The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL

TL;DR

This study maps the sky distribution of positronium (Ps) continuum emission in gamma rays using SPI/INTEGRAL, finding a bulge-dominated, circularly symmetric signal with an extent of about $8^{\circ}$ (FWHM) centered on the Galactic center. Through Richardson–Lucy imaging and maximum-likelihood multi-component fitting, the authors show that Ps continuum emission, closely following the 511 keV line, is best described by a Gaussian bulge plus CO-disk model, with a Ps fraction of $f_{PS} = 0.92 \pm 0.09$. Spectral analyses—both simplistic and rigorous—separate the Ps continuum from the Galactic continuum, yielding a Ps-continuum normalization around $3.1\times10^{-3}$ ph cm$^{-2}$ s$^{-1}$ and confirming Ps dominance (>~76–85%) in the GC emission. The results align with OSSE and SMM measurements and strengthen the interpretation that positrons originate mainly from old stellar populations, though light dark matter remains a possible alternative source. Altogether, the paper demonstrates that the Ps continuum morphology mirrors the 511 keV line, providing constraints on positron sources and the Galactic annihilation environment.

Abstract

We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESA's INTEGRAL observatory. The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8 deg FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electron-positron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92 +/- 0.09. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons.

Figures (4)

• Figure 1: A Richardson-Lucy sky map of extended emission in the summed Ps analysis intervals (the combination of the intervals 410--430, 447-465, and 490--500 keV). The contour levels indicate intensity levels of $10^{-2}$, $10^{-3}$, and $10^{-4}$ ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$. Details are given in the text.
• Figure 2: A fit of the SPI result for the diffuse emission from the GC region ($|l|, |b| \le 16^\circ$) obtained with a spatial model consisting of an $8^\circ$ FWHM Gaussian bulge and a CO disk. In the fit a diagonal response was assumed. The spectral components are: 511 keV line (dotted), Ps continuum (dashes), and power-law continuum (dash-dots). The summed models are indicated by the solid line. Details of the fitting procedure are given in the text.
• Figure 3: A fit of the GC spectrum obtained from OSSE data by Kinzer99 to the SPI result for the diffuse emission from the GC region ($|l|, |b| \le 16^\circ$) obtained with a spatial model consisting of an $8^\circ$ FWHM Gaussian bulge and a CO disk. Details are given in the text.
• Figure 4: Comparison of the SPI result for the diffuse emission from our Galaxy obtained with a spatial model consisting of an $8^\circ$ FWHM Gaussian bulge and a CO disk (data points) with the best fit model for the emission from the inner Galaxy obtained with SMM data by Harris90 (the solid and dashed lines represent their best fit model and its total uncertainty). Details are given in the text.