Newly discovered monster black hole so close to the Earth, it is ‘practically in our back yard’

A physics professor at The University of Alabama in Huntsville has discovered a supposed "monster" black hole that is 12 times heavier than the sun.

"It is closer to the sun than any other known black hole, at a distance of 1,550 light years," Dr. Sukanya Chakrabarti, the Pei-Ling Chan Endowed Chair in the Department of Physics at UAH, a part of the University of Alabama System, said in a statement. "So, it's practically in our backyard."

To find the black hole, which is a mammoth task, Chakrabarti and a national team of scientists dug deep into a treasure trove of data comprising nearly 200,000 binary stars - that was released over the summer from the European Space Agency's Gaia satellite mission.

"We searched for objects that were reported to have large companion masses but whose brightness could be attributed to a single visible star," Chakrabarti said. "Thus, you have a good reason to think that the companion is dark."

The Automated Planet Finder in California, Chile's Giant Magellan Telescope, and the W.M. Keck Observatory in Hawaii provided the measurements of the sources that were final contenders.

The research was submitted to the Astrophysical Journal.

Spectroscopic measurements confirmed the Gaia solution

"The pull of the black hole on the visible sun-like star can be determined from these spectroscopic measurements, which give us a line-of-sight velocity due to a Doppler shift," said Chakrabarti.

The professor explained that analyzing the line-of-sight velocities of the visible star, which is similar to our sun, can help explain how enormous the black hole companion is, along with the period of rotation and the orbit.

"These spectroscopic measurements independently confirmed the Gaia solution that also indicated that this binary system is composed of a visible star that is orbiting a very massive object," she said.

The majority of the black holes in binary systems are bright and visible in X-rays due to interaction with the black hole. This mostly happens when the black hole swallows the other star. "As the stuff from the other star falls down this deep gravitational potential well, we can see X-rays," Chakrabarti said.

Closer to the sun than other known black holes

The "monster" black hole discovered is on a long-period orbit of 185 days. "It's pretty far from the visible star and not making any advances toward it," said Chakrabarti.

A million visible stars have gigantic black hole companions in our galaxy. But, Chakrabarti stressed that the Gaia mission largely helped in narrowing down their search.

"There are currently several different routes that have been proposed by theorists, but noninteracting black holes around luminous stars are a very new type of population," Chakrabarti said. "So, it will likely take us some time to understand their demographics, and how they form, and how these channels are different – or if they're similar – to the more well-known population of interacting, merging black holes."

Study abstract:

We describe the discovery of a solar neighborhood (d=474~pc) binary system consisting of a main-sequence sunlike star and a massive non-interacting black hole candidate. We selected this system from the \textit{Gaia} DR3 binary catalog based on its high mass ratio and location close to the main sequence. The spectral energy distribution (SED) of the visible star is well described by a single stellar model, indicating that no contribution from another luminous source is needed to fit the observed photometry. We derive stellar parameters from a high S/N Magellan/MIKE spectrum, classifying the star as a main-sequence star with Teff=5972 K, logg=4.54, and M=0.91~\msun. The spectrum also shows no indication of a second luminous component. We have measured radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over the past three months, which we use to determine the spectroscopic orbit of the binary. We show that the velocity data are consistent with the \textit{Gaia} astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of the astrometric and spectroscopic data, we derive a companion mass of 11.9+2.0−1.6\msun. We conclude that this binary system harbors a massive black hole on an eccentric (e=0.45±0.02), long-period (185.4±0.1 d) orbit. The main-sequence star that orbits this black hole is moderately metal-poor ([Fe/H]=−0.30), on a Galactic orbit similar to thin disk stars. Our conclusions are independent of \cite{ElBadry2022Disc}, who recently reported the discovery of the same system, and find a marginally lower companion mass than we do here.