Omega Centauri’s black hole, record-energy neutrino, and GaN semiconductors for nuclear reactors

Evidence for central black hole in Omega Centauri; undersea telescope spots most energetic neutrino; GaN semiconductors to monitor nuclear reactors.

The likely location of an intermediate-mass black hole at the heart of the Omega Centauri star cluster. | Photo Credit: European Space Agency/Hubble/AFP

Hubble data reveals intermediate-mass black hole in Milky Way

A NEW study, led by Maximilian Häberle of the Max Planck Institute for Astronomy, Heidelberg, Germany, has found evidence of what astronomers had been suspecting for some time: Omega Centauri, a cluster in the Milky Way of about 10 million stars and about 18,000 light years from the earth, may contain a central black hole.

Häberle and his collaborators analysed more than 500 images of Omega Centauri taken by the Hubble Space Telescope over two decades, mostly to calibrate its instruments. This resulted in the identification of what could be the strongest candidate yet for an intermediate-mass black hole (IMBH) that is at least 8,200 times the solar mass. The work was published in a recent issue of Nature.

An IMBH is an object that was puzzlingly missing in the mass range between the “supermassive” black holes that are believed to be at the centre of most galaxies and weigh as much as millions of suns and much smaller ones that weigh just as little as a few suns. The discovery of the IMBH seems to be the “missing link” between its stellar and supermassive kin. The supermassive black hole at the centre of our galaxy is about 27,000 light years away.

Omega Centauri, astronomers believe, is the core of a small, separate galaxy whose evolution was cut short when the Milky Way swallowed it. The subsequent stellar dynamics is such that there was no way for what was its central black hole to grow.

The team reconstructed the movement of over 1,50,000 stars in the cluster. While most stars moved as theoretical models predicted, seven stars, all near the centre, were moving too fast to be explained by the cluster’s gravity alone. This led to the conclusion that the stars were accelerated by the gravity of a massive object, and since the images did not include any visible object at the likely location, it was inferred to be a black hole.

An artist’s impression of KM3NeT showing its many strings anchored to the seabed of the Mediterranean. The small optical modules (the 43 cm diameter glass spheres with optical sensors and electronics inside) can be seen in comparison to the kilometre-sized telescope. The yellow object in the front is a buoy.  | Photo Credit: Wikimedia Commons

First ultra-high-energy neutrino detected

THE Cubic Kilometre Neutrino Telescope (KM3NeT), a European observatory still under construction at the bottom of the Mediterranean, spotted what could be the most energetic neutrino ever detected.

The neutrino physicist João Coelho, of the AstroParticle and Cosmology Laboratory, Paris, reported this on June 18 at the recently held Neutrino 2024 conference in Milan, Italy. For a decade or more, physicists believed that such ultra-high-energy neutrinos—elusive near-zero mass subatomic particles travelling at nearly the speed of light—should exist following some of the universe’s most cataclysmic events, such as growth spurts of supermassive black holes in distant galaxies. KM3NeT’s Astroparticle Research with Cosmics in the Abyss (ARCA) detector, a cubic kilometre–sized telescope searching for distant neutrino sources, is an array of optical modules on “strings” attached to the 3,500-metre-deep sea-floor south-east of Sicily. It has been optimised to detect neutrinos in the tera, or trillion (10¹²), electronvolt (TeV) to the peta (10¹⁵) electronvolt (PeV) energy range. ARCA has been collecting data since the mid-2010s. It currently has 28 strings and will have a total of 230 strings by 2028.

Francis Halzen, a neutrino physicist at the University of Wisconsin-Madison and principal investigator of IceCube, the neutrino detector in Antarctica that until now had the capability to detect such high-energy particles, told the journal Nature that the detection was “a fantastic event” and that it highlighted ARCA’s potential.

Coelho stated that the particle had an energy of 10s of PeVs, which makes it the most energetic neutrino detected so far. He said details such as the precise direction from which the particle had come and when the observation occurred would be given in a paper to be published in the near future.

Kyle Reed, of Oak Ridge National Laboratory, at The Ohio State University Nuclear Reactor laboratory, where researchers recently demonstrated that a transistor made with GaN successfully maintained operations near the reactor’s core. | Photo Credit: Michael Huson/The Ohio State University

A semiconductor up close to monitor nuclear reactor cores

A TINY sensor is used in large nuclear reactors to monitor the cooling system to enhance safety. Researchers at Oak Ridge National Laboratory (ORNL), Tennessee, US, have developed intense radiation-resistant electronic hardware to make those basic sensors more accurate.

The new hardware is based on gallium nitride (GaN) semiconductors. The researchers recently demonstrated that a transistor made with GaN successfully maintained operations near the core of a nuclear reactor operated at The Ohio State University.

GaN is a wide-bandgap (WBG) semiconductor. WBG semiconductors are materials that have larger bandgaps than conventional semiconductors like silicon. While the latter have a bandgap of 0.6–1.5 eV, the former have bandgaps above 2 eV. Generally, properties of WBG semiconductors fall between those of conventional semiconductors and insulators. WBG devices can operate at much higher frequencies, temperatures, and irradiation rates.

Although GaN has earlier been tested in the ionising radiation of space, it had not been tested for the more intense radiation of neutron bombardment. “We are showing that it is great for this neutron environment,” said Kyle Reed of ORNL.

This development could be a game-changer in the safety monitoring of nuclear facilities. The data gathered by sensors provide early warnings about wear and tear on equipment to prevent broader failures. Currently, this sensing data is processed at a distance through metres of cable connected to silicon-based electronics.

“When you have lengthy cables, you end up with a lot of noise, which can interfere with the accuracy of the sensor information. By placing electronics closer to a sensor, you increase its accuracy and precision,” Reed said. The GaN transistors were able to handle over 100 times higher accumulated radiation dose than a silicon device.

According to Dianne Ezell of ORNL, the material should be capable of surviving for at least five years, the normal maintenance window. After the GaN device was exposed to days of much higher radiation levels within the core itself, the team concluded that the transistors would exceed that requirement. Reed, however, said that the testing also showed that heat seemed to be more harmful to GaN than radiation, so the team now plans to measure how GaN reacts to heat alone.