A 17 minute spin? (APOD 11 Dec 2008)
Posted: Thu Dec 11, 2008 4:54 pm
http://apod.nasa.gov/apod/ap081211.html
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A spin measurement of the black hole?
http://www.mpe.mpg.de/www-ir/GC/gc.html
<<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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http://en.wikipedia.org/wiki/Sagittarius_A*
<<Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). Sagittarius A* is most likely to be the location of a supermassive black hole, as is hypothesized to be at the centers of many spiral and elliptical galaxies.
Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry (VLBI). The images obtained have been consistent with the Sagittarius A* radio emissions being associated with the accretion disc and relativistic jets of a supermassive black hole. In September 2008 VLBI directly imaged the edge of the event horizon. The measurement had a resolution with an angular diameter of 37 μas (with an estimated error of +16 & -10). At a 26,000 light-year distance, this yields a diameter of 44 million kilometers. For comparison, the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its closest. Thus, Sagittarius A* is half the diameter of the orbit of Mercury.
Sagittarius A* has a mass estimated at 3.7 million solar masses. Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher than previous estimates. "This density…would rule out most alternatives to a black hole for Sgr A* because other concentrations of matter would have collapsed or evaporated on timescales that are short compared with the age of the Milky Way."
"Given these data only gross deviations in the behaviour of gravity itself from the behaviour predicted by general relativity can invalidate the case for black holes."
However what is seen is not strictly the black hole itself. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself emits only Hawking radiation at a negligible temperature, on the order of 10−14 kelvins.
After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the object's mass at 4.31 million solar masses.>>
------------------------------------------------------
A spin measurement of the black hole?
http://www.mpe.mpg.de/www-ir/GC/gc.html
<<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!>>
------------------------------------------
http://en.wikipedia.org/wiki/Sagittarius_A*
<<Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). Sagittarius A* is most likely to be the location of a supermassive black hole, as is hypothesized to be at the centers of many spiral and elliptical galaxies.
Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry (VLBI). The images obtained have been consistent with the Sagittarius A* radio emissions being associated with the accretion disc and relativistic jets of a supermassive black hole. In September 2008 VLBI directly imaged the edge of the event horizon. The measurement had a resolution with an angular diameter of 37 μas (with an estimated error of +16 & -10). At a 26,000 light-year distance, this yields a diameter of 44 million kilometers. For comparison, the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its closest. Thus, Sagittarius A* is half the diameter of the orbit of Mercury.
Sagittarius A* has a mass estimated at 3.7 million solar masses. Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher than previous estimates. "This density…would rule out most alternatives to a black hole for Sgr A* because other concentrations of matter would have collapsed or evaporated on timescales that are short compared with the age of the Milky Way."
"Given these data only gross deviations in the behaviour of gravity itself from the behaviour predicted by general relativity can invalidate the case for black holes."
However what is seen is not strictly the black hole itself. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself emits only Hawking radiation at a negligible temperature, on the order of 10−14 kelvins.
After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the object's mass at 4.31 million solar masses.>>
