In ‘normal’ astronomy, we are used to dealing with photons that can be captured. Radio waves in antennae (for those with enormous wavelengths) or satellite dishes, which also account for submilimetre and microwaves. Then onto more normal telescopes that deal with infrared, visible light and ultraviolet. Grazing mirrors for x-rays that would otherwise pass through ordinary mirrors and simple digital cameras to deal with gamma rays that escape any other way of capturing and focusing them.
But photons aren’t the only things that come at us from the sky, I speak of course of Cosmic Rays. This is a catch all phrase for all kinds of ionising radiation that comes at us from the skies. These include the very highest energy particles from the Sun and even higher energy particles from elsewhere in the cosmic that are able to penetrate both the solar system’s magnetic field (the heliosphere) and the Earth’s field (the magnetosphere) to strike directly at our atmosphere.
Primary cosmic rays tend to be electrons, positrons (anti-electrons) and a variety of atomic nuclei, mostly protons (singly ionised hydrogen) and helium ions. When they strike the atmosphere, these can create showers of particles in the same way that atom smashers on the ground do, except these incomers do it at far higher energies.
Protons and other atomic nuclei are nearly two thousand times heavier than electrons and positrons. This means if they hit a particle at a given speed, the particle knows about it. They can steal electrons off other atoms and become neutral particles. As they do this and the electron slowly makes its way to the ground state, it releases extra energy as light, producing a signature that can be detected by ordinary astronomical means. The light is Doppler shifted by the velocity of the proton, so whilst ordinary emissions might just be red light, superfast protons heading in our direction can release ultraviolet, x-ray or even gamma ray particles. But where would these superfast particles be produced.
In 1949 Fermi (the physicist) suggested they could be produced in the energetic deaths of high mass stars – supernovae. It has been difficult to confirm this as charged cosmic rays bounce off every magnetic field going and so their own origins are lost – like a foggy room that’s been lit by a bulb, the position of the bulb might not be easy to see as the light diffuses.
In 2010 Fermi (the gamma ray telescope) took images of supernova remnants and saw the suggestion of gamma ray sources within them (see video below). To understand what’s happening here, imagine the protons shot away from the dying star as huge jets or lobes of plasma. These slam into the cold dense matter outside of the remnant and charge exchange occurs as well as the emission of Brehmastrallung, or stopping energy, in which charged particles slowed by electromagnetic fields release photons to conserve momentum. In the bit pointing our way, this sends gamma rays towards us for Fermi (the telescope) to feast on and the ideas of Fermi (the physicist) to be validated on.
Of course, not all cosmic rays come to us charged. Some ENAs – energetic neutral atoms – have been detected. As they aren’t deflected by magnetic fields, they can be far more specific about their origins – although they usually travel a relatively (on a cosmic scale) small difference before charge cosmic rays or high energy photons reionise them.
IBEX is a space telescope that collects these rather than photons or charged particles. The telescope took a good look around the sky from its position far above the ground and saw something unusual – a giant ribbon of ENA emission. This ribbon has been tentatively identified as a flux tube – a giant magnetic field line – outside of our heliosphere (the region dominated by the Sun’s influences). The mechanism for this kind of thing generating a ribbon is a low energy analogue of how those gamma rays Fermi (the telescope) has been looking at – our solar wind and plasmas from other stars (the interstellar flow) meet the magnetic field and get trapped, spinning round and round the line. Eventually, charge exchange occurs and the now neutral particle is no longer confined to the field line – it is electromagnetically neutral. This means the field line, believed to thread through a region of stuff the solar system is ploughing through known as the Local Fluff, ‘shines’ in emitted neutral particles, without much in the way of any form of light to augment its signal.
Of course, some particles are even more elusive than even ENAs. Dark Matter is generally assumed to take the form of ‘cold’ (relatively low velocity) particles called WIMPs – Weakly Interacting Massive Particles. In some theories of particle physics, WIMPs are their own antiparticles and will annihilate other WIMPs when they meet, releasing energy. Observations suggest there are areas where dark matter has coalesced and in a recent paper, some physicists suggest that this could be a precursor of star formation in some cases. In this methodology, the clumps of dark matter would produce gravity to attract dust as well as thermal energy from annihilations to spark off a star’s thermonuclear engine. Dark stars, as they are called, would be cooler than ordinary stars, emitting less energy per unit area, which in turn means there would be less radiation blowing away material that would otherwise collect onto the star. These together would allow the stars to grow to 100,000-10 million solar masses. These large sizes would mean that the stars, although they emit less light per unit area, would be pretty bright due to their large overal surface area, allowing them to be detectable by the James Webb Space Telescope. Furthermore, a black hole created by the collapse of one of these would easily form the basis of one of the supermassive black holes we see today.