Constraints on positron annihilation kinematics in the inner Galaxy
  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 $4\pm6$ km/s/deg. The 511 keV line width
is constant as a function of longitude at $2.43\pm0.14$ 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.
