Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a "photo-ionized" plasma here on Earth because it requires an extremely hot source of radiation.
But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.
The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.
"We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively]," explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.
The beams are fired at a tiny plastic capsule, causing it to implode. "As it shrinks, a hot and dense plasma core forms inside the capsule," says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.
Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.
Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions. However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.
The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having "great potential for further development," because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.
Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.
