On the Origin of the 511 keV Emission in the Galactic Centre

TL;DR

This paper explores the origin of the 511 keV positron annihilation emission in the Galactic Centre, proposing that faint X-ray binaries with jets could be a significant source, and discusses related multi-wavelength phenomena.

Contribution

It introduces the hypothesis that faint, accretion-powered X-ray binaries with jets may produce the observed positron emission, expanding the potential sources beyond the brightest binaries.

Findings

Positron emission may originate from pair-dominated jets in faint X-ray binaries.

The population of faint X-ray sources is much larger than previously associated with the emission.

Unresolved problems remain in linking bright X-ray binaries to the 511 keV emission.

Abstract

Diffuse 511 keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 years. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ~70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the WMAP `haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511 keV emission with the brightest X-ray binaries.