[Paper Review] Measuring the smearing of the Galactic 511 keV signal: positron propagation or supernova kicks?

Thomas Siegert, Roland M. Crocker+5 more|arXiv (Cornell University)|Sep 8, 2021

Astrophysics and Cosmic PhenomenaPhysics and Astronomy111 references14 citations

TL;DR

This study analyzes 15 years of INTEGRAL/SPI gamma-ray data to measure the spatial smearing of the Galactic 511 keV positron annihilation signal, finding a characteristic smearing scale of 150 ± 50 pc. The results favor positron propagation over supernova kicks as the origin of the smearing, with injection energies ≤1.4 MeV pointing to nucleosynthetic sources like 26Al or 44Ti in massive or core-collapse supernovae.

ABSTRACT

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.

Motivation & Objective

• To determine the origin of the spatial smearing observed in the Galactic 511 keV positron annihilation signal.
• To test whether the smearing is caused by positron propagation through the interstellar medium or by supernova kicks at source formation.
• To constrain the injection energy of positrons based on their propagation length scale.
• To assess whether the signal traces the boxy bulge and nuclear stellar bulge, and how it compares to stellar and gas templates.
• To evaluate competing models—propagation vs. kick scenarios—using spectrally resolved ortho-positronium continuum and 511 keV line data.

Proposed method

• Performs a 3D spatial analysis of 15 years of INTEGRAL/SPI data in the energy range 200–508 keV, including the 511 keV line and ortho-positronium continuum.
• Uses a 3D smoothing kernel to measure the characteristic length scale of signal smearing, comparing observed morphology to stellar, gas, and dark matter templates.
• Applies spectral decomposition to separate the 511 keV line from the ortho-positronium continuum, improving statistical quality and discriminant power.
• Compares the observed signal morphology to templates of the boxy bulge (BB), nuclear stellar bulge (NB), CO, HI, and dark matter profiles using χ² minimization.
• Employs parametrized models of interstellar medium conditions (CNM, WNM, MM, WIM) to estimate propagation distances and infer injection energies.
• Uses instrumental exposure maps and background scaling (BG scaling) across 8 energy bands to account for detector response and background contributions.

Experimental results

Research questions

• RQ1 What is the characteristic length scale over which the 511 keV signal is spatially smeared, and what does this imply about positron propagation or source kinematics?
• RQ2 Does the 511 keV signal trace the boxy bulge and nuclear stellar bulge, and how does this constrain the origin of the positrons?
• RQ3 Can positron propagation from disc sources explain the observed morphology, especially given the high bulge-to-disc luminosity ratio?
• RQ4 Is the observed smearing better explained by positron diffusion in the ISM or by supernova kicks at the time of positron production?
• RQ5 What is the upper limit on the injection energy of positrons consistent with the observed smearing scale and ISM conditions?

Key findings

• The 511 keV signal is spatially smeared over a characteristic length scale of 150 ± 50 pc, indicating that positrons do not annihilate in situ at their sources.
• The signal traces both the boxy bulge and the nuclear stellar bulge, confirming its association with the old stellar population in the Galactic center.
• The observed smearing is inconsistent with a supernova kick scenario requiring different kick velocities in the nucleus (≳50 km s⁻¹) versus the wider bulge (≲15 km s⁻¹).
• Positron propagation with injection energies ≤1.4 MeV can explain the observed morphology, pointing to nucleosynthetic sources such as ²⁶Al or ⁴⁴Ti.
• Using canonical ISM phases, the allowed injection energy range is 0.13 ± 0.02 MeV, but extrapolation to higher densities in the nuclear and boxy bulge increases the upper bound to 1.4 MeV.
• The systematic uncertainty in propagation distance estimates is a factor of four due to uncertainties in magnetic field scattering, supporting a conservative upper limit of 0.4–1.4 MeV for injection energy if the same source population is responsible in both bulge components.