Combined Maximum Likelihood Fitting and Analytical Modeling of the Interstellar Neutral Gas Flow as Observed by IBEX

The Interstellar Boundary Explorer (IBEX) observes neutral interstellar gas flow distributions at the Earth's orbit providing the most detailed information on the physical parameters (flow direction, flow speed, and temperature) of the surrounding interstellar medium (ISM) and the interaction with the heliospheric boundary. IBEX has observed interstellar neutral H, He, O, and Ne and has completed its fourth observation season of the ISM flow. The maximum likelihood method has been used for fitting the ISM flow distributions due to its substantial advantages over least square fitting procedures when dealing with low counting statistics which is often the case with the minor species. IBEX is a spinning spacecraft with the spin axis aligned with the Earth-Sun line at the beginning of each orbit, providing observations near the perihelion of the interstellar atom trajectories. However, over the course of the orbit, the spin axis pointing drifts away from the Sun. As a result, the ISM flow observables, such as ISM peak rate and location in latitude, vary slowly over the course of the orbit. The spin axis drift is inserted into an analytical model of the ISM flow (Lee et al. 2012) and fitted to each IBEX orbit such that the ISM flow observables can be determined at the location where the spin axis is aligned with the Sun-Earth line, the perihelion of the interstellar trajectories. This is an improvement of the previous analysis of the IBEX ISM flow observations using the analytical model (Möbius et al. 2012), where the ISM flow observables were interpolated or extrapolated to the location where the spin axis is aligned with the Sun-Earth line. The application of the maximum likelihood method and the use of the analytical model of the ISM flow will be described along with a discussion of the He distribution results and plans for the O (+Ne) distribution.