— Jets emerge from the vicinity of a black hole or neutron star, which orbits a massive regular star in the LS 5039 system. Some of the gamma rays produced in the system are transformed into particles of matter and antimatter through collisions with ultraviolet photons (Illustration: HESS Collaboration/R Hynes)
Brightness changes of clock-like regularity have been found in a source of gamma rays for the first time. Studying these variations should help astronomers solve the mystery of how these very high energy gamma rays are produced. They also provide the first clear sign outside the laboratory of light being converted into particles of matter and antimatter.
Gamma rays coming from a system called LS 5039 were first detected in 2005 by the High Energy Stereoscopic System. HESS is an observatory in Namibia that detects gamma rays by observing their effects on particles in Earth's atmosphere.
The LS 5039 system consists of a compact object either a black hole or neutron star orbiting a star with 20 times the mass of the Sun. The gamma rays are thought to be emitted by electrons accelerating to very high speeds, beyond anything achievable in the most powerful particle physics experiments on Earth. But exactly where and how the electrons get accelerated to such high speeds in this system is a mystery.
Now, further HESS observations have revealed that the gamma rays are varying with a timescale of 3.9 days, the same as the period of the compact object's orbit around the massive star. The research effort was led by Felix Aharonian of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
This result suggests the gamma rays are produced in a very small region close enough to the massive star and its compact companion to be influenced by their relative positions, says team member Mathieu de Naurois of the Laboratoire de Physique Nucléaire et de Hautes Energies in Paris, France.
"The fact that we see a periodicity in this system means that the gamma rays cannot come from very far away they must come from inside the system," he told New Scientist.
Markus Boettcher at Ohio University in Athens, US, who is not a member of the HESS team, agrees. He says the observed changes in the brightness of the gamma rays as the compact object moves in its orbit provide may help determine what accelerates the electrons to produce the gamma rays. "We know that the distance between the star and the compact object definitely plays a role in the efficiency of the acceleration," he told New Scientist.
De Naurois says the indication that the gamma rays are produced very close in fits well with the idea that the compact object is a neutron star and that the gamma rays are generated when electrons are accelerated in its powerful magnetic field.
By contrast, the result casts doubt on the idea that the electrons are accelerated in jets extending far out into space from the compact object, which in this scenario would most likely make it a black hole. The gamma rays could still be produced in jets, he says, but it would have to happen only at their base, very close to the compact object.
The periodic brightness dips themselves provide the first evidence outside of the laboratory of gamma rays, which are a form of light, being converted into particles of matter and antimatter, de Naurois says. The creation of matter and antimatter from pure light is predicted by Einstein's relativity, but until now, no clear sign of this had been observed outside of particle accelerators on Earth.
Some of the gamma rays produced near the compact object collide with photons of ultraviolet light from the massive star and are converted into pairs of electrons and their antimatter counterparts, called positrons, he says.
These collisions become more frequent as the compact object gets closer in its orbit to the massive star, removing more of the gamma rays and causing the periodic brightness dips seen from Earth, he says.
"This is the first time that we see light absorbing light through this pair-creation process" outside of Earth-based experiments, he says.
A similar effect is thought to be responsible for absorbing some of the microwave radiation left over from the big bang, Boettcher says, but the HESS results are a clearer sign of the phenomenon.
Its occurrence in the binary system was expected from theory "but it is still interesting to see something actually happening", he says.
Journal reference: Astronomy & Astrophysics (DOI: 10.1051/0004-6361:20065940)
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