Rotating Supermassive Black Holes (3) ...

The following figure illustrates schematically what we believe the central engine of an AGN would look like if we could turn the radiation off and clear away the gas and dust.

The central engine consists of a rotating, supermassive black hole surrounded by a thin accretion disk. The accretion disk is very hot because of collisions in the rapidly swirling gas that it contains. Because it is hot, it radiates strongly in the ultraviolet (see the adjacent box).

Accretion Disk Temperatures

Typical estimates for the temperature of the accretion disk in a billion solar mass black hole are about 10,000 K. By the Wien law, this corresponds to a peak wavelength of 3000 Angstroms, which lies in the UV region of the spectrum. This temperature is lower than the accretion disk temperature for stellar black holes, which radiate more in the X-ray region because they are hotter. Generally, the accretion disk temperature depends inversely on the mass of the black hole so smaller ones have hotter accretion disks.

Accretion Disk Photons
Photons from the accretion disk are responsible for producing much of the continuum observed from AGNs, either directly, or by heating surrounding matter which then re-radiates the energy at longer wavelengths. Photons emitted by the accretion disk also ionize atoms in the nearby clouds of gas where velocities are very high and produce the broad line emission spectrum of the AGN. Finally, they ionize clouds further away from the central engine where velocities are lower and this produces the narrow lines of the emission spectrum. As we have already seen, whether both broad and narrow emission lines are visible to an external observer will depend on the location of the observer relative to the plane of the accretion disk and the torus that may surround it.

Magnetic Field and Jets
A magnetic field cannot be anchored to the black hole itself because the event horizon pinches off any magnetic field lines that cross it. However, the whirling plasma of the accretion disk lies outside the event horizon and it could have a strong magnetic field. The accretion of matter by the rotating black hole can lead to ejection at velocities approaching the speed of light (relativistic velocities) along the poles of the black hole for the portion of the matter that does not cross the event horizon. The rotating magnetic field of the accreting disk can be carried away with the ejected matter, leading to bipolar jets perpendicular to the accretion axis. These jets contain charged particles moving at near light velocity and twisting and spiraling magnetic fields. The magnetic fields probably play major roles in focusing and confining the relativistic jets into the narrow cones that are observed, but the details of how this happens are not well understood. If the jets are not at right angles to our line-of-sight, we refer to the one pointed more toward us as the jet and the other as the counterjet. Because of relativistic beaming effects, the counterjet may be faint and difficult to see.

Synchrotron Emission from the Jets
The charged particles in the jet (primarily electrons since they are most easily accelerated) spiral around the field lines at relativistic velocities. As we discussed in Chapter 5, they will emit synchrotron radiation because of the accelerated spiral motion. The resulting synchrotron spectrum is nonthermal and partially polarized. The nonthermal emission is also strongly focused in the forward direction by the relativistic beaming mechanism that we discussed in conjunction with pulsars in Chapter 22, and fluctuations of the jet in time will also be compressed into shorter apparent periods by relativistic effects. For an observer in the general direction of a jet, these effects will exaggerate both the apparent intensity and the time variation of the nonthermal emission. Thus, we believe that the nonthermal part of the continuum emission observed for AGNs originates largely in the synchrotron radiation produced in the jets (as noted above, the thermal continuum is probably produced in the accretion disk and the surrounding matter that it heats). We therefore refer to the jets produced by the central engine as relativistic synchrotron jets.