Star in tight orbit around black hole

Astronomers have spotted what they think is a star in the closest known orbit around a black hole – just 2.5 times the separation between the Earth and the Moon. Located in the 47 Tucanae globular cluster about 14,800 light-years away, the white dwarf is seen to oscillate in X-ray brightness with a period of about 28 min – which astronomers believe corresponds to its orbit around a black hole. The observation was made using NASA's Chandra X-ray Observatory and NuStar telescope along with the Australia Telescope Compact Array. "This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disc of matter around the black hole before falling in," says team member Arash Bahramian, from the University of Alberta in Canada and Michigan State University in the US. The team believes that the system could have been formed when a black hole smashed into a red giant star. Gas from the red giant was ejected during the collision, creating the white dwarf, which was drawn closer to the black hole over time. The binary system is called X9 and will be described in Monthly Notices of the Royal Astronomical Society and on arXiv.

Iron nanocubes detect nitrogen dioxide

Nanometre-sized cubes of iron similar to those used to decorate the ancient Lycurgus cup could be used to sense the presence of nitrogen dioxide. The famous cup was made in the 4th century by Roman artisans who embedded iron nanoparticles in glass to create structural colour that changes hue depending on which way light is shone through it. Now, Jerome Vernieres and colleagues at the Okinawa Institute of Science and Technology in Japan have come up with a way of making large numbers of iron nanoparticles that are all the same size – something that had proven difficult to do in the past. This uniformity is important because it should allow the nanoparticles to be used to detect specific molecules such as nitrogen dioxide. The team's manufacturing process involves firing an argon plasma at a piece of iron, which knocks out iron atoms that then join together to form nanoparticles. A magnetic field is used to achieve precise control over the plasma, which allows the team to create nanoparticles of a specific size. Once it had made its nanoparticles, the team noticed that the electrical resistance of the tiny cubes changed in the presence of nitrogen dioxide. The researchers then joined forces with others at the University of Toulouse in France to create a prototype nitrogen-dioxide sensor that they say could be useful for a range of applications including the diagnosis of asthma and detecting environmental pollution. The research is described in Advanced Functional Materials.

Born's rule prevails in five-path interferometer

An important tenet of quantum mechanics is that interference always occurs between pairs of paths in an interferometer – and that higher-order interference effects between more than two paths do not occur. This is a result of Born's rule, which was put forth by Max Born in 1926 and defines how the result of a measurement on a quantum system is related to its wave function. Any deviation from Born's rule would identify a significant flaw in quantum theory and therefore be of great interest to physicists. Now, Thomas Kauten and colleagues at the University of Innsbruck and University of Vienna in Austria have put Born's rule to the test in a five-path interferometer. By implementing single-photon detection, the team was able to run the interferometer in the "quantum regime" with one photon at a time passing through it. The researchers were able to exclude the existence of higher-order interference effects in this quantum regime to an uncertainty of 2 ×10–3, which they say is much better than previous attempts. The research is described in New Journal of Physics.

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