![]() The researchers produced the picture with observations from the Event Horizon Telescope, a worldwide network of radio telescopes that link together to form a single Earth-sized virtual instrument. “That is what we’ve measured in our image.” “The cool thing about Sgr A* is that we know its mass with great accuracy so we know exactly what Einstein’s theory of relativity should predict for how big the shadow in the center should be - around 50 micro-arcseconds in angular size or 60 million kilometers across,” Issaoun said. Issaoun also pointed out that the new view further cements Einstein’s theories on gravity and relativity. “These properties, this knowledge of the fundamental properties of the black hole will help us study the astrophysics of the black hole in more detail later on,” Issaoun said. She said the new image reveals some key details about the black hole that were previously unknown, including that one side of the black hole is almost directly facing Earth. NASA Einstein Fellow at the CfA Sara Issaoun worked on observations and imaging for the EHT team and discussed the image at the European press conference in Germany. The more voracious a black hole, the stronger the radiation that erupts from it.“While M87 had one of the biggest black holes in the universe and it launches a jet that pierces its entire galaxy, Sgr A* is giving us a view into the much more standard state of black holes, quiet and quiescent,” said Johnson. When the matter is finally swallowed, a burst of X-rays and gamma rays results. Super-massive black holes are not born so big but, thanks to their tremendous gravitational pull, they grow over time by sucking up the gas and matter around them. The high state or 'activity' of black holes is closely linked to the way in which they grow in size. However, by showing how high-energy radiation is reflected and reprocessed by the cloud, Integral allowed scientists to reconstruct for the first time the hectic past of Sgr A*. The flash was so powerful that the cloud became fluorescent in the X-rays and was even seen with X-ray telescopes before Integral. "We are now seeing an echo from a sort of natural mirror near the galactic centre - the giant cloud Sgr B2 simply reflects gamma rays emitted by Sgr A* in the past," says Revnivtsev. This powerful radiation is absorbed and then re-emitted by the gas in Sgr B2, but this process leaves behind an unmistakable signature. ![]() The team were able to unveil the history of Sgr A* thanks to a cloud of molecular hydrogen gas, called Sgr B2 and located about 350 light-years away from it, which acts as a living record of the hectic black hole's past.īecause of its distance from the black hole, Sgr B2 is only now being exposed to the gamma rays emitted by Sgr A* 350 years ago, during one of its 'high' states. This gamma-ray radiation is a direct consequence of Sgr A*'s past activity, in which gas and matter trapped by the hole's gravity are crushed and heated until they radiate X-rays and gamma rays, just before disappearing below the 'event horizon' - the point of no return from which even light cannot escape. As Revnivtsev explains, "About 350 years ago, the region around Sgr A* was literally swamped in a tide of gamma rays." This result has been obtained by a international team of scientists led by Dr Mikhail Revnivtsev (Space Research Institute, Moscow, Russia, and Max Planck Institute for Astrophysics, Garching, Germany). Data clearly show that it interacted violently with its surroundings, releasing almost a million times as much energy than it does today. However, a new investigation with ESA's gamma-ray observatory Integral has revealed that in the past Sgr A* has been much more active. In spite of its enormous mass of more than a million suns, Sgr A* appears today as a quiet and harmless black hole. Astronomers call it Sgr A* (pronounced 'Sagittarius A star') from its position in the southern constellation Sagittarius, 'the archer'. Our galaxy too, the Milky Way, hosts a super-massive black hole at its centre. Most galaxies harbour a super-massive black hole in their centre, weighing a million or even a thousand million times more than our Sun. Scientists expect that it will become active again in the future. New observations with Integral, ESA's gamma-ray observatory, have now revealed that 350 years ago the black hole was much more active, releasing a million times more energy than at present.
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