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The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990s with OSSE/CGRO showed that the emission is strongly concentrated towards the Galactic bulge. In the 2000s, the SPI instrument aboard ESAs INTEGRAL gamma-ray observatory allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge/disk luminosity ratio is larger than observed in any other wavelength. This mapping prompted a number of novel explanations, including rather exotic ones (e.g. dark matter annihilation). However, conventional astrophysical sources, like type Ia supernovae, microquasars or X-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, contrary to the rather well understood propagation of high energy (>GeV) particles of Galactic cosmic rays, understanding the propagation of low energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still remains a formidable challenge. We review the spectral and imaging properties of the observed 511 keV emission and we critically discuss candidate positron sources and models of positron propagation in the Galaxy.
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The balloon-borne Compton Spectrometer and Imager (COSI) had a successful 46-day flight in 2016. The instrument is sensitive to photons in the energy range $0.2$-$5$ MeV. Compton telescopes have the advantage of a unique imaging response and provide the possibility of strong background suppression. With its high-purity germanium detectors, COSI can precisely map $gamma$-ray line emission. The strongest persistent and diffuse $gamma$-ray line signal is the 511 keV emission line from the annihilation of electrons with positrons from the direction of the Galactic centre. While many sources have been proposed to explain the amount of positrons, $dot{N}_{mathrm{e^+}} sim 10^{50},mathrm{e^+,yr^{-1}}$, the true contributions remain unsolved. In this study, we aim at imaging the 511 keV sky with COSI and pursue a full-forward modelling approach, using a simulated and binned imaging response. For the strong instrumental background, we describe an empirical approach to take the balloon environment into account. We perform two alternative methods to describe the signal: Richardson-Lucy deconvolution, an iterative method towards the maximum likelihood solution, and model fitting with pre-defined emission templates. Consistently with both methods, we find a 511 keV bulge signal with a flux between $0.9$ and $3.1 times 10^{-3},mathrm{ph,cm^{-2},s^{-1}}$, confirming earlier measurements, and also indications of more extended emission. The upper limit we find for the 511 keV disk, $< 4.3 times 10^{-3},mathrm{ph,cm^{-2},s^{-1}}$, is consistent with previous detections. For large-scale emission with weak gradients, coded aperture mask instruments suffer from their inability to distinguish isotropic emission from instrumental background, while Compton-telescopes provide a clear imaging response, independent of the true emission.
The signature of positron annihilation, namely the 511 keV $gamma$-ray line, was first detected coming from the direction of the Galactic center in the 1970s, but the source of Galactic positrons still remains a puzzle. The measured flux of the annihilation corresponds to an intense steady source of positron production, with an annihilation rate on the order of $sim10^{43}$~e$^{+}$/s. The 511 keV emission is the strongest persistent Galactic $gamma$-ray line signal and it shows a concentration towards the Galactic center region. An additional low-surface brightness component is aligned with the Galactic disk; however, the morphology of the latter is not well constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne soft $gamma$-ray (0.2--5 MeV) telescope designed to perform wide-field imaging and high-resolution spectroscopy. One of its major goals is to further our understanding of Galactic positrons. COSI had a 46-day balloon flight in May--July 2016 from Wanaka, New Zealand, and here we report on the detection and spectral and spatial analyses of the 511 keV emission from those observations. To isolate the Galactic positron annihilation emission from instrumental background, we have developed a technique to separate celestial signals utilizing the COMPTEL Data Space. With this method, we find a 7.2$sigma$ detection of the 511 keV line. We find that the spatial distribution is not consistent with a single point source, and it appears to be broader than what has been previously reported.
The imaging spectrometer SPI on board ESAs INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. The 511 keV line is clearly detected from the Galactic disk; in addition, there is a tantalizing hint at possible halo-like emission. The available data do not yet permit to discern whether the emission around the bulge region originates from a halo-like component or from a disk component that is very extended in latitude.
The annihilation of cosmic positrons ($e^+$) with electrons in the interstellar medium (ISM) results in the strongest persistent gamma-ray line signal in the sky. For 50 years, this 511 keV emission has puzzled observers and theoreticians. A key issue for understanding $e^+$-astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (< 10 MeV). We want to shed light on how $e^+$s propagate and the resulting morphology of the emission. We approach this positron puzzle by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and source populations. By dissecting the 511 keV emission as measured with INTEGRAL/SPI, we derive spectra for individual regions in the sky. The centroid energies are converted into Doppler-shifts, representing the line-of-sight velocity along different longitudes. This results in a longitude-velocity diagram of $e^+$-annihilation. We also determine Doppler-broadenings to study annihilation conditions as they vary across the Galaxy. We find line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements, and measurements of radioactive Al-26. The velocity gradient in the inner 60 deg is determined to be $4pm6$ km/s/deg. The 511 keV line width is constant as a function of longitude at $2.43pm0.14$ keV. The positronium fraction is found to be 1.0 along the galactic plane. The weak signals in the disk leave open the question whether $e^+$-annihilation is associated with the high velocities seen in Al-26 or rather with ordinarily rotating components of the Galaxys ISM. We confirm previous results that $e^+$s are slowed down to the 10 eV energy scale before annihilation, and constrain bulk Doppler-broadening contributions to <1.25 keV. Consequently, the true annihilation conditions remain unclear.
The 511 keV line from positron annihilation in the Galaxy was the first $gamma$-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in $gamma$-ray astronomy. With a large flux of $sim$10$^{-3}$ $gamma$/cm$^{2}$/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at $>50sigma$ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in $beta^+$-decay of nucleosynthesis products, such as $^{26}$Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.
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