For more than 40 years, scientists have known that the center of the Milky Way is rich in antimatter, but they have disagreed on its origins. Now, a paper in Nature Astronomy claims to have the answer.
The center of our galaxy produces an astonishing dose of gamma-rays. These are attributed to matter and antimatter particles annihilating each other, converting their mass to energy in the form of high-frequency photons. Where then did the antimatter come from? There is still plenty of head-scratching among cosmologists as to why the universe contains so much more matter than antimatter, but given that it does, the existence of any substantial amount of antimatter takes some explaining.
The supermassive black hole at the center of the galaxy was an early favored culprit, and dark matter has also been blamed. However, according to Dr Roland Crocker of the Australian National University, these can be ruled out. Instead, the source is a subgroup of Type Ia supernovae, occurring when two white dwarf stars collide.
The collisions produce the titanium isotope 44. Titanium-44 decays to scandium and then to calcium, emitting positrons – the antimatter equivalent of electrons – in the second decay. The half-life of these two processes is barely 60 years, so we might expect the number of gamma-rays to surge after supernovae and drop off quickly thereafter.
However, Crocker told IFLScience that the positrons hang around for roughly a million years before they collide with ordinary matter in the interstellar medium, causing their annihilation. Consequently, we're still seeing plenty of the resulting gamma-rays, even though there has not been a supernova of this (or any) type in our galaxy for centuries.
The paper rejects some alternative theories. Dark matter would be expected to produce higher energy positrons than we witness. Crocker told IFLScience: “There have been attempts to model dark matter that avoid this.” However, he thinks these fail Occam's razor, relying on overly complex explanations. Type II supernovae produce enormous amounts of nickel-56, much of which release positrons during decay, but according to Crocker this happens so early during explosions that the positrons become trapped, never reaching the interstellar medium and dispersing their gamma-rays through the galaxy.
Not all Type Ia supernova do the trick, however. Titanium production requires “an unusual amount of high density helium,” Crocker told IFLScience, something that only occurs when a binary system contains two white dwarf stars, with masses between 1.4 and 2.0 times that of the Sun. These gradually coalesce and explode, but only after the larger star has captured the smaller one's helium. Despite these specific requirements, such events, known as SN 1991bg-like supernova, are common enough to account for the astonishing 1043 positron annihilations that occur every second in our galaxy.
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