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Fermi bubbles are burps from a star-eating black hole
Last year, astronomers analysing data from Fermi Gamma Ray Telescope made an extraordinary announcement: two giant bubbles emanating from the centre of the galaxy, stretching some 20,000 light years above and below the galactic plane. Today, scientists from The University of Hong Kong say the bubbles are the remnants of stars that have been eaten by the supermassive black hole at the centre of the galaxy.Our galaxy’s supermassive black hole is huge (4 million times more massive than the Sun), and a star falls into it every 1000 years or so. When this happens, part of the star is devoured by the black hole, while the rest is burped back out into space in the form of high energy protons that heat up the gas and dust surrounding the black hole creating an expanding bubble of high energy electrons.
This cannot expand far in the plane of the galaxy where it is absorbed. But the electrons can travel far into the space above and below the galactic plane, creating the gamma ray bubbles seen by Fermi. This explains why the edge of the bubble is so well defined. But it also explains another of the great puzzles that astronomers are sweating over: the strange energy distribution of cosmic rays.
It’s easy to imagine that higher energy cosmic rays ought to be rarer than lower energy ones. But when astronomers plot the number of cosmic rays against energy, there are far more high energy ones than there ought to be. These form a ‘knee’ in the graph, hence the name of the problem. The new model explains this knee: the extra high energy cosmic rays must be protons created during this star-eating process that have made their way to Earth. So it is the sheer size and energy of the black hole burp that generates the extra high energy protons in the spectrum.
Ref: arxiv.org/abs/1109.6087: “Fermi Bubbles as a Result of Star Capture in the Galactic Center”