Discovery of structure of radio source from a pulsar orbiting a massive star

The research was carried out by Javier Moldón, Marc Ribó and Josep Maria Paredes, of the Department of Astronomy and Meteorology at the University of Barcelona and the UB Institute of Cosmos Sciences, together with Simon Johnston, of the Australia Telescope National Facility (Australia) and Adam Deller, of the National Radio Astronomy Observatory (USA), and in it they studied the only gamma-ray binary that is known to be formed of a pulsar (PSR B1259-63; that is, a neutron star with a radius of some 10 km that is spinning extremely fast) and a massive star (LS 2883), which is 30 times the mass of the Sun.

As the researchers from the UB explain, it is the first time that anyone has been able to observe the morphology, at different positions in the orbit, of the radio source of a gamma-ray binary in which the pulsar has known properties. The results show how the emission forms a type of cometary tail which moves around as the pulsar traces out its orbit. They have thus been able to see that the radio source is up to ten times larger than the orbit of the binary system.

The radio emission is produced during the periastron passage of the system -- which is the point at which the two components of the binary system are at their closet to each other -- once every 3.4 years. It has been shown that the radio emission is due to the synchrotron radiation produced by electrons that escape from the binary system at relativistic speeds of up to 100,000 km per second. This has allowed limits to be placed on the magnetization, which is essential for understanding the relativistic winds emitted by pulsars.

The PSR B1259-63/LS 2883 system is 7,500 light years away, in the direction of the constellation of Centaurus. The pulsar's orbit is 14 times larger than Earth's orbit around the Sun, but because of its extreme eccentricity, the pulsar passes within just 0.9 AU (astronomical units: Earth-Sun distance) during periastron. At such short distances the powerful wind from the massive star, travelling at over 1,000 km per second, collides with the wind from the pulsar, which is less dense but which travels at 100,000 km per second. This shock of winds accelerates particles that emit photons throughout the whole electromagnetic spectrum through synchrotron emission and inverse Compton scattering. The new radio observations directly show the radiation in the tail of particles accelerated in the shock, which spreads out over some 120 UA. This has allowed the researchers to infer the conditions under which the acceleration of the particles in produced in the region of the shock.

The observations of the binary system PSR B1259-63/LS 2883 were performed using the Australian Long Baseline Array (LBA) made up of five antennas separated by distances of up to 1,500 km. Using interferometry techniques, this network allowed the researchers to explore spatial scales of the order of 0.02 seconds of arc, an unprecedented resolution for observations of this binary system. To give an idea of the resolution involved, it corresponds to distinguishing features just 40 metres long on the surface of the Moon observed from Earth.