2023-11-21 11:48:55
The rotation speed of a black hole is considered to be an important parameter that affects the environment around the black hole. For this reason, it is important to accurately calculate the rotation speed of a black hole. A research team led by Ruth A. Daly of Pennsylvania State University analyzed X-ray and radio observation data of the supermassive black hole Sagittarius A* (Sagittarius A*), which exists at the center of the Milky Way galaxy. The rotation parameter representing the speed was calculated to be 0.90±0.06. This value is close to the upper limit of the theoretical rotation speed of a black hole.[▲ Figure: Image of Sagittarius A* (Credit: EHT Collaboration)]■ “Rotation parameter” that represents the rotation of a black hole Various celestial bodies, including the Earth, undergo rotational motion called rotation. Although their rotational speeds vary, all celestial bodies have physical limits. In the case of planets such as the Earth and stars such as the Sun, the limit of their rotation is that their rotational speed is so high that they break into pieces due to centrifugal force. On the other hand, in the case of a “black hole”, the situation is different from other celestial bodies. A black hole is not a celestial body made up of some kind of object, but is a space-time property defined by the “event horizon,” so it is not possible to think of a limit to its rotational speed in the same way as matter. The event horizon is the boundary surface from which any object or energy that enters inside cannot escape to the outside once more, even at the speed of light, and is the basis of the property of black holes that even light cannot escape. Masu. When we solve the general theory of relativity for a rotating black hole, the event horizon of a black hole that rotates faster than a certain speed disappears. Since black holes exist in space-time inside the event horizon, it is thought that black holes cannot exist under conditions where the event horizon disappears . This is expressed by a numerical value called “rotation parameter”, which is represented by the symbol a_*. A black hole that does not rotate at all has a rotation parameter of 0, while at a limit value where the event horizon disappears, the rotation parameter becomes 1. . In other words, a black hole that can exist has a rotation parameter between 0 and 1. *The hypothesis that there is no black hole whose event horizon has disappeared, that is, a naked singularity, is called the “cosmic censor hypothesis.” However, the Cosmic Censor Hypothesis has neither been proven nor disproved. Many black holes that have been discovered rotate at high speeds with a rotation parameter close to 1, but this is true for “stellar mass black holes” that have a mass several times that of the Sun. The rotational parameters of the “supermassive black holes” that exist at the centers of many galaxies are unknown in many cases. ■Rotational parameters of Sagittarius A* revealed Daly and other research teams attempted to calculate the rotational parameters of Sagittarius A*, a supermassive black hole located at the center of the Milky Way Galaxy. Since it is not possible to directly observe the rotation of the black hole itself, we will calculate the rotation parameters of Sagittarius A* by observing the radiation from the accretion disk, which is the material surrounding the black hole. Daly and colleagues estimated the rotational speed of each accretion disk from the X-ray and radio observation results of Sagittarius A*, and calculated the rotational parameters of Sagittarius A* from these values. As a result, the rotation parameter was 0.90±0.06. This is very close to 1, indicating that Sagittarius A* is rotating at a speed close to the theoretical limit. ■The rotation parameters of supermassive black holes are important for galaxies What can we learn by knowing the rotation parameters of supermassive black holes? For example, in past studies, the rotation parameter of Sagittarius A* was estimated to be a rather small value of 0.44. On the other hand, this is contradictory because it is believed that supermassive black holes tend to increase their rotational speed by sucking in matter around them, that is, their rotational parameter tends to increase. However, in this study, although the research method was different from the study that estimated 0.44, the results were more consistent. Additionally, the environment around a black hole is very different when it is rotating and when it is not rotating. The rotation of a black hole affects things such as radiation from its accretion disk, which in turn affects a larger area, such as the evolution of galaxies. The large rotational parameter of Sagittarius A* may have implications for the environment and evolution of many galaxies thought to contain supermassive black holes. SourceRuth A. Daly, et al. “New black hole spin values for Sagittarius A* obtained with the outflow method”. (Monthly Notices of the Royal Astronomical Society)Brian Koberlein. “The Milky Way’s Black Hole is Spinning as Fast as it Can ”. (Universe Today) Y. Kato, et al. “Measuring spin of a supermassive black hole at the Galactic center — implications for a unique spin”. (Monthly Notices of the Royal Astronomical Society: Letters) Written by Riri Ayaka
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