Ribbon of enhanced ENA flux whose full width at half maximum (FWHM) is comparable to the sensor’s field of view.

We have undertaken a campaign to improve the quality of IBEX sky maps by applying advanced statistical science algorithms, to extract spatial features more accurately, and to improve the angular resolution.  We present Theseus, a two-stage statistical map estimation procedure. Theseus takes IBEX-Hi direct event data as input and produces an estimated sky map with uncertainty. In the first stage, Theseus uses an ensemble of non-parametric regression models to estimate an initial map at the intrinsic instrument angular resolution. In the second stage, Theseus leverages regularization to deconvolve the instrument angular response, resulting in a sharpened map. Sky map uncertainty estimates are computed via bootstrapping.  The resulting maps should lead to the new heliospheric science not possible with current IBEX-Hi sky maps.  We show that higher-resolution maps generated by the Theseus algorithm allow considerably clearer profiles for the Ribbon than the standard 6-degree resolution map data product. Theseus may also be an important tool for analysis of data collected by the Interstellar Mapping and Acceleration Probe (IMAP) ENA imagers.