Astronomers have discovered that the black hole at the heart of the Andromeda galaxy is so weak that it's nearly inexplicable. So is the Milky Way's black hole. But lately the Andromeda black hole, M31, has been exploding with energy.
First of all, being a weak black hole just means that these black holes aren't shooting out a ton of X-rays, which is surprising given that they are surrounded by clouds of gas that presumably they're consuming.
Said Zhiyuan Li of the Harvard-Smithsonian Center for Astrophysics:
The black holes in both Andromeda and the Milky Way are incredibly feeble. These two 'anti-quasars' provide special laboratories for us to study some of the dimmest type of accretion even seen onto a supermassive black hole.
This feeble state is what Andromeda's M31 is usually in. But recently, say astronomers, the black hole has gotten erratic. According to a release from Harvard:
[A] decade-long study by Chandra reveals 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.
After the outburst, M31* entered another relatively dim state, but was almost ten times brighter on average than before 2006. The outburst suggests a relatively high rate of matter falling onto M31* followed by a smaller, but still significant rate.
"We have some ideas about what's happening right around the black hole in Andromeda, but the truth is we still don't really know the details," said Christine Jones, also of the CfA.
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.
Above you can see an optical view of the Andromeda Galaxy, with an inset of images from around M31. On the left is a composite of images from before 2006 (the lights are X-ray emissions), and on the right you can see a new source of X-ray emissions after 2006.
Image: X-ray (NASA/CXC/SAO/Li et al.), Optical (DSS)