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

¹ Centre d'Étude Spatiale des Rayonnements, 9 Av. du Colonel Roche, 31028 Toulouse Cedex, France
² American University of Sharjah, College of Arts & Sciences, Physics Unit, Sharjah, UAE

Corresponding author: P. von Ballmoos, pvb@cesr.fr

Received: 2 August 2002
Accepted: 10 October 2002

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 (, ), in addition to the emission from the galactic bulge and disk (Purcell et al. [CITE]; Cheng et al. [CITE]; Milne et al. [CITE]; Milne et al. [CITE]). This Positive Latitude Enhancement (PLE) was first attributed to an “annihilation fountain" in the Galactic center (Dermer & Skibo [CITE]) 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.