S301 Star Reveals Black Hole's Spin

Traveling at 25,000 km/s, or 8% of the speed of light, S301 orbits closer to the galactic center than Uranus does to the Sun. Researchers noted, "We thought this was the closest star to the center of our galaxy, the Milky Way. Turns out [it is S301]."

S301 is an extraordinarily dim object, about 2 billion times fainter than the brightest stars visible in the night sky, yet it orbits closer to the galactic center than Uranus does to the Sun, moving at an incredible speed of 25,000 km/s, or 8% of the speed of light, all while maintaining a stable orbit without being torn apart.

While black holes are defined by their mass, charge, and spin, only the mass of Sgr A* has been directly measured until now, a finding recognized with a Nobel Prize in 2020.

The Gravity Collaboration's discovery of S301 is crucial because it provides the means to directly measure the black hole's spin, which offers vital data about its historical growth and the surrounding environment.

Black holes are not static; they spin, inheriting the angular momentum from the stars that formed them, much like an ice skater speeds up by pulling their arms in as density increases rotation.

For supermassive black holes, spin and mass are the primary indicators of their growth history, with a consistent inflow of gas from one direction tending to increase spin, while chaotic gas accretion results in a lower net spin.

Gas channeled through spiral arms into the black hole may drive faster rotation in galaxies that have not undergone mergers. The team explained, "So, we're talking about pure flat spiral disc galaxies that somehow funnel gas to their black hole, maybe along the spiral arms, always from the same direction."

The prediction is that gas channeled to the black hole, potentially along spiral arms consistently from the same direction, leads to a faster rotation of the central black hole.

The Innermost Stable Circular Orbit (ISCO) is a frequently used metric for measuring the spin of supermassive black holes, where a faster black hole spin allows material to orbit closer to the event horizon before being trapped.

Accretion discs around black holes produce X-ray emissions that are Doppler shifted and smeared by the black hole's strong gravity, but current X-ray observations are indirect and require complex disc models.

In 1918, Joseph Lense and Hans Thirring theorized that spinning objects, in addition to curving space, also drag it, similar to an object spinning in a thick liquid like honey.

This phenomenon, known as frame-dragging or the Lense-Thirring effect, causes satellite orbits around Earth to precess, an effect that was later confirmed by LAGEOS missions.

To accurately measure frame-dragging, scientists must track a specific object orbiting very close to a massive body, where the effect is most pronounced.

Measuring the spin of black holes in distant galaxies is challenging because their accretion discs often appear as single points, prompting scientists to seek bright light signatures from stars torn apart by tidal forces.

The Milky Way's central black hole, Sgr A*, has a mass equivalent to 4.3 million Suns, a finding that earned a Nobel Prize in 2020.

While the star S2 was previously deemed too distant to detect frame-dragging effects, the Gravity Collaboration recently identified S301 as the key to direct spin measurement due to its unprecedented proximity to Sgr A*.

The Gravity Collaboration utilizes the Very Large Telescope Interferometer (VLTI), located in Chile, which combines light from four 8-meter wide telescopes to create a virtual telescope aperture approximately 130 meters wide.

This advanced system delivers a resolution 15 times greater than a single 8-meter telescope, enabling precise measurement of light arrival times to map stellar positions within a tiny 0.02-degree square patch at the galactic center.

The Gravity Collaboration's best model indicates S301 completes an orbit around the black hole every 8.7 years, reaching a maximum speed of 25,000 km/s (8% of the speed of light), and at its closest approach, it is 1.78 billion km from the black hole, slightly further than Saturn's distance from the Sun.

The close proximity of S301 to the black hole subjects it to frame dragging, a phenomenon essential for measuring the black hole's spin, but scientists need to observe how the star's orbit precesses over time.

A proper measurement of the Lense-Thirring frame dragging effect requires data from two full orbits of S301, which researchers estimate will take another 10 to 15 years to collect, combining VLTI data with future observations from the 39-meter Extremely Large Telescope (ELT).

Current observations have only covered half of S301's orbit, and accurate measurement requires monitoring the star for two complete orbits, a data collection process expected to span another 10 to 15 years.

The Gravity Collaboration will continue monitoring the galactic center with the VLTI and will significantly benefit from the Extremely Large Telescope (ELT) when it comes online in the late 2020s or early 2030s.

The ELT, with its impressive 39-meter wide primary mirror, will offer a much larger light-collecting area than the VLTI, providing increased light sensitivity crucial for discovering fainter stars influenced by frame-dragging effects.

All new data from the VLTI and the future ELT are expected to allow the Gravity Collaboration to constrain S301's orbit with enough precision to directly determine the Milky Way's black hole spin for the first time.

This groundbreaking measurement will reveal crucial clues about the history of how the Milky Way's supermassive black hole has grown to become over 4 million times heavier than the Sun.