  Positron annihilation line at 511~keV is a known component of the gamma-ray
diffuse emission. It is believed to be produced in the Galaxy, but there could
be possible extragalactic contribution as well. E.g., positrons can be produced
in jets of active galactic nuclei (AGN) and after that accumulate and gradually
annihilate in hot gaseous halos around galaxies. In this work we test this
hypothesis in application to an individual object -- the Andromeda galaxy (M31)
which is close and has a supermassive black hole in its center, which powered
an AGN before. We compute the growth history of the supermassive black hole in
M31, relate it to the evolution of jet luminosity and estimate the positron
content in its halo. We calculate the 511~keV photon flux due to positron
annihilation which should be observed at Earth and find the value of around
$10^{-4}$ photon cm$^{-2}$s$^{-1}$. It is very close to the observational
limits ($<10^{-4}$photon cm$^{-2}$s$^{-1}$) set by the INTEGRAL/SPI in the
assumption of the point source, so further observations would be able to
constrain leptonic models of the jets and propagation of cosmic rays in the
circumgalactic medium of large spiral galaxies.
