Positron annihilation kinematics with INTEGRAL/SPI: rotation or not rotation
The 511 keV gamma-ray line signal from electron-positron (e--e+)
annihilation in the Milky Way (bottom right) is one of the big unsolved
mysteries in astrophysics: where is this large amount of antimatter
particles coming from, and why is the sky looking so differently from
any other wavelength (top right)?
While potentially any object in the Galaxy can produce positrons, the
true source(s) has not been identified. In fact, going from the sheer
number of positrons that are annihilating (about 5⨉1043 e+/sec), every
candidate source population and combinations thereof could explain the
signal - however, also even more positrons that we are seeing.
Once having escaped from their local production environment, these
positrons have to be considered cosmic rays, which means they propagate
a certain distance from their source until they annihilate with
electrons from ambient atoms. This propagation of relativistic positrons
towards kinetic energies of eV (known from the "ortho-positronium
continuum", dashed blue, bottom right) is a crucial point in
understanding why the Galactic Centre (bulge) is so bright compared to
the Galactic Plane (disk), in which most of the sources are expected.
This cooling is governed by the density structure and electro-magnetic
fields and imprint on the velocities at which positrons annihilate. As
the Milky Way is rotating, the Doppler effect should lead to
line-shifts, which, integrated over the entire bulge, should make the
511 keV appear broader. This effect adds to the broadening which is
determined by the temperature and ionisation fraction of the
interstellar medium in which the positrons annihilate (FWHM = 2.06±0.08
keV).
With more than 10 years of INTEGRAL/SPI data, it was possible for the
first time to constrain this velocity curve in the inner part of the
Milky Way (left): while being consistent with the Galactic rotation
speed of about 220 km/sec (blue-shaded band), the data (black) also
allows for higher velocities, as could be expected from the prominent
positron source, 26Al (orange), as well as for a flat rotation curve
(511 keV velocity gradient 4±6 km/sec/deg). Both higher and lower
velocities would be surprising as they can only be explained by special
configurations, like a preferred direction similar to 26Al (see
INTEGRAL POM Jan 2014), nearby emission regions (same velocity as the Sun), or
dispersion dominated halo kinematics.
Measuring the conditions at which positrons annihilate must always take
into account the kinematics of where positrons annihilate.
