Cosmic rays were discovered almost 100 years ago (1912), yet astrophysicists are still uncertain about where they come from or how they acquire their extremely high energies. Research recently published gives strong evidence that the Cygnus X region, which contains hundreds of very hot, massive, young stars, is a source of cosmic rays and has the means to accelerate them to high energy.
A great deal is known about cosmic rays. They are not electromagnetic radiation (such as gamma rays) but instead consist of charged particles of ordinary matter – electrons, protons, or other atomic nuclei. Of nuclei heavier than hydrogen or helium, the elements represented in cosmic rays occur in proportions close to, but not quite the same as, what is found in typical interstellar gas. A few heavier elements are overrepresented.
The amount of kinetic energy carried by most cosmic rays can range up 1000 TeV (1015 eV), but a small number may be up to 1021 eV. The energy of the highest energy cosmic rays exceeds what could be produced by any known source within our galaxy, so the source is unknown, but likely to be a very energetic active galaxy.
Since cosmic rays are charged particles, their trajectories are bent and twisted by galactic magnetic fields, so there’s no direct way to identify their place of origin by the direction from which they arrive. Possible sources have to be examined individually to determine their ability to produce cosmic rays. For lower energy cosmic rays (under 1000 TeV) the possible sources inside our galaxy include supernova remnants and clusters of very hot, young stars. The new research gives evidence for one instance of the latter.
Along their way here, cosmic rays collide frequently with photons and other interstellar particles, generating very energetic gamma rays, just as regularly happens as a result of collisions in particle accelerators. Like other forms of electromagnetic radiation, gamma rays are unaffected by magnetic fields, so their source is easy to localize in space. Supernova remnants, such as the Crab Nebula, may emit gamma rays and may also be a source of cosmic rays. However, if a large diffuse source of gamma rays is found, and the source cannot be narrowed down to a specific object like a supernova remnant, then the gamma-ray emissions may be a byproduct of highly accelerated charged particles. The question then is about the mechanism for the acceleration
The new research provides good evidence that one such gamma-ray source – the active star-forming region Cygnus X – is probably a significant origin of cosmic rays. Cygnus X is a large region, more than 160 light-years in extent, containing hundreds of very hot, luminous, young stars of spectral class O and spectral class B. O-type stars have masses from 15 to 90 M⊙ (solar masses). The range for B-type stars is 2 to 15 M⊙. Except for the most massive such stars (more than about 30 M⊙), the life expectancy of stars large enough to expire as core collapse supernovae (those of more than ~8 M⊙) is at least 10 millions years. Most of the O and B stars in Cygnus X are young (~5 million years), so few in the area could have accelerated charged particles in supernova explosions or left remnants that might continue to emit gamma rays.
The more massive O and B stars are rare for several reasons. One is their relatively short lifetimes. But a more significant reason here is that their very high luminosity generates intense stellar winds that heat surrounding gas and expel it from their general vicinity. Since star formation requires large amounts of cold gas, once a small number of very large stars have formed, additional star formation is shut off. Even so, if enough gas was present to begin with, groups of tens or hundreds of O-type and B-type stars can form simultaneously. Such groups are called OB associations. One of these in Cygnus X (the Cygnus OB2 association) has 500 stars with mass of 10 M⊙ or more. Another nearby association (NGC 6910) has 75 hot, massive stars.
Cygnus X is at a distance of about 4500 light-years. There is a gamma-ray source (a supernova remnant) associated with the star named (coincidentally) γ Cygni, but that object seems to be somewhat more distant than Cygnus X. The stellar-mass black hole Cygnus X-1 is also in the same part of the sky, but recently determined to be about 6060 light-years distant from us. The research used the Fermi Gamma-ray Space Telescope to confirm that Cygnus X is a significant source of gamma rays. The shape of the gamma-ray spectrum is more like what would be expected from cosmic rays than from the few supernova remnants in the area.
If supernova remnants are unlikely to be what accelerates charged particles sufficiently to produce gamma radiation, the most probable alternative is the turbulent stellar winds from the O and B stars. Large cavities of lower density gas were carved out of Cygnus X early in the lives of the massive stars by ionization, radiation pressure, and coalescing stellar winds. Shock waves of compressed gas form the outer boundaries of these cavities, and charged particles inside these cavities tend to remain confined inside them. The cavity boundaries can be traced through 8 μm infrared emissions.
Tangled magnetic fields in the turbulent plasma may also help confine the charged particles and accelerate them in the same way as magnetic fields within man-made particle accelerators do. Computer modeling of the physics inside the Cygnus X cavities suggests that diffusion lengths of charged particles will be much shorter than in the normal interstellar medium, so that protons could be confined within the cavities for ~100,000 years. The simulations yield proton energies peaking in the 10 to 100 GeV range, but extending to 150 TeV. This is consistent with the observed gamma-ray spectrum from Cygnus X.
One important observation from the research is that the gamma radiation from Cygnus X is concentrated within the cavities rather than being spread evenly throughout the area. In addition, the gamma-ray spectrum is largely the same in all cavities. This is the strongest evidence that the environment inside the Cygnus X cavities – rather than discrete objects – is responsible for the acceleration of charged particles.
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