How does a black hole bend time?

Gamma rays show: the center of the Milky Way was active even more recently

Cambridge (USA) - The supermassive black hole in the center of our Milky Way galaxy has only recently devoured large amounts of matter and emitted huge jets of matter in the process - possibly a few hundred thousand years ago. This is shown by observations made with the gamma-ray satellite Fermi. For the first time, the measurements show the traces of so-called gamma jets, high-energy gamma radiation from the matter ejected from the black hole, report the astrophysicists in the “Astrophysical Journal”.

Gamma jets

"These weak jets are the afterimage of something that existed millions of years ago," explains Meng Su of the Harvard-Smithsonian Center for Astrophysics in Cambridge, USA. “This is a strong indication of an active galaxy nucleus in the Milky Way in the relatively recent past.” Large galaxies such as the Milky Way harbor black holes in their centers with a mass that is a million to a billion times the mass of our sun. Incoming matter ensures spectacular activity - the galaxy nuclei sometimes shine brighter than the rest of the star system, and huge jets of matter reach far out of the galaxy into space.

In contrast, the center of the Milky Way is currently calm. But in 2010 measurements with the Fermi satellite showed large "gamma bubbles" above and below the galactic center, large bubble-shaped regions that emit high-energy gamma radiation. The astrophysicists interpreted the bubbles as an indication of an earlier activity of the central black hole of the Milky Way. This thesis is now confirmed with the discovery of Su and his colleague Douglas Finkbeiner. The observations of the two researchers show elongated, thin structures - called jets - that protrude 27,000 light years from the galactic center and run through the bubbles. Such jets are known from other galaxies. However, it is the first time that jets could also be detected in the range of high-energy gamma radiation.

The gamma jets are inclined by 15 degrees to the polar axis of the Milky Way. Su and Finkbeiner see this as an indication of an inclination of the accretion disk around the black hole. Incoming matter collects in this disk before it falls into the black hole. “The rotation of the black hole bends the accretion disk,” explains Finkbeiner. Magnetic fields deflect part of the incident matter and accelerate it along the polar axes. Su and Finkbeiner cannot yet say when the center of the Milky Way was last active. “The jets may switch themselves on and off again and again, depending on whether matter falls into the black hole or not,” says Finkbeiner. In order to explain the observed traces of past activity, the researchers estimate that at least an amount of matter equivalent to 10,000 solar masses must have fallen into the black hole