Pulsar and Black Hole / Copyright Ralph Eatough/MPIfR

Black holes recalculated

We know quite a bit about Sagittarius A*(SgrA*), the black hole at the center of our solar system. It is an extremely massive black hole that spins around itself at a very high speed. This information is obtained from the effects it has on its surrounding stars - a finding for which Reinhard Genzel and Andrea Ghez were even awarded the Nobel Prize in 2020. A team from the Universities of Bremen and Bielefeld has now gone one step further and has been able to provide mathematical proof that, with the help of a pulsar, the rotation of a black hole could be calculated much more precisely than was previously the case.

Eva Hackmann and Bilel Ben-Salem from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen have devoted themselves to the idea of using a pulsar as a measuring device for a black hole. A pulsar is a neutron star and thus one of the most compact objects after a black hole: It has approximately the mass of our sun, but compressed to a radius of only about 20 kilometers. Another peculiarity: It emits radio waves with a very regular radiation frequency and rotates -similar to a lighthouse, which emits a recurring cone of light. However, the pulsar rotates up to a dizzying 700 times per second. Using this precise cosmic clock to more accurately determine a black hole is the basis of the two scientists' scientific paper, which appeared in the Monthly Notices of the Royal Astronomical Society (MNRAS) on August 16, 2022.

Mass, angle, rotational velocity

If we were to detect a pulsar in close orbit around the supermassive black hole SgrA* at the center of our Milky Way, effects of the black hole's strong gravity on the pulsar's rays could be observed and thus its characteristics of mass, rotational velocity and angle could be precisely defined. These include the effect of frame dragging, which means that space-time is dragged along by the rotation of the black hole and thus the orientation of the rotation axis can be calculated. The team was able to demonstrate that this effect was overestimated in previous calculations and now offers -based on Einstein theory - a much more accurate way to measure the characteristics of a black hole.

In their work, the authors performed their calculations on the basis of Einstein's general theory of relativity and compared their results with the conclusions of two other well-known papers derived on the basis of the post-Newtonian approximation. The latter is based on Newton's theory of gravity and its enhancement by corrections from general relativity. Eva Hackmann and Bilel Ben-Salem concluded that their method is more precise and reliable, especially in the region of strong gravity near a supermassive black hole, where the post-Newtonian approach becomes vague.
The paper appears at the close of the graduate program "Models of Gravity." The project is a collaboration between the Center for Applied Space Technology and Microgravity (ZARM) at the University of Bremen and the Universities of Oldenburg, Bielefeld, Hannover, Jacobs University and the Niels Bohr Institute in Copenhagen.

Link to the publication: https://academic.oup.com/mnras/article/516/2/1768/6673442

Scientific inquiries:
Eva Hackmann

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