Probing Black Hole Gravity

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Science  24 Aug 2012:
Vol. 337, Issue 6097, pp. 916-917
DOI: 10.1126/science.1227083

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Black holes (BHs) are among the most amazing predictions of Einstein's general relativity theory. In isolation, BHs are describable simply by their mass and spin rate and are practically invisible—only observed as they feed on surrounding gas, such as when a star gets close enough and is disrupted and ripped apart by the tidal forces of a supermassive BH (SMBH) with the mass of millions to billions of Suns. Such tidal disruption events or flares have been observed only a few times. On page 949 of this issue, Reis et al. (1) report on the discovery of another tidal disruption event (called Sw J1644+57 for the event as measured by Swift, NASA's space-based observatory) by a previously dormant BH. The crucial discovery is that this particular flare has an x-ray light curve with a statistically significant highly coherent oscillation. The frequency of this so-called quasi-periodic oscillation (QPO) is so high, at about 1/200 Hz, for a SMBH that the emission must occur due to some physical process operating close to the BH's event horizon. The implication is that the physical processes take place in the strong-field gravity regime and that these QPO observations might allow one to probe or even test Einstein's general relativity theory.