A black hole is so simple (at least in traditional theories) that it can be completely described by just three parameters: its mass, its spin, and its electric charge. Even though it may have formed out of a complex mix of matter and energy, all the other specific details are lost when it collapses to a singular point. Astronomers are working to measure the spins of black hole in active galaxies in order to probe the connections between spin and jet properties.
One method for measuring black hole spin is X-ray spectra, by looking for distortions in the atomic emission line shapes from the very hot gas in the accreting disk of material around the black hole. Effects due to relativity in these extreme environments can broaden and skew intrinsically narrow emission lines into characteristic profiles that depend on the black hole spin value.
CfA astronomers Guido Risaliti, Laura Brenneman, and Martin Elvis, together with their colleagues, used joint observations from the NuSTAR and XMM-NEWTON space missions to examine the time-varying spectral shape of highly excited iron atoms in the nucleus of the galaxy NGC 1365, a well-studied active galaxy about sixty-six million light-years away and known for exhibiting time-variable line profiles. The team obtained four high quality observations of the source, catching it over an unprecedented range of absorption states, including one with very little line-of-sight absorption to the central nucleus. All the observations, despite the range of absorptions, displayed hallmarks of the innermost regions of the accretion flow. There have been disagreements within the community about the reliability of attributing observed line shapes to the black hole spin (rather than to other effects in the nucleus), but this new result not only demonstrates that it is possible, it shows that even single-epoch observations are likely to provide reliable measurements, making the task of studying other such systems more efficient.