(CNA) — An international research team led by NTHU has discovered a rare intermediate-mass black hole candidate currently devouring a star. Often described as the “missing link” in the black hole family, intermediate-mass black holes are extremely rare in astronomical observations. The team’s findings were published in The Astrophysical Journal and featured on NASA’s official website.

In a press release on the 24th, NTHU noted that black holes are among the universe’s most mysterious objects. They do not emit light themselves, and their existence can only be inferred from changes in surrounding matter. Variations in temperature and luminosity during accretion — when a black hole “feeds” — serve as key indicators.

Currently, two main types of black holes have been confirmed: stellar-mass black holes and supermassive black holes. Direct evidence for the in-between category — intermediate-mass black holes — has been nearly absent.

Professor Kuo-Hsing Chiang of NTHU’s Institute of Astronomy led the team that identified a rare candidate in the process of consuming a star. Known as NGC 6099 HLX-1, the source lies in the constellation Hercules, about 450 million light-years from Earth. It was first observed in 2009 and experienced a dramatic flare in 2012, with its brightness increasing more than 100-fold before gradually fading in subsequent years.

According to first author Yi-Chi Chang, a graduate student at NTHU’s Institute of Astronomy, such extreme outbursts are usually associated with supermassive black holes at galactic centers. However, this object, located in the outskirts of its host galaxy, released an unusually strong X-ray signal consistent with theoretical predictions of intermediate-mass black holes, making it a compelling candidate.

Unlike supermassive black holes that constantly accrete matter and shine brightly, or stellar-mass black holes that are abundant and easier to detect, intermediate-mass black holes are elusive, revealing themselves only briefly when “feeding” and emitting strong X-ray bursts.

To confirm the nature of the object, the team conducted three years of observations and analysis using the Hubble Space Telescope, NASA’s Chandra X-ray Observatory, and the European Space Agency’s XMM-Newton telescope. Results showed that in 2012, the X-ray signal from NGC 6099 HLX-1 reached a temperature of about 3 million degrees Celsius — consistent with the high-energy process of a black hole tearing apart a star. Surrounding the object is a dense cluster of stars, described as a “buffet” providing the black hole with a steady food supply.

In addition to X-ray data, optical observations offered further clues. Blue light was traced to temperature variations from rapidly rotating material near the black hole, while red light was linked to the dense stellar population around it. Together, these pieces of evidence strongly support the classification of the object as an intermediate-mass black hole candidate, though further observations will be needed for confirmation.