Constraints on positron annihilation kinematics in the inner Galaxy

Abstract

The annihilation of cosmic positrons ($e^{+}$) with electrons in the interstellar medium (ISM) results in the strongest persistent gamma-ray line signal in the sky. For 50 years, this 511 keV emission has puzzled observers and theoreticians. A key issue for understanding $e^{+}$-astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (< 10 MeV). We want to shed light on how $e^{+}$s propagate and the resulting morphology of the 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 source populations. By dissecting the 511 keV emission as measured with INTEGRAL/SPI, we derive spectra for individual regions in the sky. The centroid energies are converted into Doppler-shifts, representing the line-of-sight velocity along different longitudes. This results in a longitude-velocity diagram of $e^{+}$-annihilation. We also determine Doppler-broadenings to study annihilation conditions as they vary across the Galaxy. We find line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements, and measurements of radioactive Al-26. The velocity gradient in the inner 60 deg is determined to be $4\pm 6$ km/s/deg. The 511 keV line width is constant as a function of longitude at $2.43\pm 0.14$ keV. The positronium fraction is found to be 1.0 along the galactic plane. The weak signals in the disk leave open the question whether $e^{+}$-annihilation is associated with the high velocities seen in Al-26 or rather with ordinarily rotating components of the Galaxy's ISM. We confirm previous results that $e^{+}$s are slowed down to the 10 eV energy scale before annihilation, and constrain bulk Doppler-broadening contributions to <1.25 keV. Consequently, the true annihilation conditions remain unclear.

arXiv comment8 pages, 5 main figures, 7 pages appendix with 6 auxiliary figures, accepted for publication in A&A

References in corpus (29)

• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Variable Stars in Galactic Globular Clusters
• Mapping the Three-Dimensional Density of the Galactic Bulge with VVV Red Clump Stars
• A faint type of supernova from a white dwarf with a helium-rich companion
• The all-sky distribution of 511 keV electron-positron annihilation emission
• 3D maps of the local ISM from inversion of individual color excess measurements
• Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
• Stringent Constraint on Galactic Positron Production
• Spectral analysis of the Galactic e+e- annihilation emission
• Positron annihilation spectrum from the Galactic Center region observed by SPI/INTEGRAL
• Gamma-ray spectroscopy of Positron Annihilation in the Milky Way
• Helium Shell Detonations on Low Mass White Dwarfs as a Possible Explanation for SN 2005E
• Unifying boxy bulge and planar long bar in the Milky Way
• The lives and deaths of positrons in the interstellar medium
• Before the Bar: Kinematic Detection of A Spheroidal Metal-Poor Bulge Component
• Kinematics of massive star ejecta in the Milky Way as traced by 26Al
• On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL
• Radioactive 26Al from the Scorpius-Centaurus Association
• Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL, revisited: annihilation in a cooling ISM?
• Post-merger evolution of carbon-oxygen + helium white dwarf binaries and the origin of R Coronae Borealis and extreme helium stars
• INTEGRAL/SPI ground calibration
• INTEGRAL/SPI $\gamma$-ray line spectroscopy
• Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations
• Thermonuclear detonations ensuing white dwarf mergers
• RR Lyrae Variables in the Globular Clusters of M31: a First Detection of Likely Candidates
• Background modelling for $\gamma$-ray spectroscopy with INTEGRAL/SPI
• Positron Annihilation on Polycyclic Aromatic Hydrocarbon molecules in the Interstellar Medium
• Direct imaging of a massive dust cloud around R Coronae Borealis
• Astrophysical signatures of leptonium

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