Annu. Rev. Astron. Astrophys. 1992. 30: 311-358
Copyright © 1992 by . All rights reserved

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Gravitational lenses can be used as probes of the sources that they magnify. In this section, we review what has been learned about quasars and galaxies in this way, and also describe how lensing can sometimes interfere with our attempts to study distant regions of the universe.

6.1 Quasar Emission Regions

As discussed in Section 3.5, gravitational microlensing leads to significant flux variation only if the angular size of the source is smaller than the Einstein radius of the lens. This allows interesting constraints to be set on the sizes of quasar emission regions (Sanitt 1971). Based on the variability observed in Q2237+031, the linear size of the continuum emission region in this source is estimated to be ltapprox 1015 cm - the first direct estimation of the size of a quasar (Wambsganss et al. 1990b, Witt et al. 1991, Webster et al. 1991). Interestingly, this size is not consistent with a blackbody spectrum from a standard accretion disk model of the quasar (Rauch & Blandford 1991) and suggests the presence of a non-thermal component. Future multi-color observations should lead to additional information on the structure of this quasar (Wambsganss & Paczynski 1991). Interesting effects are also possible in the broad line emission, particularly if it arises in compact clouds orbiting the quasar nucleus (Nemiroff 1988b, Schneider & Wambsganss 1990). Differential magnification can cause variation in the line profiles between macroimages and this can, in principle, be used to probe the cloud kinematics.

Since the strongest microlensing events, the so-called high-amplification events (HAE), are due to the source crossing a caustic, either a fold or a cusp, and since the generic structure of caustics is well-understood, it may be possible in favorable cases to invert the light curve to obtain the one-dimensional structure of the source (Kayser et al. 1986, Grieger et al. 1986, 1988, Refsdal 1990, Grieger 1990, cf also Refsdal 1966b). If observations are done simultaneously from Earth and one or more spacecraft in the solar system, then it will be possible to distinguish between microlensing and intrinsic source variability and also to determine the source profile as well as the lens velocity.

6.2 Magnified Images of Galaxies and Radio Sources

In the really spectacular examples of gravitational lensing of resolved sources, such as the blue luminous arcs and radio rings, some parts of the source are magnified by quite large factors. When a successful model is developed, this automatically provides a reconstruction of the source with particularly high resolution information in those parts of the source that lie close to a caustic. The very fact that the arcs are highly magnified allows spectroscopy to be carried out on 27m galaxies, three magnitudes fainter than normal. Several arc redshifts have been thus determined. An even more impressive example is the arc in Abell 2390, where a velocity variation of 378 km s-1 has been measured along the length of the arc (Pello et al. 1991). By combining a knowledge of the velocity width with the central surface brightness, it might be possible to measure the Hubble constant, or more realistically, the evolution of the Tully-Fisher relation (Soucail & Fort 1991).

Occasionally, a star in the source galaxy may come close enough to a caustic to be brightened for a few hours by a huge factor of up to 107 or more (Miralda-Escudé 1991b). Other examples of gravitational lensing give more modest magnifications but may still be useful. VLBI imaging of the core-jet region of a lensed radio quasar (Q0957+561), HST imaging of the fuzz around a lensed optical quasar or its line emitting regions, and the imaging of Lyman-alpha emission regions (Q2016+112) are examples of this. It has been suggested that some examples of superluminal velocities seen in the VLBI jets of extragalactic radio galaxies may be the result of gravitational magnification of more modest velocities (e.g. Chitre & Narlikar 1979), and that rapid variability may be associated with microlensing, if the sources are intrinsically extremely compact (Gopal-Krishna & Subramanian 1991, Subramanian & Gopal-Krishna 1991).

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