In this artists impression of a tidal disruption, an unfortunate star has wandered too close to the supermassive black hole in the right resulting in a tidal disruption. The tidal disruption will produce radio and x-ray emission (shown in blue), and a long stream of debris glowing in optical light, which Gaia can detect. Image credit: NASA/CXC/U. Michigan/J. Miller et al.; Illustration: NASA/CXC/M. Weiss.
Black holes are among the most mysterious and fascinating objects in the Universe. Their gravity is so strong that not even light can escape from their surface. But what are they, and why can they be seen as Gaia Alerts?
What is unique about black holes is not how much matter they contain, but rather how small a volume they are squeezed into. A black hole can have the mass of the Sun, but only have a radius of a few kilometres.
We think that a supermassive black hole lies at the heart of every large galaxy. In the centre of our own galaxy, the Milky Way, is a monster black hole called Sagittarius A*, which is around two million times the mass of the Sun. Using high resolution images taken of the centre of the Milky Way, astronomers have seen stars orbiting around Sagittarius A*. Even though the black hole itself cannot be observed, the orbit of these stars around a single point provides evidence for its presence.
In our own galaxy, the stars close to Sagittarius A* are happily orbiting it, and will continue to do so for many millions of years to come. But sometimes, a star close to a massive black hole will be knocked off its orbit by another passing star. If this happens, and the unlucky star passes too close to the black hole, it can be torn apart by the incredibly strong gravity of the black hole. This rare event is known as a Tidal Disruption Event or TDE.
TDEs are rare, perhaps happening only once every hundred thousand years in a typical galaxy. But if one does happen, it can give rise to a burst of light that Gaia can spot. When a star passes too close to a massive black hole and becomes a TDE, roughly half the matter in the star will fall into the black hole, and half will be flung out into the galaxy. The matter that falls into the back hole will not fall in directly, but will spiral around the black hole beforehand (similar to how water circles around the drain in the sink). As this material spirals around it will emit X-rays, radio waves and visible light, giving rise to a transient which Gaia can see.
Tidal disruption events are important as they are one of the few ways to study supermassive black holes in other galaxies. By looking at tidal disruption events, it is possible to work out the mass of the black hole in the centre of the galaxy, and even whether or not it is spinning.
Along with tidal disruption events where a supermassive black hole swallows a star, black holes can devour clouds of gas that lie in the centres of galaxies. If this happens, a disk of gas will form around the back hole as the material spirals in. This disk will be heated, and emit radiation in the form of light and X-rays. This process will last for many millions of years, although variations in the supply of gas to the black hole will cause the disk to brighten or fade over much shorter timescales.
A supermassive black hole which is currently being fuelled with gas is referred to as an Active Galactic Nucleus. Gaia will spot many of these, as they change brightness over months and years, as the flow of gas varies. The excellent resolution of Gaia is then critical for distinguishing these objects (which can only occur in the centres of galaxies) from supernovae (which will be at a slightly offset position).