How massive is that black hole?

doi:10.1126/science.abk3451

GSTDTAP > 气候变化

DOI	10.1126/science.abk3451
	How massive is that black hole?
	Paulina Lira; Patricia Arevalo
	2021-08-13
发表期刊	Science
出版年	2021
英文摘要	A black hole is a point in space that is a cosmic sink—the gravitational attraction is so strong that not even light can escape. A supermassive black hole (SMBH) is enormous, with a mass on the order of millions to billions of times that of the Sun ([ 1 ][1]). It's not clear how such an entity arises. It could be the result of the merger of smaller black holes or the collapse of either a stellar cluster or large gas clouds. Every large galaxy is thought to contain a SMBH. To understand how they arise, we need to know how massive they are. On page 789 of this issue, Burke et al. ([ 2 ][2]) describe a method to make this determination on the basis of radiation emissions from the accretion disks of SMBHs. The approach also shows a connection between SMBHs and much less massive objects, such as white dwarf stars. As matter is gravitationally attracted (accreted) toward a massive object, such as a star or black hole, it cannot fall toward the central body in a straight line unless it was initially at rest and nothing perturbs its trajectory. Any deviation from a straight path will force the falling material to start orbiting the massive object or swing past it and leave it behind. Gas that settles into an orbit around a massive central object has notable properties: Through viscous interactions of its individual particles, it can slow down the falling speed and dissipate this kinetic energy as well as angular momentum. Viscosity will allow matter to gradually spiral down and feed the central object. This system is called an accretion disk, and it is the mechanism that feeds growing protostars ([ 3 ][3]), stars with close binary companions ([ 4 ][4], [ 5 ][5]), and SMBHs in the center of galaxies ([ 6 ][6], [ 7 ][7]). Through this mechanism, it is believed that SMBHs grew from small “seeds” in the early Universe to the colossi observed today. Accretion disks around SMBHs can be extremely bright because of the high temperatures that are reached as the gas slows down through viscosity. These disks, not much larger than the Solar System, can outshine the whole galaxy tha t hosts them, in the optical, ultraviolet, and x-ray regions of the electromagnetic spectrum. It is therefore at these wavelengths that astronomers can study the accretion processes and determine the properties of the disks and of their central SMBHs. Methods to measure the mass of a SMBH are often indirect, such as scaling relations with large-scale properties of their host galaxies ([ 8 ][8], [ 9 ][9]). Also, methods used now often can only be applied to the nearest objects (about tens of megaparsecs from Earth) ([ 10 ][10]), require very special conditions (such as having very dense disks of molecular gas orbiting the black hole and positioned exactly edge-on as seen from Earth) ([ 11 ][11]), or require large amounts of telescope time ([ 12 ][12]). It also can be difficult to apply these methods to fainter accretion disks. Thus, new methods to determine SMBH masses are very much needed. Burke et al. present a new method to determine the mass of SMBHs by examining the temporal variations of the emission from their accretion disks. Because accretion disks are small and extremely energetic, they are prone to instabilities that change the amount of radiation that they produce in a stochastic manner, a trademark that has been known since the discovery of the so-called quasars (very energetic and distant accreting SMBHs) ([ 13 ][13]). The stochasticity of the variable emission makes it difficult to isolate characteristic time scales of an accretion disk, such as a particular time associated with the orbital periods or any other “beat” signal from the structure of the disk (such as density waves or an orbiting hot spot in the disk that creates a periodic luminosity change). The exact nature of the observed variations is not well understood and is an area of very active research. The method presented by Burke et al. uses the flux variability in radiation emission of 67 well-observed accreting SMBHs to determine the time scale—days, weeks, months, or years—on which the fluctuations become noticeably smaller. They found that this “damping” time scale correlates well with the masses of the SMBHs (that were determined by other means) over an impressive range from 10,000 to 10 billion solar masses. The damping time scale (of the order of several hundreds of days) can only be measured accurately if the lengths of the observations are about 10 times longer than the time scale itself. This requires dedicated monitoring campaigns that track the brightness changes in these sources stretching for many years. This apparently important restriction is at present being overcome by large monitoring surveys, which are an accumulating time series of the luminosity of millions of objects in the sky, making this method applicable on a massive scale. As with the general SMBH flux variability, researchers have been struggling to interpret the meaning of this damping time scale. The findings of Burke et al. indicate that correlation of this time scale with physical properties such as the mass of the SMBH should provide further insight on the nature of both phenomena. As Burke et al. propose, other properties will likely play a role in determining the time scales of the fluctuations, such as the rate at which mass has been transferred to the SMBH by the accretion disk and the SMBH spin, which determines the innermost radius of the disk ([ 14 ][14]). One of the most interesting aspects of the study of Burke et al. is that it extends its findings to much less massive objects, such as white dwarf stars, which emit radiation through a similar accretion disk mechanism and can be regarded as miniature accreting SMBHs. The authors found that stellar black holes and SMBHs follow almost the same damping time scale–mass relation. That the same relation extends through many orders of magnitude suggests that the physics of accretion disks is, at least in some aspects, scalable and proves that the variations that we see are indeed produced intrinsically by the accretion process. 1. [↵][15]Event Horizon Telescope Collaborationet al., Astrophys. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/335868
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Paulina Lira,Patricia Arevalo. How massive is that black hole?[J]. Science,2021.
APA	Paulina Lira,&Patricia Arevalo.(2021).How massive is that black hole?.Science.
MLA	Paulina Lira,et al."How massive is that black hole?".Science (2021).