Two quasiperiodic oscillations from a supermassive black hole
NASA/ESA HST
In 1986, astronomers discovered that x rays streaming from some binary star systems exhibit quasiperiodic oscillations. The first QPOs were found in systems that contain a white dwarf, a neutron star, or a stellar-mass black hole along with a normal, close-by star. Surrounding the compact star is a rotating disk of plasma—an accretion disk—that transports material from the star’s outer envelope and delivers it to the compact object. Although astronomers have yet to reach consensus on the mechanism behind QPOs, they agree that the oscillations arise in the accretion disk.
Two QPO frequency ranges were identified: low (a few millihertz to 30 Hz) and high (100 Hz and above). Because the supermassive black holes in some galaxies also have accretion disks, astronomers expected to see QPOs in those galaxies too, but at lower, scaled-down frequencies because of the systems’ greater bulk. The first galactic QPO was discovered 10 years ago. A second, more tentative galactic QPO was reported in 2016. Now Stefania Carpano of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and Chichuan Jin of China’s National Astronomical Observatories in Beijing have found two QPOs in a galaxy known as XMMU J134736.6+173403.
Finding the QPOs and determining their periods of 23.8 hours and 71.4 hours took a campaign with NASA’s Swift satellite that lasted 106 days and included 29 separate observations. The ratio of the QPOs’ periods, 1:3, is not a numerical coincidence. As plasma spirals through the disk toward the black hole, it oscillates radially in one of several possible resonances with the local, Keplerian rotation. The rate at which a supermassive black hole spins provides clues about how and when the black hole formed. From the resonant QPOs of XMMU J134736.6+173403, it’s possible to derive the spin of its black hole using general relativity, but doing so requires a more accurate determination of the black hole mass than existing observations can provide. (S. Carpano, C. Jin, Mon. Not. R. Astron. Soc., 2018, doi:10.1093/mnras/sty841 .)
