Detecting compact objects such as black holes , white dwarfs , strange ( Quark ) stars and neutron stars by means of their gravitational lensing effect on an observed companion in a binary system has already been suggested almost four decades ago .
However , these predictions were made even before the first observations of gravitational lensing , whereas nowadays gravitational microlensing surveys towards the Galactic bulge yield almost 1000 events per year where one star magnifies the light of a more distant one .
With a specific view on those experiments , we therefore carrry out simulations to assess the prospects for detection of the transient periodic magnification of the companion star , which lasts typically only a few hours binaries involving a main-sequence star .
We find that the effect is practically independent of the distance of the binary system from the observer , but a limit to its detectability is given by the achievability of dense monitoring with the required photometric accuracy .
In sharp contrast to earlier expectations by other authors , we find that main-sequence stars are not substantially less favourable targets to observe this effect than white dwarfs , not only because of a better achievable photometry on the much brighter targets , but even more due to the fact that there are \ga 10 ^ { 4 } times as many objects that can be monitored .
The requirement of an almost edge-on orbit leads to a probability of the order of 3 \times 10 ^ { -4 } for spotting the signature of an existing compact object in a binary system with this technique .
Assuming an abundance of such systems about 0.4 per cent , a high-cadence monitoring every 15 min with 5 per cent photometric accuracy would deliver a signal rate per target star of \gamma \sim 4 \times 10 ^ { -7 } ~ { } \mbox { yr } ^ { -1 } at a recurrence period of about 6 months .
With microlensing surveys having demonstrated the capability to monitor about 2 \times 10 ^ { 8 } stars , one is therefore provided with the chance to detect roughly semi-annually recurring self-lensing signals from several compact compacts in a binary system .
These must not be mistaken for similar signatures that arise from isolated planetary-mass objects that act as gravitational lens on a background star .
If the photometric accuracy was pushed down to 0.3 per cent , 10 times as many signals would become detectable .