In the Shadow of the Accretion Disk: Higher Resolution Imaging of the Central Parsec in NGC 4261
In the Shadow of the Accretion Disk: Higher Resolution Imaging of the Central Parsec in NGC 4261
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在吸积盘的阴影下：NGC 4261 中央秒差距的高分辨率成像
DOI：
10.1086/320979
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发表时间：
2001
期刊：
影响因子：
--
通讯作者：
D. Meier
中科院分区：
文献类型：
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作者：
Dayton L. Jones;A. Wehrle;G. Piner;D. Meier
吸积盘内部秒差距的物理条件被认为是围绕活动星系核中心黑洞运行的，可以通过对来自射电反喷流的背景辐射的吸收（盘中电离气体的吸收）进行成像来探测。我们报告的高角分辨率VLBI观测附近（~40 Mpc）的射电星系NGC 4261，确认自由自由吸收的无线电发射从一个反喷流的几何薄，近边盘在1.6，4.8，和8.4 GHz。正如预期的那样，吸收的角宽度和深度似乎随着频率的降低而增加。假设内盘倾角和张角与较大半径时相同，我们得到了在盘半径约为0.2 pc时的平均电子密度约为104 cm-3。热气体和磁盘中的磁场之间的压力平衡意味着在0.2 pc的半径处的平均场强为10-4 G。这是迄今为止最接近自由-自由吸收测量的银河系外吸积盘绕着一个质量确定的黑洞运行的条件。如果一个标准的对流为主的吸积流存在于盘中心，那么薄和厚盘区域之间的过渡必须发生在半径小于0.2 pc（4000史瓦西半径）。
The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption (by ionized gas in the disk) of background emission from a radio counterjet. We report high angular resolution VLBI observations of the nearby (~40 Mpc) radio galaxy NGC 4261 that confirm free-free absorption of radio emission from a counterjet by a geometrically thin, nearly edge-on disk at 1.6, 4.8, and 8.4 GHz. The angular width and depth of the absorption appears to increase with decreasing frequency, as expected. We derive an average electron density of ~104 cm-3 at a disk radius of about 0.2 pc, assuming that the inner disk inclination and opening angles are the same as at larger radii. Pressure balance between the thermal gas and the magnetic field in the disk implies an average field strength of 10-4 G at a radius of 0.2 pc. These are the closest in free-free absorption measurements to date of the conditions in an extragalactic accretion disk orbiting a black hole with a well-determined mass. If a standard advection-dominated accretion flow exists in the disk center, then the transition between thin and thick disk regions must occur at a radius less than 0.2 pc (4000 Schwarzschild radii).