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Register a new userThe sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI
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Added by
Georg Weidenspointner
Publication date
2007
fields
Physics
and research's language is
English
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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.
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We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESAs 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.
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 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.
During its first year in orbit the INTEGRAL observatory performed deep exposures of the Galactic Center region and scanning observations of the Galactic plane. We report on the status of our analysis of the positron annihilation radiation from the 4th Galactic quadrant with the spectrometer SPI, focusing on the sky distribution of the 511 keV line emission. The analysis methods are described; current constraints and limits on the Galactic bulge emission and the bulge-to-disk ratio are presented.
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