Using binary gravitational lenses, Stephen Hardy and Mark Walker have shown how we can tell what sort of objects these lenses really are.
By watching carefully some tens of millions of stars near the centre of our Galaxy, astronomers have picked up the brief flashes of light which signify that these distant stars are being magnified by lenses - gravitational lenses - along the way. Nearly a hundred such examples have now been catalogued, but we still know very little about the objects which are responsible for the observed gravitational lensing effects. It seems most likely that the lenses are themselves ordinary, if relatively faint, stars in the central regions of our Galaxy, but this idea currently lacks clear observational support. Stephen Hardy and Mark Walker have suggested a new way in which the nature of the lenses might be pinned down.
Their idea makes use of the small number of cases in which the lens is composed of two objects bound together: a binary system. In these cases the magnification (brightening) of the background star is extraordinarily sensitive to the alignment between the source, the lens and the observer; so much so, in fact, that observers on different continents see quite different degrees of brightening at the same instant. Stephen and Mark showed that this effect ought to be readily measurable, so that a small telescope in Chile, say, could show a lensed source brightening a minute or so before this was observed in Australia.
From existing data we can infer the likely speed of any lens at a given distance from us and, in combination with the overall duration of the lensing event (which is easily found), the extra measurement which can be made on binary lenses should give us enough information to uniquely determine where and what these lenses really are.
A complex quintuple image of the Sun (in H 
emission)
as it would appear if the Sun was gravitationally lensed by a
binary star system.
The undistorted image, with a large filamentary prominence,
is shown in the inset.
Two of the individual images are almost merged in the bottom right
of the figure but they can be distinguished by looking for the
prominence.