Astronomers Find a Sun-like Star Orbiting a Nearby Black Hole

In 1916, Karl Schwarzchild theorized the existence of black holes as a solution to Einstein’s field equations for his general theory of relativity. In the mid-20th century, astronomers first began detecting black holes using indirect methods, which consisted of observing their effects on surrounding objects and space. Since the 1980s, scientists have studied supermassive black holes (SMBHs), which reside at the centers of the most massive galaxies in the universe. And in April 2019, the Event Horizon Telescope (EHT) collaboration released the first image ever taken of an SMBH.

These observations are an opportunity to test the laws of physics under the most extreme conditions and provide insight into the forces that shaped the universe. According to a recent study, an international research team relied on data from ESA Gaia Observatory to observe a sun-like star with strange orbital properties. Based on the nature of its orbit, the team concluded that it must be part of a binary black hole system. This makes it the closest black hole to our solar system and implies the existence of a sizeable population of dormant black holes in our galaxy.

The research was led by Kareem El-Badry, a Harvard Society Fellow astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CfA) and the Max Planck Institute for Astronomy (MPIA). He was joined by researchers from CfA, MPIA, Caltech, UC Berkely, the Flatiron Institute’s Center for Computational Astrophysics (CCA), the Weizmann Institute of Science, the Observatoire de Paris, MIT’s Kavli Institute for Astrophysics and Space Research, and several universities . The paper describing their findings will be published in the Monthly Bulletins of the Royal Astronomical Society.

As El-Badry explained to Universe Today via email, these observations were part of a broader campaign to identify dormant black holes that are companions to normal stars in the Milky Way. “I’ve been searching for dormant black holes for the past four years using a wide range of datasets and methods,” he said. “My previous attempts have revealed a diverse menagerie of binaries masquerading as black holes, but this was the first time the search has borne fruit.”

For this study, El-Badry and his colleagues relied on data provided by the European Space Agency (ESA) Gaia Observatory. This mission has spent nearly a decade measuring the positions, distances, and proper motions of nearly 1 billion astronomical objects, including stars, planets, comets, asteroids, and galaxies. By tracking the movement of objects as they orbit the center of the Milky Way (a technique known as astrometry), the Gaia Mission aims to create the most accurate 3D space catalog ever created.

For their purposes, El-Badry and his colleagues studied all 168,065 stars in Gaia Data Release 3 (GDR3) that appeared to have two-body orbits. Their analysis found one particularly promising candidate, a G-type (yellow star) designated Gaia DR3 4373465352415301632 – for their purposes the team called it Gaia BH1. Based on the observed orbital resolution, El-Badry and his colleagues determined that this star must have a black hole companion. Said El-Badry:

“The Gaia data constrains how the star moves across the sky by tracing an ellipse as it orbits the black hole. The size of the orbit and its period give us a constraint on the mass of its invisible companion – about 10 solar masses. To confirm that the Gaia solution is correct and to rule out non-black hole alternatives, we observed the star spectroscopically with several other telescopes. This tightened our constraints on the companion’s mass and proved that it really is ‘dark’.”

Artist’s rendering of ESA’s Gaia Observatory with the Milky Way in the background. Photo credit: ESA

To confirm their observations, the team analyzed radial velocity measurements of Gaia BH1 from multiple telescopes. These included the WM Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES), the MPG/ESO Telescope’s Fiber-fed Extended Range Optical Spectrograph (FEROS) spectrograph, the Very Large Telescope’s (VLT) X-shooter spectrograph, and the Gemini Multi-Object Spectrographs (GMOS), the Magellan Echellette (MagE) spectrograph, and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).

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Similar to the hunt for exoplanets (Doppler spectroscopy), the spectra provided by these instruments allowed the team to observe and measure the gravitational forces affecting its orbit. These follow-up observations confirmed Gaia BH1’s orbital detachment and that a companion of about ten solar masses was orbiting with it. As El-Badry suggested, these results could represent the first black hole in the Milky Way that has not been observed due to its X-ray emissions or other energy releases:

“Models predict that the Milky Way contains about 100 million black holes. But we only observed about 20 of them. All of the previous ones we’ve observed are in “X-ray binaries”: the black hole eats away at a companion star and glows brightly in X-rays as that material’s gravitational potential energy is converted into light. But these are just the tip of the iceberg: a far larger population may be lurking, hidden in more distant binaries. The discovery of Gaia BH1 sheds early light on this population.”

If confirmed, these results could mean that there is a robust population of dormant black holes in the Milky Way. This refers to black holes that are not identifiable by bright disks, bursts of radiation, or high-speed jets emanating from their poles (as is often the case with quasars). If these objects are ubiquitous in our galaxy, the implications for stellar and galactic evolution could be profound. However, it’s possible that this particular dormant black hole is an outlier and not indicative of a larger population.

Artist’s impression of the triple system HR 6819, consisting of an inner binary with a star (orbit in blue) and a newly discovered black hole (orbit in red) and another star in a broader orbit (also in blue). Photo credit: ESO

To verify their results, El-Badry and his colleagues look forward to the yet-to-be-decided Gaia Data Release 4 (GDR 4), which will include all data collected during the five-year nominal mission (GDR 4 ). This release will contain the most up-to-date astrometric, photometric, and radial velocity catalogs for all observed stars, binaries, galaxies, and exoplanets. The fifth and final version (GDR 5) will contain data from the nominal and extended mission (the full ten years).

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“Based on the BH companion occurrence rate implied by Gaia BH1, we estimate that the next Gaia data release will enable the discovery of dozens of similar systems,” said El-Badry. “With just one object, it’s difficult to know exactly what it says about the population (it could just be a weirdo, a coincidence). We are excited about the demographic studies we can do with larger samples.”

Further reading: arXiv

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