What could be causing gamma-ray photons to be streaming from the galactic core with a precise energy of 511 keV (8 X 10-14 joules)? It’s an interesting question, one tackled by Ian O’Neill on his astroENGINE site, as posted by 21st Century Waves in this week’s Carnival of Space. O’Neill notes the defining nature of this energy level, which turns out to be the exact rest mass energy of a positron, the antimatter equivalent of an electron. That fact suggests the annihilation of positrons in the galactic center, but what’s causing it?
The usual suspects just don’t fit, as O’Neill is quick to note:
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?
Seong Chan Park (Seoul National University) and team may have an answer in their speculation about the existence of a new particle called the millicharged fermion. Now we’re in dark matter territory — the millicharged fermion has a history running back twenty years in dark-matter speculations. And the mechanism may work, for the particle is calculated to decay into an electron and positron, leading to quick annihilation. Moreover, the tiny charge of the millicharged fermion would make it all but transparent to detection efforts. Dark indeed.
Intriguing work, and good reason to keep following ESA’s INTEGRAL mission, launched in 2002, which focuses on gamma-ray sources (absorbed by the atmosphere, gamma-rays must be studied from space). But don’t assume gamma-ray bursts (GRBs) are the cause of the unusual emissions. They’re short-lived and can’t account for the renewable nature of what INTEGRAL has found flowing from the galactic bulge. As with dark matter speculation itself, the galactic core turns out to be a good place for what O’Neill calls “…new particle physics and some lateral thinking…”