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

Abstract

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• , b ≈ 7 • ), in addition to the emission from the galactic bulge and disk (Purcell et al. 1997;Cheng et al. 1997;Milne et al. 2000;Milne et al. 2001). This Positive Latitude Enhancement (PLE) was first attributed to an "annihilation fountain" in the Galactic center (Dermer & Skibo 1997) 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.

Possible Evidence for MeV Dark Matter in Dwarf Spheroidals

“…At this temperature, direct annihilations are important. If dust is present at even half of the standard interstellar gasto-grain ratio, the annihilation line remains narrow (less than about 2 keV for a grain fraction greater than one tenth of the local interstellar medium value) [32]. Hence the 511 keV line flux should be approximately 4 times greater than in the case of annihilation through positronium formation, as in the case of the galactic bulge where the dominant component of the diffuse interstellar gas is assumed to be at T ∼ 10 4 K.…”

Positron transport in the interstellar medium

“…Positron annihilation in the Galactic center (GC) region is now a firmly established source of radiation, which has been observed since the early seventies in several balloon and satellite experiments (see von Ballmoos et al 2003, Jean et al 2004, Diehl et al 2006 for reviews). Despite significant progress in observational capabilities, the origin of Galactic positrons remains an open question.…”

Early SPI/INTEGRAL constraints on the morphology of the 511 keV line emission in the 4th galactic quadrant

“…The OSSE observations suggest at least two emission components, one being a spheroidal bulge and the other being a galactic disk component. Indications of a third component situated above the galactic plane have resulted in various speculations about the underlying source (von Ballmoos et al 2003), yet the morphology and intensity of this component is only poorly determined (e.g. Milne et al 2001).…”