Spotting A Black Hole Swallowing a Star

A unique astronomical event - an incredibly bright and long lived burst of gamma rays - was probably the result of a black hole destroying a star, according to research published in the journal Science this week.

On the 28th of March this year, NASA's swift satellite observed a bright flash of gamma rays emanating from a distant galaxy, around 3.8 billion light years away.   Gamma Ray Bursts like this are associated with some of the most energetic and destructive events in our universe, such as the supernova and collapse of a massive star into a black hole or neutron star.

This flash, now known by the catchy name of SW 1644+57, was unusual.  Normal gamma rays bursts are intense, but short lived, and followed by an "after-glow" of lower intensity emission.  The event in March continued for weeks, outlasting any previous burst, and rather than fading away quietly, it was followed by a number of less intense bursts of radiation - almost like an earthquake and its aftershocks.

To determine exactly what caused this unique event, it was vital to determine whereabouts in the galaxy it had occurred.  Andrew Levan and colleagues at Warwick University used subsequent observations taken by the Hubble Space Telescope and Chandra x-ray observatory to confirm that the event had taken place at the heart of the galaxy.

This left only one logical conclusion as to the cause of the event - they had witnessed a star about the size of our Sun being torn apart by a massive black hole.  The enormous gravitational pull of a black hole at least a million times the mass of the Sun would cause the star to initially distort into a banana shape, before being ripped into a ring of material, raining down onto the black hole.

Spinning black holes tend to radiate this energy in two highly columnated beams, one from each pole, and this time, the Earth happened to fall right in the firing line.  Dr Levan now hopes to look at historical data to find evidence of off-axis events, where the column of light is not aimed at the Earth, to find out how rare these extraordinary events really are.