Sagittarius A* for 16 years, Gillessen et al. estimated the object's mass at solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009. Reinhard Genzel, team leader of the research, said the study has delivered "what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt." On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sgr A*, according to astronomers. On 13 May 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object. Central black hole In a paper published on October 31, 2018, the discovery of conclusive evidence that Sagittarius A* is a black hole was announced. Using the GRAVITY interferometer and the four telescopes of the Very Large Telescope (VLT) to create a virtual telescope in diameter, astronomers detected clumps of gas moving at about 30% of the speed of light. Emission from highly energetic electrons very close to the black hole was visible as three prominent bright flares. These exactly match theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*. In July 2018, it was reported that S2 orbiting Sgr A* had been recorded at , or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about (approximately 1,400 Schwarzschild radii) from Sgr A*. At that close distance to the black hole, Einstein's theory of general relativity (GR) predicts that S2 would show a discernible gravitational redshift in addition to the usual velocity redshift; the gravitational redshift was detected, in agreement with the GR prediction within the 10 percent measurement precision. Assuming that general relativity is still a valid description of gravity near the event horizon, the Sagittarius A* radio emissions are