Sunday, April 7, 2013

Black Hole Munches On Orphan Planet

A black hole in galaxy NGC 4845, 47 million light-years away, has ‘awoken’ from 20-30 years of inactivity to consume an object of 14–30 Jupiter masses (a brown dwarf or large gas planet) that strayed too close. The sneaky snack was observed by ESA's Integral space observatory, with follow-up observations from ESA's XMM-Newton, NASA's Swift and Japan's MAXI X-ray monitor on the International Space Station.

Astronomers were observing a different galaxy using Integral when a bright X-ray flare was noticed, coming from a different location in the same field-of-view. The astronomers then used XMM-Newton to confirm the origin confirmed as NGC 4845; a galaxy never before detected at high energies. The emission was traced back from its maximum in January 2011; this was when the galaxy brightened by a factor of a thousand and then subsided.

The astronomers were able to determine the source of the emission by analysing the characteristics of the flare: it came from material surrounding the galaxy’s central black hole while it tore apart and consumed an object with 14-30 Jupiter masses. Though this size range matches brown dwarfs, it is possible the object could have had a lower mass of just a few times that of Jupiter. Recent research suggests there may be many ‘orphan’ free-floating planetary-mass objects in galaxies, ejected from their parent planetary systems by gravitational interactions to wander alone.

The black hole at NGC 4845’s centre is estimated to have a mass around 300,000 times that of the Sun. The way the emission brightened and decayed showed that the orphaned object did not have a quick ‘death’: there was a delay of 2–3 months between the object being disrupted and debris heating up in the vicinity of the black hole. The astronomers estimate that only the external layers of the object were consumed by the black hole, which amounts to about 10% of the object’s total mass. A denser core was left orbiting the black hole.

This feeding event can be seen as a precursor to a similar event expected in the supermassive black hole at the centre of the Milky Way Galaxy. This time it is not likely to be an ‘orphaned’ planet, but a compact cloud of gas amounting to just a few Earth masses; it has been observed spiralling towards the black hole and is thought to meet its doom soon, maybe even this year. Events like these aid astronomers in understanding how different types of objects are torn apart by black holes of various size.

Watch a reconstruction of the object being consumed slowly here: The image is a still from the video.
"Tidal disruption of a super-Jupiter in NGC 4845," by M. Nikolajuk and R. Walter is published in Astronomy & Astrophysics, April 2013.
Image and video credit: ESA

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