Is the recent detection of very-high-energy gamma rays from the galactic centre revealing the presence of dark matter? Or, is dark matter at the origin of the electron-positron annihilation in our galaxy? The possibility is not excluded, but it would imply that dark-matter particles are even more exotic than previously thought.

It is well known that positrons annihilate with electrons near the centre of our galaxy; the associated emission line at 511keV was detected more than 30 years ago. The spatial extent of the emission as seen by the European Space Agency's INTEGRAL gamma-ray satellite is smooth and corresponds roughly to the bulge of our galaxy. Such a more or less spherical cloud of positrons agrees well with the expected distribution of dark matter in our galaxy, so it has been suggested that light-weight dark-matter annihilation could produce the observed positron population (Boehm et al. 2004).

Is it possible, however, that dark-matter particles in the 1-100MeV range could decay into electron-positron pairs without leaving a detectable signal other than the 511 keV line? This question has indeed been addressed by some researchers, who show that such a decay would inevitably produce gamma rays via an internal "bremsstrahlung" process (Beacom et al. 2004). This emission should have been detected by the high-energy instruments of the Compton Gamma-Ray Observatory, unless the dark-matter particles have masses below about 20MeV.

Another claim for the possible detection of dark matter has followed the discovery of very-high-energy gamma rays emitted by the galactic centre. The gamma rays, at an energy above 100GeV, are detected by ground-based telescopes observing the faint Cherenkov light emitted by the electromagnetic shower that results from the interaction of a gamma-ray photon with the terrestrial atmosphere.

The first measurements by the American Whipple telescope and the Australian- Japanese CANGAROO observatory had relatively large uncertainties on the actual position of the gamma-ray source. The results were therefore compatible with a diffuse emission as expected from the annihilation of dark-matter particles. Recent measurements by the European-African High-Energy Stereoscopic System (HESS), a new array of four Cherenkov telescopes in Namibia (CERN Courier November 2002 p7), have shown with a more than 10 times better spatial aCCEuracy that the emission is really bound to the galactic centre.

If dark-matter particles are responsible for these gamma rays observed by HESS, then they must have masses of more than 12TeV and be very concentrated within a few tens of light-years from the galactic centre (Horns 2004). Although it cannot be firmly excluded that dark matter is at the origin of the galactic gamma-ray emission observed at 511 keV and/or at TeV energies, this would imply particle masses either much below (<20MeV)>12TeV) the expectations of most models of non-baryonic dark matter. Fortunately, other less-exotic phenomena oCCEurring in supernovae or black holes are promising alternatives to solve the gamma-ray mystery in the heart of our galaxy.

Further reading
C Boehm et al. 2004 Phys. Rev. Lett. 92 101301.
J F Beacom et al. 2004 astro-ph/0409403.
D Horns 2004 astro-ph/0408192.

Compiled by Marc Türler, INTEGRAL Science Data Centre