Kavli Affiliate: Shunsaku Horiuchi
| First 5 Authors: Thomas Siegert, Roland M. Crocker, Oscar Macias, Fiona H. Panther, Francesca Calore
| Summary:
We use 15 years of $gamma$-ray data from INTEGRAL/SPI in a refined
investigation of the morphology of the Galactic bulge positron annihilation
signal. Our spatial analysis confirms that the signal traces the old stellar
population in the bulge and reveals for the first time that it traces the boxy
bulge and nuclear stellar bulge. Using a 3D smoothing kernel, we find that the
signal is smeared out over a characteristic length scale of $150 pm 50,$pc,
suggesting either annihilation in situ at astrophysical sources kicked at
formation or positron propagation away from sources. The former is disfavoured
by its requiring kick velocities different between the Galactic nucleus
($gtrsim 50,mathrm{km,s^{-1}}$) and wider bulge ($lesssim
15,mathrm{km,s^{-1}}$) source. Positron propagation prior to annihilation
can explain the overall phenomenology of the 511 keV signal for positrons
injection energies $lesssim 1.4,$MeV, suggesting a nucleosynthesis origin.
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