Black holes as telescopes

Bending of light in the presence of a black hole

What can you do with a black hole? A first answer will be like, not much, and you better keep your distance!

It turns out that you can use the black hole in the center of our Galaxy as a giant telescope.

At least this is the idea of Amitai Bin-Nun of the University of Pennsylvania in his publishes paper in Physical Review D, to use the black hole from our very own Galaxy center (called Sgr A*) to observe a lensing effect caused by the black hole on the light of young Wolf-Rayet stars.

The general concept is to observe this young stars which are part of the stellar population in the central parsec of the Milky Way. As the light from them passes across the optic axis (line connecting the lens and the observer) it forms a secondary image.

Forming the secondary image

Until here nothing special, we already observe this secondary images on other astronomical pictures in other galaxies. The twist comes when you relate the properties of this second images with the metric of the black hole, then the brightness of this second image, if observed, can separate the type of black holes and can show how the space and time around our black hole in the center of the Galaxy is warped. Also many modified gravity theories can be tested by the analysis of the lensing effect caused by the Sgr A* on the light from the surrounding stars.

All we have to do is to wait until 2018 when one of the candidate stars will have its secondary image at its peak brightness. Well, maybe not that long, as new experiments planned for the future like MICADO (Multi-AO Imaging Camera for Deep Observations) at the European Extremely Large Telescope experiment will have the resolution to identify other candidates for such lensing effect.

In conclusion, if you would like to test your version of the metric of a black hole, now you can use the black hole in the center of our Galaxy, Sgr A* to do so.