The large image here shows an optical view, with the Digitized Sky Survey, of the Andromeda Galaxy, otherwise known as M31. The inset shows Chandra X-ray Observatory images of a small region in the center of Andromeda. The image on the left shows the sum of 23 images taken with Chandra's High Resolution Camera (HRC) before January 2006 and the image on the right shows the sum of 17 HRC images taken 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 from 1999 to the beginning of 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.
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.