Message #24445

[Raven]

A spin measurement of the black hole?

Light curves of the Sgr A* NIR flares in 2002 and 2003, observed with NACO/VLT. The L'-band flare on August 30, 2002, was only partially covered by observations. Gaps in the time series of the H-band flare on May 10, 2003, and of the KS-band flare on June 15, 2003, are due to sky observations and instrument failure, respectively. For comparison, the emission of the steady emission of the star S1 near Sgr A* is shown in all the plots (light grey data points). Arrows in the plots of the two KS-band flares indicate substructure peaks of the flares. Both KS-band flares show very similar quasi-periodicity, although the second flare was observed more than 24 h after the first one and must thus have been an unrelated event. The upper right panel shows the normalised power spectrum of the two KS-band flares. Both of them show a significant peak at a frequency corresponding to time scales of 16.8±2.0 min. In both cases, the power spectrum of S1 does not show such a frequency.

The two K-band flares observed on the 15th and 16th of June 2003 are the flares that were completely covered by observations. Although they happened more than 24 hours apart and thus appear to be unrelated events, they both show a striking quasi-periodicity of the flare with a period of about 17 min. Of all possible periodic processes near a black hole (acoustic modes of a thin disk, Lense-Thirring precession, precession of orbital nodes, orbital motion), the period of matter circling the black hole near the last stable orbit is the shortest one. The observed period of 17 min is so short, however, that the only reasonable explanation is that the oscillations are produced by Doppler boosting of hot gas near the last stable orbit of a spinning (Kerr) black hole. The spin of the black hole will allow for a last stable orbit closer to the event horizon and thus with a shorter orbital frequency. From the observed 17 min period we estimate that the supermassive black hole Sgr A* has a spin that is half as big as the maximum possible spin of such an object. Additional observations of flares and their quasi-periodicity will be needed in order to confirm this result. Should the quasi-periodicity indeed be an intrinsic feature of the flares then this will mean that the era of black hole physics has begun with the properties of black holes accessible to direct measurements!


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last update: 20/11/2006, editor of this page: Thomas Ott

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