Andromeda’s black hole wakes up

Since 2006 the supermassive black hole at the heart of the Andromeda Galaxy, M31, has stirred from its slumber and unleashed violent outbursts that have been chronicled by the Chandra X-ray Observatory, the observations from which were presented yesterday at the 216th American Astronomical Society meeting in Miami, Florida.

An optical view of M31 taken by the Digitized Sky Survey, while inset are before and after X-ray images by Chandra of the centre of the Andromeda Galaxy. Before 2006 there were only three X-ray sources visible, all which are believed to by X-ray binaries. Only after the 2006 outburst does M31* become visible, just below and to the right of the central source. Image: NASA/CXC/SAO/Li et al/DSS.

Every large galaxy contains a supermassive black hole, and these black holes grow partly by feeding off interstellar gas and stars that gets too close. There are two levels to this accretion; there is high-rate accretion such as in quasars where vast amounts of material is being consumed, and there is low rate accretion where the material trickles into the maw of the black hole in fits and spurts. It is this low level accretion that M31’s black hole, designated M31*, experiences. In fact, M31* is the second quietest supermasive black hole known, and the quietest happens to be the black hole in our Galaxy, Sagittarius A*. But on 6 January 2006, M31* woke up.

Since late 1999, NASA’s Chandra X-ray Observatory has made 40 observations of the region around M31* with its High Resolution Camera, and 58 measurements with its Advanced CCD Imaging Spectrometer (ACIS). “Before 2006 we could barely see the black hole,” says Zhiyan Li of the Harvard–Smithsonian Center for Astrophysics, who was presenting the findings in Miami. “After 2006 it became very prominent, fluctuating around an average luminosity ten times higher than before.”

Our Galaxy’s black hole also undergoes outbursts, but whereas it has steady and very low background activity, M31* shows a dramatic difference in activity before and after 2006. The activity has continued at a slightly smaller rate into the present day, and on a whole is possibly the result of a star or large cloud of gas being consumed by the black hole. What powered the violence of the initial outburst, however, is still uncertain. “We only have a tentative understanding of this phenomenon,” says Li. “It could be explained by the periodic ejection of relativistic jets of plasma.” These jets would be driven by pent up magnetic energy stored in the vicinity of the black hole, where the magnetic fields are very strong, and Li likens it to the release of a coronal mass ejection on the Sun.

In the early Universe a process called feedback, where conditions around black holes allowed large amounts of radiation to be transported out into the galaxy rather than down into the black hole, heated the interstellar gas and even blew some of it clean out of their galaxies, preventing further star formation. The M31* outburst is one of the first examples of feedback continuing on a very small scale in quiet galaxies that populate the Universe today.