[Paper Review] The positron density in the intergalactic medium and the galactic 511 keV line

A. Vecchio, Vincent, A. C.+2 more|arXiv (Cornell University)|Apr 1, 2013

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper proposes that positrons produced in extragalactic active galactic nuclei (AGN) jets and radio lobes escape into the intergalactic medium (IGM), where they are later accreted by the Milky Way, contributing to the observed 511 keV gamma-ray line emission. Using the cosmic radio background as a proxy, the authors estimate an IGM positron-to-electron ratio of up to ~10⁻⁵, showing that a fraction as low as ~10⁻⁶ is sufficient to explain the observed 511 keV luminosity, with a spherical accretion model reproducing the bulge-dominated emission profile.

ABSTRACT

The 511 keV electron-positron annihilation line, most recently characterized by the INTEGRAL/SPI experiment, is highly concentrated towards the Galactic centre. Its origin remains unknown despite decades of scrutiny. We propose a novel scenario in which known extragalactic positron sources such as radio jets of active galactic nuclei (AGN) fill the intergalactic medium with MeV e+e- pairs, which are then accreted into the Milky Way. We show that interpreting the diffuse cosmic radio background (CRB) as arising from radio sources with characteristics similar to the observed cores and radio lobes in powerful AGN jets suggests that the intergalactic positron-to-electron ratio could be as high as 10^{-5}, although this can be decreased if the CRB is not all produced by pairs and if not all positrons escape to the intergalactic medium. Assuming an accretion rate of one solar mass per year of matter into the Milky Way, a positron-to-electron ratio of only 10^{-7} is already enough to account for much of the 511 keV emission of the Galaxy. A simple spherical accretion model predicts an emission profile highly peaked in the central bulge, consistent with INTEGRAL observations. However, a realistic model of accretion with angular momentum would likely imply a more extended emission over the disk, with uncertainties depending on the magnetic field structure and turbulence in the galactic halo.

Motivation & Objective

• To investigate whether positrons produced in extragalactic sources like AGN jets could account for the Galactic 511 keV line emission.
• To estimate the intergalactic positron-to-electron ratio based on observed cosmic radio background (CRB) properties.
• To model the accretion of IGM positrons into the Milky Way and assess their contribution to the 511 keV annihilation signal.
• To evaluate whether a spherical accretion model can reproduce the observed bulge-dominated morphology of the 511 keV emission.
• To explore the implications of magnetic fields and angular momentum in shaping the spatial distribution of positron annihilation in the Galaxy.

Proposed method

• Estimate the IGM positron-to-electron ratio by modeling the cosmic radio background (CRB) as synchrotron emission from relativistic electron-positron pairs in AGN jets and lobes.
• Use the observed CRB spectrum (T_radio ∝ ν^−2.6) and luminosity function extrapolations to infer the total pair population in the IGM.
• Apply energy-dependent cooling timescales for positrons in the IGM, showing that intermediate-energy positrons survive over cosmological timescales.
• Model spherical accretion of IGM material into the Milky Way, assuming a 1 M☉/yr accretion rate, to compute positron annihilation rates in the bulge.
• Use the positronium formation fraction (100%) from Jean et al. (2006) to compute the expected 511 keV line luminosity from annihilation.
• Compare the predicted emission profile from the spherical model with INTEGRAL/SPI observations, particularly the bulge concentration and latitude dependence.

Experimental results

Research questions

• RQ1 Can the cosmic radio background be explained by synchrotron emission from extragalactic electron-positron pairs, and what does this imply for the IGM positron density?
• RQ2 What is the maximum possible intergalactic positron-to-electron ratio consistent with CRB observations?
• RQ3 Is a positron fraction of ~10⁻⁶ in the IGM sufficient to account for the observed 511 keV luminosity in the Milky Way?
• RQ4 Can a simple spherical accretion model reproduce the observed spatial concentration of the 511 keV emission toward the Galactic bulge?
• RQ5 How would angular momentum and magnetic fields in the galactic halo affect the predicted morphology of the 511 keV emission?

Key findings

• The cosmic radio background implies a maximum IGM positron-to-electron ratio of approximately 10⁻⁵, assuming the CRB arises from synchrotron-emitting AGN jets and lobes.
• A positron fraction as low as ~10⁻⁶ in the IGM is sufficient to account for the observed 511 keV luminosity of ~2×10⁴³ positrons per second.
• A spherical accretion model predicts a 511 keV emission profile highly peaked in the Galactic bulge, consistent with INTEGRAL/SPI observations.
• The observed bulge concentration is robust in the spherical model, but realistic accretion with angular momentum would likely spread the emission over the disk, reducing central luminosity.
• Magnetic field structures in the galactic halo may further influence positron trajectories and annihilation locations, potentially enhancing bulge emission if a dipole component exists.
• Future small-scale radio fluctuations measurements could test whether cosmological AGN populations are responsible for the CRB and, by extension, the IGM positron population.