Wednesday, November 9, 2011

Flares from a Supermassive Black Hole

Supermassive black holes are known to exist in the cores of many galaxies. On average, each supermassive black hole in a typical galaxy disrupts a passing star once every 10,000 years or so. An event like this generates a spectacularly bright flare that lasts for months as the disrupted star forms an accretion disc around the supermassive black hole. In the Milky Way galaxy, a supermassive black hole named Sagittarius A* sits in its center. Sagittarius A* is estimated to have over 4 million times the mass of the Sun. On a daily basis, Sagittarius A* is observed to emit tiny flares that lasts for only a few hours each time. These flares are billions of times smaller in amplitude when compared to a flare produced by the disruption of a star.

In a recently published paper by Kastytis Zubovas, et al. 2011, it is postulated that the tiny flares produced by Sagittarius A* on a daily basis are caused by the tidal disruption of asteroids rather than stars. There are vastly more asteroids than stars and asteroids are also much smaller than stars. This explains why the flares produced by Sagittarius A* are much more frequent but are much less luminous than a flare generated by the disruption of a star.

The total luminosity of Sagittarius A* in its quiescent state is approximately 300 times the luminosity of the Sun. This remarkably low luminosity is believed to be powered by a very tenuous quasi-spherical accretion flow of gas into the supermassive black hole. The quiescent state of Sagittarius A* is punctured a few times each day by tiny flares. These flares have luminosities ranging from 3 to 100 times greater that the quiescent state of Sagittarius A* in both X-rays and near infrared.

The minimum size of an asteroid that is necessary to produce an observable flare from Sagittarius A* is estimated to be around 10 kilometres. An asteroid gets tidally disrupted in the vicinity of Sagittarius A* when it passes close enough to the supermassive black hole such that the asteroid’s own gravity becomes unable to hold the asteroid together. This causes the asteroid to break up into smaller fragments that are bound by chemical forces rather than by gravity where the maximum size for such a fragment is probably less than 1 kilometre.

In order for an asteroid to be tidally disrupted, it has to come within 1 AU of Sagittarius A* where 1 AU is basically the average distance of the Earth from the Sun. Sagittarius A* is a supermassive black hole and any object orbiting it within 1 AU will be travelling at an immense velocity of over 60,000 kilometres per second. Since Sagittarius A* is surrounded in its immediate vicinity by a very tenuous gaseous accretion flow, the fragments of a tidally disrupted asteroid will get vaporised by friction with the surrounding gas as they plough through at such incredible speeds. The energy released from the vaporisation of these asteroid fragments is sufficient to produce the observable flares from Sagittarius A*. As the tenuous gaseous accretion flow around Sagittarius A* extends beyond 1 AU, an asteroid passing Sagittarius A* beyond 1 AU and remains intact will still have its surface layers vaporised.

The disruption of a planet around Sagittarius A* is expected to occur much less frequently, on the order of one every thousand years or so. Since a planet is much more massive than an asteroid, a flare produced from the vaporised fragments of a tidally disrupted planet is expected to be millions of times more luminous than from an asteroid. In fact, an observed X-ray echo from a giant molecular cloud that is located a few hundred light years away from Sagittarius A* points towards the possibility that a large flare may have been produced from the disruption and subsequent vaporisation of a planet, occurring approximately 300 years ago.