Models for the positive latitude e{^-}e{⁺} annihilation feature

Abstract

Galactic maps of e^-e⁺ annihilation radiation based on CGRO-OSSE, SMM and TGRS data have indicated the existence of an extended component at positive Galactic latitudes (l~ -2 ^deg, b~ 7 ^deg), in addition to the emission from the galactic bulge and disk (Purcell et al. \cite{Purcell97}; Cheng et al. \cite{Cheng97}; Milne et al. \cite{Milne00}; Milne et al. \cite{Milne01}). This Positive Latitude Enhancement (PLE) was first attributed to an ``annihilation fountain" in the Galactic center (Dermer & Skibo \cite{Dermer97}) but has since been the object of several models. After discussing the observational evidence for the PLE, we investigate various models for the PLE: besides the scenarios proposed in the literature, we have introduced a number of models requiring relatively modest positron rates due to a local origin of the e^-e⁺ emission (local galactic-, solar system-, earth- and spacecraft-environment origins). The various scenarios for the PLE are constrained in the light of the latest OSSE-SMM-TGRS data analysis results: we have looked at the possible positron production mechanisms as well as the annihilation conditions in the different physical environments (temperature and dust grain content) proposed for the positive-latitude region. By constraining those parameters, based on the recent limits for the line width and the positronium fraction, we found that some of the models can essentially be discarded. A number of other scenarios will have to await further measurements and maps, such as will be possible with INTEGRAL's SPI and IBIS instruments. We present a table/checklist of model-falsification criteria.