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Dormant black hole discovered in neighboring galaxy

An astronomer team of "black hole police" may have been able to debunk a slew of other purported black holes, not this one.

(CN) — Astronomers mostly agree that formation of a black hole begins in the collapse of a star, but due to the nature of black holes, confirmation about the in-between remains elusive. One hurdle has been identified in determining if a particular observation is truly a black hole or rather some other difficult to discern cosmic structure.

A team that has gained a reputation for debunking possible black holes has published a study on Monday in the journal Nature Astronomy that concludes that an unidentified discovery in a two-object system could only be a black hole. Beyond that, the astronomers were also able to determine that the star that created the black hole may have collapsed without the supernova that normally accompanies the death of a star.

“As a researcher who has debunked potential black holes in recent years, I was extremely skeptical regarding this discovery,” says Tomer Shenar, a Marie-Curie Fellow at Amsterdam University in the Netherlands and lead author of the study, in press release.  Shenar and his team, comprised mostly of PhD researchers at the Institute of Astronomy at Belgium's KU Leuven, have spent the last several years evaluating the veracity of other astronomers' claims of black holes. The team collaborated with an independent researcher who had developed an especially fearsome reputation for debunking imposter black holes, Kareem Al-Badry with the Harvard/Smithsonian Center for Astrophysics, who confirmed that, based on the data, that there was no other option than to conclude that the discovery was indeed a black hole binary.

The study confirms the presence of VFTS 243, a large but quiet stellar-mass black hole within a binary system in a nearby galaxy. This is the first dormant stellar-mass binary unambiguously detected outside of our own Milky Way. The massive x-ray binary, found in the Tarantula Nebula of the Large Magellanic Cloud, consists of a star 25 times larger than our own Sun in orbit with an initially unidentified companion.

Black holes are formed as stars eventually reach the end of their life cycle and collapse. Most detectable stellar black holes exist in an x-ray binary system, where the black hole pulls gas and material from its accompanying star.

Astronomers usually rely on the x-ray radiation generated from this pull to identify black holes, but authors of the study explain that these binaries are interesting for their relative lack of x-rays. Though x-ray ‘quiet’ binaries like VFTS 243 are considered fairly rare, astronomers do still expect to see them, leading to some of these x-ray dormant binaries being sometimes mistaken for containing a black hole.

Astronomers used six years of data collected from the Very Large Telescope in Chile in collaboration with the European Southern Observatory (ESO) to prove a large O-type blue star and a second object nine times larger than the Sun. The team, however — living up to the nickname ‘black hole police’ — was hardly about to automatically assume that the mystery object was a black hole for certain.

Many of the the prior non-starter black holes debunked by the team turned out to have other explanations.

"What all these "black hole imposters" had in common is that they were very unique, rare types of binaries in which one observes a star that "looks" much heavier than it actually is. The reason for that is that the star lost its outer layers to a companion and thereby lost a lot of mass. Technically, they have a spectral type of a star that would typically weight 5-10 times more than what they actually weight," Shenar explains in an email interview with Courthouse News. "Without a careful analysis, one therefore strongly estimates their mass, and in turn, one overestimates the mass of the companion, and erroneously concludes that the companion must be a black hole. Really making sure that you've found a black hole requires a careful detailed spectral analysis, which most authors simply didn't do."

In contrast, VFTS 243 had no evidence toward being one of these misleading stars, and the companion large star was proved to be rather unremarkable, with no features that might affect observation of its companion.

The team also established that this system truly was different even from previously studied dormant x-ray binaries.

"Finding black holes in binaries such as VFTS 243 provides indispensable information on the collapse process of the massive stars that formed them. There is an ongoing debate as to whether such stars explode as a supernova when they form black holes or directly collapse, and what this might depend on. This has important implications for supernova research, stellar structure and nuclear reactions within it, and importantly: the formation of black-hole mergers detected with gravitational-wave observatories. VFTS 243 tells us that the star that formed its black hole collapsed directly, with no explosion," says Shenar in an email.

It is not only the black hole of the VFTS 243 that interests astronomers. Shenar further explains, "The stellar companion in VFTS 243 is so massive (~25 Msun) that it, too, is expected to collapse into a black hole. And the orbit is tight enough that these black holes will spiral in and merge due to gravitational-wave emission, such like we see with gravitational-wave observatories. There's a huge effort to try and find progenitors of such black-hole pairs, and VFTS 243 is arguably one of the best candidate's we've found so far."

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