The central engine, and unification schemes

QSOs certainly are very bright. But are they unusually bright? Yes. The argument is neat, and based on the variability of QSOs. The luminosity of QSOs varies on a range of time-scales - hours, days, and months, depending on the wavelength at which you observe them. But variations in the X-ray properties tend to have very short time-scales, of order of minutes. Now this implies that the X-rays come from a small region - of the order of light minutes. To come to this conclusion, all we need is to say that the information about the variation has to be able to travel across the object that is producing the emission, and the fastest the information can travel is the speed of light, $c$. Hence the engine is small, of the order of light minutes. Recall that the distance to the Sun is 8 light minutes^11.7. And so we need to find a small engine, size of order a solar system, that can produce as much energy as all the stars in 100 galaxies combined.

The most efficient way we know of for producing energy is by mass accretion onto a compact object^11.8. Recall from the stellar part of this course, that stellar X-ray binaries produce their energies this way. The estimated efficiency (in terms of the rest mass energy $mc^2$ of the fuel converted into radiation) is 10 per cent, which you can compare to the second most efficient way, nuclear fusion, at 0.7 per cent.

Recall also from those lectures that there is a maximum luminosity - the Eddington luminosity - that a spherical object in equilibrium can produce. If the luminosity is higher, than the radiation pressure becomes larger than the gravitational force, and the object can no longer remain bound. Comparing the Eddington luminosity with the QSO's luminosity, gives us a lower limit to the masses involved, of order $10^8\hbox{$M_\odot$}$. Given such a small size, and such a large mass, Donald Lynden-Bell and Martin Rees suggested that accretion onto a massive black hole must be the energy source powering QSOs.

There is a problem though. A black hole is characterised by just two numbers: its mass, and its spin (i.e. its rotational energy). But there is a wide variety of QSOs out there. For example, some have jets, and strong radio sources, others don't. The time variability is by no means regular. How can that be, if there is only two parameters that characterise the engine?

The gas is thought to accrete onto the SMBH by passing through an accretion disk, where it has to loose enough angular momentum to be able to accrete. This introduces a third parameter: the orientation of the disk with respect to the observer. The most popular unification schemes^11.9 suggest that orientation effects determine many of the observed properties of the QSO. But there is by no means a simple answer to understand the wide variety of properties.