Supermassive Black Hole: Detection of Flares by Gas Accretion
2009. 5. 22


At the center of the Milky Way Galaxy, there is a supermassive black hole, named Sagittarius A star, whose mass is approximately 4 million times that of our sun. Two members of the Kyoto University Department of Astronomy, Dr. Shogo Nishiyama (a JSPS post-doctoral fellow) and Professor Tetsuya Nagata, participated in a collaboration with members of the National Astronomical Observatory of Japan, Nagoya University, Cologne University, and the Institute of Astrophysics of Andalusia, employing the Subaru Telescope to make observations of this black hole. This collaboration succeeded in measuring polarized near infrared light from continuous flare phenomena occurring in the neighborhood of this black hole.

Sagittarius A star is located at the center of the Milky Way. This region is very dense with matter, containing a high concentration of stars, dust and gas. For this reason, extracting information concerning the black hole from the great morass of signals emanating from this region is a very difficult task, which can be accomplished only by use today's largest telescopes. In addition, the dust in this region makes observations with visible light unfeasible, and thus to observe the black hole, we are forced to use other wavelengths, in particular X-rays and infrared light. Furthermore, we must employ a type of technology that allows us to remove the effect of fluctuations in the Earth's atmosphere, which are the cause of the twinkling of stars. The collaborative group mentioned above, using the Subaru Telescope, with its 8.2 m diameter mirror, a CIAO near infrared camera and an A036 adaptive optics system to compensate for atmospheric fluctuations, was able to extract extremely sharp images of Sagittarius A star from its densely crowded environment.

On the evening of May 28th, 2008, three occurrences of flare phenomena were observed. The first was recorded just after the observation was begun. This was a relatively dim flare of long duration. The second flare was the strongest, with an approximately 40 minute duration. The final flare was the shortest, brightening and then dimming over a period of 6.5 minutes. From the fact that this flare lasted 6.5 minutes, we can conclude that it took place within a region of about 10 times the black hole radius (Schwarzschild radius), which is regarded as its near neighborhood.

Comparing the variation in time of the brightness and degree of polarization of the flares with predictions obtained from a black hole accretion disk model, we are able to make a hypothesis regarding their cause. The characteristics of the time dependence of these quantities are very similar to those predicted by the model in the case that a clump of high temperature gas closely orbits the black hole at very high speed. According to the prediction obtained from this model, the main mechanisms determining the time variation of the brightness and degree of polarization of the light coming from the flares are the black hole's gravitational lensing effect and the Doppler effect resulting from the high speed orbiting of the gas. If these predictions are correct, the implication is that the observed flares are caused by relativistic phenomena in the neighborhood of Sagittarius A star.