A new study may help resolve the question of how fast the Milky Way's supermassive black hole is spinning. The black hole, called Sagittarius A* (SgrA*), has a mass of approximately 4 million times that of the Sun. The study, using NASA's Chandra X-ray Observatory and the National Science Foundation's Very Large Array (VLA), found that SgrA* is spinning rapidly. This high spin distorts space-time around SgrA* so that it looks like an American football.
This artist's illustration shows a cross-section of the supermassive black hole at the center of the Milky Way and surrounding material. The black sphere in the center represents the black hole's event horizon, the point of no return from which nothing, not even light, can escape. Looking at a rotating black hole from the side, as shown in the figure, the surrounding space-time is shaped like an American football. The yellow-orange material on either side represents gas swirling around the black hole. This material inevitably falls toward the black hole, and once it falls into the football-shaped interior, it passes through the event horizon. Therefore, the area within the shape of the football, outside the event horizon, is depicted as a cavity. The blue spheres represent jets emanating from the poles of the spinning black hole. Image source: NASA/CXC/M.Weiss
This artist's illustration depicts the results of a new study of Sagittarius A* (SgrA*), the supermassive black hole at the center of the Milky Way. The study found that SgrA* is spinning so fast that it is warping spacetime - the three dimensions of time and space - making it look more like a football.
The results were produced by NASA's Chandra X-ray Observatory and the National Science Foundation's Karl G. Jansky Very Large Array (VLA). The team used a new method, using X-ray and radio data, to determine how fast SgrA* is spinning based on the way material flows in and out of the black hole. They found that SgrA*'s rotational angular velocity is about 60% of the maximum possible value, and its angular momentum is about 90% of the maximum possible value.
Black holes have two basic properties: mass (weight) and spin (speed of rotation). Determining either of these values would give scientists a good idea of any black hole and how it behaves. Astronomers have made several estimates of SgrA*'s rotation speed in the past using different techniques, with results ranging from SgrA* not spinning at all to almost spinning at its maximum speed.
New research shows that SgrA* is actually spinning rapidly, which is causing the space-time around it to be squeezed. Shown here is a cross-section of SgrA* and the disk of material rotating around it. The black sphere in the center represents the so-called black hole event horizon, the point of no return from which nothing, not even light, can escape.
As shown in the figure, when viewing a rotating black hole from the side, the surrounding space-time is shaped like a football. The faster the spin, the flatter the football becomes.
The yellow-orange material on either side represents gas swirling around SgrA*. This material will inevitably fall towards the black hole, and once it falls inside the football shape, it will pass through the event horizon. Therefore, the area within the shape of the football, outside the event horizon, is depicted as a cavity. The blue spheres represent jets ejected from the poles of the spinning black hole. Looking down at the black hole from the top along the barrel of the jet, space-time is a circle.
The spin of a black hole can serve as an important source of energy. Rotating supermassive black holes produce collimated outflows such as jets as they extract spin energy, which requires at least some matter near the black hole. Due to the limited fuel around SgrA*, the black hole has been relatively quiet and its jets relatively weak for nearly a thousand years. However, this study shows that this may change if the amount of material near SgrA* increases.
To determine SgrA*'s spin, the authors used an empirically based technique called the "outflow method," which details the relationship between a black hole's spin and its mass, the properties of the material near the black hole, and the outflow properties. The collimated outflow produces radio waves, while the disk of gas surrounding the black hole produces X-ray radiation. Using this method, the researchers combined data from Chandra and the VLA with independent estimates of the black hole's mass from other telescopes to put constraints on the black hole's spin.
A paper describing these results, led by Ruth Daly (Pennsylvania State University), appears in the January 2024 issue of Monthly Notices of the Royal Astronomical Society.
Compiled source: ScitechDaily