Illustration of a precessing tilted inner disk around a spinning black hole
Credit: ESA/ATG medialab; Ingram et al. 2016

Kind of a Drag

Among the many surprising predictions of Einstein's theory of gravity, one of the most surprising is that spinning massive bodies exert a drag on the spacetime near the body. This unusual effect has been directly measured around the spinning earth by a satellite experiment called Gravity Probe B, even though this distortion is very weak and difficult to measure because of earth's relatively low mass and slow spin. The distortion of spacetime produced by a spinning mass should be much stronger near very massive, compact objects like spinning neutron stars and black holes. In fact, previous studies of high energy X-ray emission produced near the event horizon of a spinning black showed a nearly periodic variation in the emission consistent with the expected distortion of spacetime due to the spin of the black hole. The best evidence yet of this spacetime distortion has recently been obtained by astrophysicists. New joint observations by the XMM-Newton and NuSTAR X-ray observatories monitored the nearly periodic change in X-ray emission in a particular black hole system called H 1743-322. Analysis of the XMM and NuSTAR data showed that observed variations in the emission are caused by the distortion of spacetime near the black hole, which produces a periodic wobble in the rotation axis of the inner accretion disk. This wobble is similar to the way in which friction causes the rotation axis of a spinning top to precess as the top rotates. The image above illustrates how, as the tilted inner disk wobbles near the spinning black hole, X-ray emission from the inner disk periodically illuminates approaching (image a) and receding (image c) parts of the outer disk. Due to the Doppler shift, this variable illumination causes emission produced by iron atoms in the outer disk to show a characteristic back-and-forth motion in energy, as the approaching, then receding, parts of the disk are illuminated. This nearly periodic change in the energy of the iron line was detectd by XMM and NuSTAR. By monitoring higher-energy X-ray emission, the NuSTAR observations showed that the only plausible explanation for the observed change in the energy of the iron emission is the wobble of the inner disk produced by the twisted spacetime near the spinning black hole. An important implication of this analysis is that the spin axis of the black hole is not aligned with the rotation axis of the accretion disk, a puzzle for theorists to understand.
Published: July 18, 2016

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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 25-Jul-2016 09:34:11 EDT