Shredded star yields estimate of a black hole’s spin

Black holes weighing in at millions or billions of solar masses lurk at the center of nearly every galaxy. Studying those invisible behemoths yields information about their formation, the evolution of their host galaxies, and even such fundamental physics as the mass of the hypothetical carrier particle of gravitation (see the article by Jon Miller and Chris Reynolds, Physics Today, August 2007, page 42 ). Although researchers have reliable ways of estimating the mass of supermassive black holes, spin is far more difficult to pin down. The most useful method to date relies on analyzing the radiation scattered by a disk of hot gas swirling around a black hole. Now researchers have added a new technique to the arsenal. For the first time, they estimated the spin of a supermassive black hole by analyzing x rays emitted by the gas from a star it tore apart.

The x-ray signal came from an outburst spotted in November 2014 by the All-Sky Automated Survey for Supernovae. ASASSN-14li was quickly classified as a tidal disruption event, in which a star gets torn apart by tidal forces as it approaches a black hole (see the article by Suvi Gezari, Physics Today, May 2014, page 37 ). Dheeraj Pasham of MIT and colleagues searched for patterns in follow-up x-ray data of the event from the XMM-Newton, Chandra, and Swift space telescopes. All three data sets contained broad peaks in the power spectrum every 131 s, a quasi-periodic oscillation that persisted for at least 450 days following ASASSN-14li’s discovery. The researchers interpret that signal as evidence of stellar debris orbiting perilously close to the event horizon. Because the orbital frequency depends not just on radius and black hole mass but also on black hole spin, the researchers could infer a minimum spin parameter of 0.7, which corresponds to a tangential rotation speed of about half c.

Pasham and his team say they may have been extremely lucky: The surprisingly strong and regular x-ray emission suggests that another object, such as a white dwarf, may have been whirling around the black hole at the same time as the doomed star. But if quasi-periodic patterns in tidal disruption events are commonplace, then astrophysicists will have a valuable new method of measuring supermassive black hole spin. (D. R. Pasham et al., Science, in press, doi:10.1126/science.aar7480 .)