Astronomers find a sun-like star orbiting a nearby black hole

In 1916, Karl Schwarzschild theorized the existence of black holes as a solution to Einstein’s field equations for his theory of general relativity. By the middle of the 20th century, astronomers began to discover black holes for the first time using indirect methods, which consist in observing their effects on surrounding objects and space. Since the 1980s, scientists have studied supermassive black holes (SMBHs), which are at the center of the most massive galaxies in the universe. By April 2019, the Event Horizon Telescope (EHT) collaboration released the first ever image captured from SMBH.

These observations are an opportunity to test the laws of physics under the most extreme conditions, and they also provide insight into the forces that shaped the universe. According to a recent study, an international research team relied on data from the European Space Agency’s Gaia observatory to observe a sun-like star with strange orbital properties. Due to the nature of its orbit, the team concluded that it must be part of a black hole binary system. This makes it the closest black hole to our solar system and means there are a large number of dormant black holes in our galaxy.

The research was led by Karim El-Badri, Harvard Society Fellow in Astrophysics 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 Center for Computational Astrophysics (CCA), the Weizmann Institute of Science, the Paris Observatory, MIT’s Kavli Institute for Astrophysics and Space Research, and multiple universities. The paper describing their findings will be published in Monthly Notices of the Royal Astronomical Society.

As Al-Badri explained to Universe Today via email, these observations were part of a broader campaign to identify black hole companions lurking regular stars in the Milky Way. “I’ve been searching for latent black holes for the past four years using a wide range of datasets and methods,” he said. “My previous attempts have yielded a variety of binaries masquerading as black holes, but this was the first time the research had come to fruition.”

For this study, El-Badri and colleagues relied on data obtained by the European Space Agency (ESA) Gaia Observatory. This mission has spent nearly a decade measuring the appropriate postures, distances, and movements of nearly a billion people astronomical bodies, such as stars, planets, comets, asteroids, and galaxies. By tracking the motion of objects as they orbit around the center of the Milky Way (a technique known as astronomy), the Gaia mission aims to create the most accurate 3D space catalog ever made.

For their purposes, El-Badri and his colleagues examined all 168,065 stars in Gaia Data Release 3 (GDR3) that appear to have dual-body orbits. Their analysis found a particularly promising candidate, a G-type (yellow star) designated Gaia DR3 4373465352415301632 — for their purposes, the team named it Gaia BH1. Based on the observed orbital resolution, Al-Badri and his colleagues determined that this star must have a binary companion of black holes. Said Al-Badri:

“The Gaia data determine how the star moves in the sky, tracking an ellipse as it orbits the black hole. The size and duration of the orbit give us a constraint on the mass of its unseen companion—about 10 solar masses. In order to make sure the Gaia solution is correct and to rule out alternatives other than the black hole, we observed the star.” spectroscopically with many other telescopes. This tightened our limitations on the companion’s mass and proved to be really “dark.”

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

Similar to the method used to hunt exoplanets (Doppler Spectroscopy), the spectra provided by these instruments allowed the team to monitor and measure the gravitational forces affecting its orbit. These follow-up observations confirmed Gaia BH1’s orbital solution and that a companion with roughly ten solar masses was orbiting with it. As Al-Badri pointed out, these findings could form the first black hole in the Milky Way that has not been observed based on X-ray emissions or other energetic emissions:

Models predict that the Milky Way contains about 100 million black holes. But we’ve only observed about 20 of them. All the previous ones we’ve observed are in ‘X-ray binaries’: the black hole is eating a companion star, and it glows brightly in X-rays. Where the gravitational potential energy of this material is transformed into light. But this is only the tip of the iceberg: a much larger population may be hiding, hiding in separate dichotomies on a larger scale. The discovery of Gaia BH1 shines an early light on these populations.”

If these results are confirmed, they could mean the presence of a large number of dormant black holes in the Milky Way. This refers to black holes Inconspicuous from bright disks, bursts of radiation, or hypervelocity 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 is possible that this sleeping black hole is an anomaly and does not indicate the presence of a larger population.

To verify their findings, El-Badri and colleagues are looking forward to the Gaia Data Release 4 (GDR 4), which has yet to be dated, and which will include all data collected during the nominal five-year mission (GDR 4). This release will include the latest astronomical, photometric, and radial velocity catalogs of all observed stars, binaries, galaxies and exoplanets. The fifth and final version (GDR 5) will include data from the nominal and extended (full ten years) mission.

“Based on the incidence of BH concomitant guaranteed by Gaia BH1, we estimated that the upcoming Gaia data release will enable the discovery of dozens of similar systems,” Al-Badri said. “With just one organism, it’s hard to know exactly what that means about the population (it could just be something weird, coincidence). We’re excited about the population demographic studies that we’ll be able to do with larger samples.”