Lights out: Leicester astronomers detect star falling into a black hole

Posted by mjs76 at Aug 26, 2011 06:17 PM |
For the first time, a massive black hole has been caught in the act of swallowing up a star.
Lights out: Leicester astronomers detect star falling into a black hole

image: NASA/Goddard SFC/Swift

A massive black hole is thought to lie at the centre of every decent-sized galaxy throughout the Universe. These naturally exert a strong influence on the central region of their galaxy, and we expect that stars near to the centre will be slowly drawn into the black hole.

Although the slowly-decaying remnants of such an event have been seen on a few occasions, it is only now that we have seen the initial event that marks this stellar destruction.

What would we expect to happen when a star gets too close to a black hole? The strong gravitational force would both compress and stretch the star, heating it to very high temperatures and blowing it apart. Some part of the gas from the star would quickly spiral down into the black hole, likely forming a spinning disk as it did so (much like water going down a plug-hole), while the rest would join this disk a bit later. This is the standard theory, and is consistent with the few sparse late-time observations so far.

Artist's impression of Swift (image: NASA)

On 28 March 2011, NASA's Swift satellite discovered a new bright hard X-ray source (Swift J1644+57). Unlike the gamma-ray bursts that Swift was designed to study, this source did not fade away fast, but flared up and down, and only faded very slowly.

Observations with other facilities showed that the source was exactly in the centre of a small galaxy, some four billion light-years away. This new source was so bright that it could only come from a new ultra-fast jet directed towards Earth from a newly-formed disk around the previously unseen massive black hole at the centre of the galaxy.

The sudden onset of this new source was due to the rapid formation of a disk immediately following the initial destruction of a star by the black hole. The subsequent rapid variability of the source indicates that the black hole mass, at around 10 million times that of the Sun, is consistent with what is expected for a galaxy of this size.

Jets directed at us from black holes have been seen before, in systems called 'blazars', but these are all very long-lived and are due to a much more massive disk around the black hole. The onset of a new jet has never been seen before.

black hole eats graph540.jpg
image: NASA/Swift/Penn State

These results were published as a letter in the journal Nature on 25 August 2011. Four researchers from our Department of Physics and Astronomy - Drs Kim Page, Julian Osborne, Andy Beardmore and Phil Evans - are co-authors of the paper.

The flaring and decay of the new source, as well as its initial precise position, was measured by the Swift X-ray Telescope. The camera of this telescope was built here in Leicester (with funding from STFC), and we continue to support its operation as well as hosting the UK Swift Science Data Centre (funded by the UK Space Agency). Swift is a US/UK/Italian satellite managed by the NASA Goddard Space Flight Center, and operated by Penn State University.

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