Constraints on positron annihilation kinematics in the inner Galaxy

Thomas Siegert,Roland Diehl,Fiona H Panther,Roland M Crocker,Christoph Weinberger,Martin G H Krause,Moritz M M Pleintinger

doi:10.1051/0004-6361/201833856

Abstract

Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ-ray line signal in the sky. For the past 50 yr, this 511 keV emission – predominantly from the galactic bulge region and from a low surface-brightness disk – has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (≲10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this “positron puzzle” by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derived spectra for individual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determined Doppler-broadening from γ-ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We found line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2–3 km s−1 deg−1), and measurements of radioactive 26Al (7.5–9.5 km s−1 deg−1). The velocity gradient in the inner ±30° is determined to be 4 ± 6 km s−1 deg−1. The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave the question open of whether positron annihilation is associated with the high velocities seen in 26Al or rather with ordinarily rotating components of the Milky Way’s interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation and constrain bulk Doppler-broadening contributions to ≲1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.

Highlights

The interpretation of the morphology of γ-ray emission from positron annihilation in the Milky Way has remained in contention since the discovery of the galactic 511 keV line in the late 1960s (Haymes et al 1969; Johnson et al 1972)
The orientation of the velocity gradient is aligned with the CO signal, the spectral uncertainties from the individual regions of interest (ROIs) are too large to claim a consistent trend by using this method
This means that the annihilation conditions could be different to what has been inferred in previous studies, not considering the galactic kinematics

Summary

Introduction

The interpretation of the morphology of γ-ray emission from positron annihilation in the Milky Way has remained in contention since the discovery of the galactic 511 keV line in the late 1960s (Haymes et al 1969; Johnson et al 1972). Unlike at any other wavelength, the bulge region dominates the signal with a flux ratio between bulge and disk of 0.6 (Siegert et al 2016) (see Milne & Leising 1997; Knödlseder et al 2005; Bouchet et al 2010). With increased observing time from the γ-ray spectrometer SPI (Vedrenne et al 2003) aboard the INTEGRAL satellite (Winkler et al 2003), it has become clearer that the 511 keV disk of the Milky Way is probably not truncated beyond 20◦ longitude – as found using data from OSSE aboard CGRO (Purcell et al 1997) – but rather extended in both longitude (Bouchet et al 2010; Skinner et al 2014) and latitude (Siegert et al 2016). The main questions to be understood are the connection between the positrons seen to annihilate, their source regions within the Galaxy, and the number of positrons produced in the possible sources

Results

Discussion

Conclusion