科研之友
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摘要
We use a sample of radio-loud active galactic nuclei (AGNs) with estimated central black hole masses to explore AGN jet-formation mechanisms. The jet power of AGNs is estimated from their extended radio luminosity. We find that the jets in several AGNs of this sample are too powerful to be extracted from the standard thin accretion disks or rapidly spinning black holes surrounded by standard thin disks. If advection-dominated accretion flows (ADAFs) are present in these AGNs, their bright optical continuum luminosity cannot be produced by pure ADAFs because of their low accretion rates and low radiation efficiency, unless the ADAFs transit to standard thin disks at some radii R-tr. If this is the case, we find that dimensionless accretion rates (m) over dot = (M) over dot/(M) over dot(Edd) as high as greater than or equal to 0.05, and a transition from ADAFs to standard thin disks at rather small radii, similar to 20GM(bh)/c(2), are required to explain their bright optical continuum emission. We propose that the disk-corona structure is present in at least some AGNs in this sample. The plasmas in the corona are very hot, and the pressure scale height of the corona H-c similar to R. Powerful jets with Q(jet) similar to L-bol (bolometric luminosity) can form via the large-scale magnetic fields created by dynamo processes in the disk corona of some AGNs. The maximal jet power extractable from the corona, Q(jet)(max) less than or equal to 0.6L(c) (L-c is the corona luminosity), is expected in this jet-formation scenario. The statistical results for the sample of AGNs are consistent with the predictions of this scenario. Finally, the possibility that the jet is driven from a super-Keplerian rotating hot layer located between the corona and the cold disk is discussed. We find that, in principle, this layer could also produce a powerful jet with Q(jet) similar to L-bol.
- 出版日期2004-10-1
- 单位中国科学院; 中国科学院上海天文台
