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An international team including Lawrence Livermore National Laboratory scientists has definitively measured the spin rate of a supermassive black hole for the first time.
The findings, made by the two X-ray space observatories, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton, solve a long-standing debate about similar measurements in other black holes and will lead to a better understanding of how black holes and galaxies evolve.
The formation of supermassive black holes is thought to mirror the formation of the galaxy itself. Because of this, astronomers are interested in measuring the spin rates of black holes in the hearts of galaxies.
The observations also are a powerful test of Einstein's theory of general relativity, which holds that gravity can bend light and space-time. The X-ray telescopes detected these warping effects in the most extreme of environments, where the immense gravity field of a black hole is severely altering space-time.
NuSTAR is uniquely designed to detect the highest-energy X-ray light in great detail. For Livermore, the predecessor to NuSTAR was a balloon-borne instrument known as HEFT (the High Energy Focusing Telescope). NuSTAR takes HEFT's X-ray focusing abilities and sends them beyond Earth's atmosphere on a satellite. The optics design and the manufacturing process for NuSTAR are based on those used to build the HEFT telescopes.
NuSTAR complements telescopes that observe lower-energy X-ray light, such as the European Space Agency's (ESA's) XMM-Newton and NASA's Chandra X-ray Observatory. Scientists use these and other telescopes to estimate the rates at which black holes spin.
Previous measurements were uncertain because obscuring clouds around the black holes could, in theory, have confused the results. By working together with XMM-Newton, NuSTAR was able to see a broader range of X-ray energy, penetrating deeper into the region around the black hole. The new observations ruled out the idea of obscuring clouds, demonstrating that spin rates of supermassive black holes can be determined conclusively.
NuSTAR and XMM-Newton simultaneously observed the two-million-solar-mass supermassive black hole lying at the dust and gas-filled heart of a galaxy called NGC 1365. The results showed that the black hole is spinning close to the maximal rate allowed by Einstein's theory of gravity.
"These monsters, with masses from millions to billions of times that of the sun, are formed as small seeds in the early universe and then grow by swallowing stars and gas in their host galaxies, and/or merging with other giant black holes when galaxies collide," said Guido Risaliti, lead author of the new study from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and the Italian National Institute for Astrophysics.
"Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy."