Volume 627, July 2019
|Number of page(s)||15|
|Section||Interstellar and circumstellar matter|
|Published online||11 July 2019|
Constraints on positron annihilation kinematics in the inner Galaxy
Max-Planck-Institut für extraterrestrische Physik,
2 Excellence Cluster Universe, Boltzmannstraße 2, 85748 Garching, Germany
3 Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093, USA
4 Research School of Astronomy and Astrophysics, Australian National University, Canberra 2611, ACT, Australia
5 Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
6 School of Science, UNSW Canberra, Australian Defence Force Academy, Canberra 2612, Australia
7 ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), Canberra, Australia
Accepted: 31 May 2019
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.
Key words: ISM: kinematics and dynamics / Galaxy: structure / gamma rays: general / techniques: spectroscopic
© T. Siegert et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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