March 2, 2017

511 keV Positron/Electron Annihilation Ray Line

In 2004 and 2005, data from Integral's SPI spectrometer was used to compile the first complete map of the emission at 511 keV (energy of the annihilation positron/electron). This matter-antimatter annihilation mainly happens in the galactic nucleus. Use of the newest imaging technology as well as in-flight updates on the satellite made possible the observation of the disk's faint emission. This fits well with models recently developed to explain the distribution of antimatter within our Galaxy. Indeed, in 2007 SPI forced a rethink of some of these models, thus advancing our knowledge further. The mystery remains about what is responsible for the production of positrons: the prefered candidates are exploding massive stars called "supernovae", as well as X-ray binaries (violent pairs of stars). Perhaps this could also lead to an indirect observation of a new type of dark matter, known as Light Dark Matter (LDM).

 the first complete map of the sky
The Universe viewed through Integral: the first complete map of the sky
at the electron-positron annihilation energy scale (Credits J. Knödlseder - CESR - September 2005).
A detailed new one to come soon!

Source of antimatter identified in the Galaxy by Integral

For 30 years, we have observed a phenomenal quantity of anti-matter at the centre of our Galaxy that has long had astrophysicists excited about its origin. Observations accumulated over nearly 5 years by ESA’s Integral satellite, particularly its SPI instrument developed by CNES, have enabled an international science team to solve some of the mystery.

This team, with researchers from the CESR astrophysics laboratory at CNRS in Toulouse and from CEA (Saclay), showed that a large portion of the anti-matter created in the galactic plane comes from particular binary sources: a black hole or a neutron star which thanks to its enormous gravitational field rips matter away from its companion with a mass lower or equal to our Sun.

This discovery explains half the antimatter observed in the central bulb of the Milky Way. For the remainder, other contributors are likely, such as the supermassive black hole at the centre of our Galaxy, or supernovae. Even if the whole mystery is not resolved, more exotic hypotheses such as dark matter now seem less believable.

Carte de la voie Lactée par SPI et par IBIS
The left-hand panel shows the glow of 511 keV gamma rays coming from the annihilation of electrons by their antimatter counterparts, the positrons of the Milky Way observed by SPI. The map shows the entire sky, with the Galactic Centre at the middle. The emission can be seen extending towards the right-hand side of the map. The color code shows the intensity of the signal (white more intense). The right-hand panel shows the distribution of hard low mass X-ray binary stars detected by the IBIS/ISGRI telescope on board Integral. This stellar population has a distribution that matches the extent of the 511 keV map. (Credits: Integral CEA and CESR team)

Galactic positron annihilation not a signal of dark matter

Integral's map of 511 keV line emission has been used to rule out dark matter as the origin of galactic positron annihilation. In a paper published in Physical Review Letters Richard Lingenfelter and colleagues demonstrated that contrary to what had been assumed in recent years no exotic sources are required to explain the spatial and spectral features observed by Integral.

For many years the underlying assumption in the interpretation of the 511-keV data has been that the positrons annihilate close to their sources because the conditions they encounter are such that they cannot propagate for very long distances. Based on this assumption, the expectation is that the distribution of the radiation will closely match the distribution of the sources.

In a paper published in the 17 July 2009 issue of Physical Review Letters, Richard Lingenfelter and his colleagues James Higdon and Richard Rothschild restore order to the discussion. The main point made by the authors is that it is incorrect to assume that positrons cannot propagate over large distances, which was the central motivation behind the dark matter hypothesis. Just like cosmic ray electrons of the same energy in fact, positrons can travel unhindered over kiloparsec-scale distances because, as the authors show, their interaction with the magnetic fluctuations across most of the Galaxy is too weak to affect them significantly.

Having removed the obstacle to the propagation of the positrons, the authors are now able to explain the observed flux ratio in terms of classical sources of positrons. Additionally, allowing for the propagation of positrons makes it possible to explain another feature of the radiation, namely the ratio between the 511 line flux and the positronium continuum, an aspect neglected by the dark matter solutions to the problem. In an accompanying paper published in the Astrophysical Journal, the authors show that the observed asymmetry can be explained by the asymmetry of the Galactic spiral arm as seen from the Solar System, finally putting to rest the need to invoke an exotic explanation for the experimental observations.

References:

  • Lingenfelter, R. E., Higdon, J. C. Rothschild, R. E., "Is There a Dark Matter Signal in the Galactic Positron Annihilation Radiation?", Physical Review Letters, Volume 103, Issue 3, 031301, 2009. DOI:10.1103/PhysRevLett.103.031301
  • Higdon, J. C., Lingenfelter, R. E., Rothschild, R. E., "The Galactic Positron Annihilation Radiation and the Propagation of Positrons in the Interstellar Medium", The Astrophysical Journal, Volume 698, Issue 1, pp. 350-379, 2009. DOI:10.1088/0004-637X/698/1/350
  • Georg Weidenspointner, Gerry Skinner, Pierre Jean, Jürgen Knödlseder, Peter von Ballmoos, Giovanni Bignami, Roland Diehl, Andrew W. Strong, Bertrand Cordier, Stephane Schanne, Christoph Winkler "An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays", scientific journal Nature on January 10th, 2008
  • Knödlseder, J. et al "The all-sky distribution of the 511 keV electron-positron annihilation emission." Astronomy & Astrophysics 441 (2005): 513-532
  • Weidenspointner, G. et al "The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI." arXiv : astro-ph/0702621v1 (2007)
  • Milne, Peter A. D. "Distribution of Positron Annihilation Radiation." New Astronomy Reviews 50.7-8 (2006): 548-552

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