There is something strange happening in the core of the Milky Way. A space observatory measuring the energy and distribution of gamma-rays in the cosmos has made an unexpected (and perplexing) discovery. It would seem there is a very high proportion of gamma-ray photons emanating from our galactic core with a very distinctive signature; they have a precise energy of 511 keV (8×10-14 Joules), and there’s a lot of them. So what could possibly be producing these 511 keV gamma-rays? It turns out, 511 keV is a magic number; it is the exact rest mass energy of a positron (the antimatter particle of the electron). So this is fairly conclusive evidence that positrons are dying (i.e. annihilating) in vast numbers in our galactic nuclei. Still, this is of little help to astrophysicists as there is no known mechanism for producing such high numbers of annihilating positrons. Ideas have been put forward, but there’s a new possibility, involving some new particle physics and some lateral thinking…
So what produces gamma-rays in large numbers? The first thing that comes to mind is a gamma-ray burst, produced when a massive star dies and collapses as a supernova. But this is short-lived and not sustained. How about the supermassive black hole sitting in the middle of the Milky Way’s galactic nucleus? This theory was recently discussed on Astroengine, but the production of antimatter (i.e. positrons) is more of a slow leak than anything substantial, certainly not of the scale that is being measured. As we are dealing with gamma-rays of the exact rest mass energy as a positron, so we know that the source is some kind of positron annihillation. What could possibly be doing this?
The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a European Space Agency (ESA) mission set out to detect some of the most energetic emissions measured in the Universe. INTEGRAL is the most sensitive and advanced mission of its kind, giving us a deep look into gamma-ray sources. Little did we know the mystery target would be the centre of our own galaxy. At first, this distinct gamma-ray signal was thought to come from the short-lived positronium “element” which exists for an instant just as electron-positron pairs collide and annihilate. However there was still the problem of a production mechanism. Supernovae and black holes don’t produce enough antimatter for this to be the cause.
So the blame then fell on the common dark matter candidates, such as very light super-particles (of the weakly-interacting ilk), but again, researchers found that this explanation didn’t fit with the data nor the models. Now a group of South Korean astrophysicists think they have an answer. Seong Chan Park and his team from Seoul National University believe there may be a new particle in the playground called the “millicharged fermion.” This new particle is very lightweight and has a minuscule electrical charge, and this research indicates that it may be the root cause of the 511 keV gamma-ray photons.
“Most preferred interpretations of the 511 keV gamma rays involve the introduction of new particles, and a millicharged fermion has been proposed before, as far back as 20 years ago, as a dark-matter candidate. But in this work we have handled the urgent problem of interpreting the 511 keV anomaly using millicharge fermions, within certain physical constraints.” – Seong Chan Park.
So, although the millicharged fermion has been put forward as a possible dark matter candidate in the past, it seems that this particle lightweight may be an ideal positron generation mechanism. The millicharged fermion is calculated to decay into an electron and a positron which, as you’d expect, annihilate soon after. Or, the it could annihilate with its own antiparticle to generate an electron-positron pair that go on to annihilate. Regardless, the outcome is the same: a consistent source for the 511 keV gamma-rays.
Such a millicharged fermion would easily evade any particle detector as it has such a small charge. This by definition makes it a prime dark matter candidate, possibly constituting as much as 10% toward the missing mass of the Universe.
This is all great, but the one issue that this published work avoids is why these millicharged particles are generated within the galactic bulge. Are they trapped around the supermassive black hole? Is there some further “new physics” that needs to be invoked? Either way, I’m totally intrigued by these findings; realizing we are being bathed in photons at an exact energy and working out the source is incredible. Taking the extra step and piecing together the data with production mechanisms is worth waiting for…