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M31: Nearby Black Hole is Feeble and Unpredictable

The large image here shows an optical view, with the Digitized Sky Survey, of the Andromeda Galaxy (M31). The inset shows Chandra X-ray Observatory images of a small region in the center of Andromeda, taken before and after January 2006.

Before 2006, three X-ray sources are clearly visible in the Chandra image, including one faint source close to the center of the image. After 2006, a fourth source, called M31*, appears just below and to the right of the central source, produced by material falling onto the supermassive black hole in M31.

A detailed study of Chandra observations over ten years shows that M31* was in a very dim, or quiet, state before 2006. However, on January 6, 2006, the black hole became more than a hundred times brighter, suggesting an outburst of X-rays. This was the first time such an event had been seen from a supermassive black hole in the nearby local universe.

The outburst suggests a relatively high rate of matter falling onto M31* followed by a smaller, but still significant rate. The overall brightening since 2006 could be caused by M31* capturing winds from an orbiting star or by a gas cloud that spiraled into the black hole. The increase in the rate of material falling towards the black hole is thought to drive an X-ray brightening of a relativistic jet.

The cause of the outburst in 2006 is even less clear, but it could be due to a sudden release of energy, such as magnetic fields in a disk around the black hole that suddenly connect and become more powerful.

Just like the supermassive black hole in the center of the Milky Way, M31* is surprisingly quiet. In fact, Andromeda’s black hole is ten to one hundred thousand times fainter in X-ray light that astronomers might expect given the reservoir of gas around it. The black holes in both Andromeda and the Milky Way provide special laboratories to study the dimmest type of accretion ever seen onto a supermassive black hole.

It’s important to figure out what’s going on here because the accretion of matter onto these black holes is one of the most fundamental processes governing the evolution of galaxies. These results imply that the feeble, but erratic behavior of the black hole in the Milky Way may be typical for present-day supermassive black holes.

Source: Harvard-Smithsonian Center for Astrophysics, Chandra X-ray Observatory