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| Paper IPM / Astronomy / 11737 |
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| Abstract: | |||||||
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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 to those
experiments, we therefore carry out simulations to assess the prospects for detection of the
transient periodic magnification of the companion star, which lasts typically only a few hours
in 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 104 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 Ã 10â4 for spotting the signature of
an existing compact object in a binary system with this technique. Assuming an abundance
of such systems of 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 γ â¼ 4 à 10â7 yrâ1 at
a recurrence period of about 6 months. With microlensing surveys having demonstrated the
capability to monitor about 2 Ã 108 stars, one is therefore provided with the chance to detect
roughly semi-annually recurring self-lensing signals from several compact objects 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.
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