The most energetic light and particles in the Universe represent an enduring mystery: we don’t know where they come from.
Sure, we can trace some; but there’s more gamma radiation and neutrinos streaming through the Universe than we can account for. A lot more. And astronomers have just found an explanation for some of them: nearly dormant black holes.
This, they say, can explain the excess of ‘soft’ gamma rays in the Universe without relying on cold (nonthermal) electrons – which has always been a problematic explanation, because electrons become thermalized on timescales thought to be too short to generate high-energy particles.
Gamma rays and neutrinos are not exactly rare. Gamma radiation is the most energetic form of light in the Universe, and it has been detected at extraordinarily high energies – the teraelectronvolt range.
Neutrinos, or ghost particles, are almost massless particles streaming through the Universe, barely interacting with anything at all. Those, too, we’ve detected at high energies.
In order to obtain these energies, the photons and particles within them require the presence of a cosmic accelerator. These should be high-energy objects, such as supernova remnants, or a black hole actively devouring material.
But even once we’ve accounted for these high-energy sources, we’re still left with a gamma-ray excess in lower ‘soft’ energies, as well as a neutrino excess, that is difficult to explain.
According to a team of researchers led by astronomer Shigeo Kimura of Tohoku University in Japan, the excess may come from an unexpected source: supermassive black holes that are almost, but not quite, dormant – but nor are they entirely active.
When a supermassive black hole is active, it’s circled by an immense disk of dust and gas that is slowly being siphoned onto the black hole. The immense forces at play in the space around the black hole heat the material in the disk so that it blazes across a range of electromagnetic wavelengths, including gamma radiation.
In addition, some material is siphoned around the outside of the black hole along its magnetic field lines, which act as an accelerator, towards the poles, where it is launched into space at a significant percentage of the speed of light.
Each galaxy is thought to have a supermassive black hole in its center, but not all are active. Our galaxy’s supermassive black hole, for example, is pretty snoozy.
According to Kimura and his team, the gamma-ray excess in the lower energy range – megaelectronvolts rather than giga- or teraelectronvolts – could be produced by supermassive black holes that are accreting at such a low level as to be much dimmer to our telescopes here on Earth.
The team performed calculations, and figured out how it would work. Although there’s less material swirling around these non-active black holes, there is still some, and it still gets heated up.
In fact, this hot plasma could get up to billions of degrees Celsius – hot enough to be generating gamma radiation in the megaelectronvolt range, or what we call ‘soft’ gamma rays.
Within this plasma, protons can be accelerated to high speeds. When these high-energy protons interact with radiation and matter, they can generate neutrinos – thus also explaining the neutrino excess. And there are enough of these quiet supermassive black holes in the Universe to explain at least a significant proportion of these excess signals.
So far, it’s just a hypothesis, but the math checks out. Armed with this information, astronomers should have a better idea of what to look for in future observations – and the mystery of those inexplicable gamma rays will be closer to being solved.
The research has been published in Nature Communications.