科研之友
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摘要
The detection of a mm/Sub-mm "bump" in Sgr A*'s radio spectrum suggests that at least a portion of its overall emission is produced within a compact accretion disk. This inference is strengthened by observations of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that radiation from this source is produced on progressively smaller spatial scales with increasing frequency, the mm/Sub-mm bump apparently arises within a mere handful of Schwarzschild radii of the black hole. We have found that a small (10-Schwarzschild-radii) magnetized accretion disk can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In addition, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization, while X-ray flares detected from Sgr A* may be induced by a sudden enhancement of accretion through this disk. The hardening of the flare-state X-ray spectrum appears to favor thermal bremsstrahlung as the dominant X-ray emission mechanism during the transient event. This picture predicts correlations among the mm, IR, and X-ray flux densities, that appear to be consistent with recent multi-wavelength observations. Further evidence for such a disk in Sgr A* is provided by its radio variability. Recent monitoring of Sgr A* at cm and mm wavelengths suggests that a spectral break is manifested at 3 mm during cm/Sub-mm flares.
The flat cm spectrum, combined with a weak X-ray flux in the quiescent state, rules out models in which the radio emission is produced by thermal synchrotron process in a bounded plasma. One possibility is that nonthermal particles may be produced when the large scale quasi-spherical inflow circularizes and settles down into the small accretion disk. Dissipation of kinetic energy associated with radial motion may lead to particle acceleration in shocks or via magnetic reconnection. On the other hand, the identification of a 106-day cycle in Sgr A*'s radio variability may signal a precession of the disk around a spinning black hole. The disk's characteristics imply rigid-body rotation, so the long precession period is indicative of a small black-hole spin with a spin parameter a/M around 0.1. It is interesting to note that such a small value of a/M would be favored if the nonthermal portion of Sgr A*'s spectrum is powered by a Blandford-Znajek type of process; in this situation, the observed luminosity would correspond to an outer disk radius of about 30 Schwarzschild radii. This disk structure is consistent with earlier hydrodynamical and recent MHD simulations and is implied by Sgr A*' s mm/Sub-mm spectral and polarimetric characteristics.
For the disk to precess with such a long (106-day) period, the angular momentum flux flowing through it must be sufficiently small that any modulation of the total angular momentum is mostly due to its coupling with the black-hole spin. This requires that the torque exerted on the inner boundary of the disk via magnetic stresses is close to the angular momentum accretion rate associated with the infalling gas. Significant heating at the inner edge of the disk then leaves the gas marginally bounded near the black hole. A strong wind from the central region may ensue and produce a scaled down version of relativistic (possibly magnetized) jets in AGNs.
- 出版日期2003-9
