Fermi Bubbles Are Burps from a Star-Eating Black Hole

The following story may be recalled from almost a year ago, if only because of the eye-catching illustration:

NASA’s Fermi Telescope Finds Giant Structure in our Galaxy

NASA’s Fermi Gamma-ray Space Telescope has unveiled a previously unseen structure centered in the Milky Way. The feature spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy.

“What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center,” said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who first recognized the feature. “We don’t fully understand their nature or origin.”

Since gamma rays are the most energetic form of electromagnetic radiation (far more potent than X-rays), the existence of large, though diffuse, clouds of them above and below the plane of our galaxy wasn’t something to be taken lightly.

Now there is a proposed explanation: indigestion from star consumption by the Milky Way’s central black hole:

Fermi Bubbles Are Burps from a Star-Eating Black Hole – Technology Review

Last year, astronomers analysing data from NASA’s orbiting Fermi Gamma Ray Telescope made an extraordinary announcement. They said that Fermi had spotted two giant bubbles emanating from the centre of the galaxy, stretching some 20,000 light years above and below the galactic plane.

These bubbles are clearly some kind of shockwave in which high energy electrons interact with photons, giving up their energy in the form of gamma rays.

But what could have caused such a shockwave, which is many times bigger than astronomers would expect to see from a supernova?

Today, Kwong Sang Chen at The University of Hong Kong and a few pals say think they know. They say the bubbles are the remnants of stars that have been eaten by the supermassive black hole at the centre of the galaxy.

The researchers believe than their model also helps explain a completely different issue: the energy distribution of very high-energy cosmic rays.

The proposed explanation for the gamma-ray bubbles is based on stars falling into the black hole at a rate estimated to be in the range of one star every ten to a hundred thousand years. Stars falling into a black hole don’t go quietly. Although most of the matter disappears into the black hole, large amounts of energy are also released. Space around the black hole is filled with diffuse hydrogen gas. The energy released creates a series of shock waves (every 10,000+ years). The shock waves separate the electrons and protons of the hydrogen atoms and accelerate both types of particles to very high energies.

The high-energy electrons scatter off lower energy photons (ordinary light) and kick the photons up to gamma-ray strength, by the process known as inverse Compton scattering. That explains the size and shape of the observed gamma-ray bubbles.

At the same time, some protons are also accelerated up to energies above 1015 eV (electron-volts) – which is the limit on the energy of cosmic ray protons that can be produced from supernova remnants (such as the Crab Nebula). So the proposed explanation of the gamma-ray bubbles can also explain some ultra-high-energy cosmic rays, which have been another puzzle for a long time.

It seems that a case of a star in the process of falling into a black hole has actually been observed directly, very recently. The event looked much like an ordinary gamma-ray burst, except that the peak emissions lasted much longer than normal.

First time ever: scientists see jets as black hole swallows a star

[T]he authors think that we’re looking down the barrel of a near relativistic jet originating from a black hole, but we can’t be looking at the steady inflow of gas seen in most cases. So they suggest that Swift was looking straight down onto a black hole as a star got close enough to be torn apart and swallowed. This would explain the suddenness of the event’s onset. They suggest that the lack of signal in the visible spectrum is probably a product of some dust sitting between us and Sw 1644+57, which absorbed the event’s output in that area of the spectrum.

The energies involved also let them estimate the acceleration the jets were providing to particles within them, and find that it comes out to be around 1020 electronVolts, which is in the neighborhood of the highest energy cosmic rays observed.

Further reading:

Fermi Bubbles as a Result of Star Capture in the Galactic Center

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