Rapidly rotating black hole accreting matter
ESA’s XMM-Newton and NASA’s NuSTAR have detected a rapidly rotating
supermassive black hole in the heart of spiral galaxy NGC 1365. The rate
at which a black hole spins encodes the history of its formation. An
extremely rapid rotation could result from either a steady and uniform
flow of matter spiralling in via an accretion disc (as shown in this
artist impression) or as a result of the merger of two galaxies and
their smaller black holes.
Also depicted in this image is an
outflowing jet of energetic particles, believed to be powered by the
black hole’s spin. The regions near black holes contain compact sources
of high energy X-ray radiation thought, in some scenarios, to originate
from the base of these jets. The nature of the X-ray emission enables
astronomers to see how fast matter is swirling in the inner region of
the disc, and ultimately to measure the black hole's spin rate. Download Hi-Res (955.29 kB)
A rapidly rotating supermassive black hole has been found in the heart
of a spiral galaxy by ESA’s XMM-Newton and NASA’s NuSTAR space
observatories, opening a new window into how galaxies grow.
Supermassive black holes are thought to lurk in the centre of almost all
large galaxies, and scientists believe that the evolution of a galaxy
is inextricably linked with the evolution of its black hole.
How fast a black hole spins is thought to reflect the history of its
formation. In this picture, a black hole that grows steadily, fed by a
uniform flow of matter spiralling in, should end up spinning rapidly.
Rapid rotation could also be the result of two smaller black holes
merging.
On the other hand, a black hole buffeted by small clumps of material
hitting from all directions will end up rotating relatively slowly.
These scenarios mirror the formation of the galaxy itself, since a
fraction of all the matter drawn into the galaxy finds its way into the
black hole. Because of this, astronomers are keen to measure the spin
rates of black holes in the hearts of galaxies.
One way of doing so is to observe X-rays emitted just outside the ‘event
horizon’, the boundary surrounding a black hole beyond which nothing,
including light, can escape.
In particular, hot iron atoms produce a strong signature of X-rays at a
specific energy, which is smeared out by the rotation of the black hole.
The nature of this smearing can then be used to infer the spin rate.
Using this technique, previous observations have suggested there are
extremely rapidly spinning black holes in some galaxies. However,
confirming the spin rate has been very difficult, because the X-ray
spectrum can also be smeared out by absorbing clouds of gas lying close
to the disc. Until now, telling the two scenarios apart has been
impossible.
For roughly 36 hours in July 2012, ESA’s XMM-Newton and NASA’s NuSTAR –
the Nuclear Spectroscopic Telescope Array – simultaneously observed the
spiral galaxy NGC 1365. XMM-Newton captured the lower energy X-rays,
NuSTAR the higher energy data.
The combined data proved to be key to unlocking the puzzle. A spinning
black hole model makes a clear prediction for the ratio of high-energy
to low-energy X-rays. The same is true for an absorbing cloud of gas.
But importantly, the predictions are different and the new data agree
only with a rapidly spinning black hole. This suggests that the galaxy
has grown steadily with time, with material streaming uniformly into the
central black hole.
However, astronomers cannot yet rule out a single large event where two
galaxies and their black holes subsequently merged, producing a sudden
acceleration of the resulting supermassive black hole.
“But we can completely rule out the absorption model,” says Guido
Risaliti, INAF – Osservatorio Astrofisico di Arcetri, Italy, who led the
investigation.
“Now that we know how to measure black hole spin rates for certain, we
can more confidently use them to infer the evolution of their host
galaxies.”
Measuring black hole spins also provides a new way to test general
relativity. Published in 1915, general relativity is Albert Einstein’s
description of gravity. It predicts effects that are most easily seen in
extremely strong gravitational fields, such as those found near black
holes, and NGC 1365’s black hole is spinning almost as fast as
Einstein's theory of gravity will allow.
“Both physics and astrophysics benefit from this result,’ says Dr
Risaliti, who is already applying the X-ray measurement technique to
different galaxies.
“The result is a great example of the synergy that can be achieved when
complementary space missions are used together. It would have been
impossible to achieve this work without the two spacecraft working in
tandem,” says Norbert Schartel, ESA XMM-Newton project scientist.
Notes for Editors
“A rapidly spinning supermassive black hole at the centre of NGC 1365” by G. Risaliti et al. is published in Nature 28 February 2013; doi:10.1038/nature11938.
The European Space Agency's X-ray Multi-Mirror Mission, XMM-Newton, was
launched in December 1999. It is the biggest scientific satellite to
have been built in Europe and uses over 170 wafer-thin cylindrical
mirrors spread over three high throughput X-ray telescopes. Its mirrors
are among the most powerful ever developed. XMM-Newton's orbit takes it
almost a third of the way to the Moon, allowing for long, uninterrupted
views of celestial objects. The scientific community can apply for
observing time on XMM-Newton on a competitive basis.
NuSTAR is a Small Explorer mission led by the California Institute of
Technology in Pasadena and managed by NASA's Jet Propulsion Laboratory,
also in Pasadena, for NASA's Science Mission Directorate in Washington.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Guido Risaliti
INAF–Osservatorio Astrofisico di Arcetri
Tel: +39 055 2752286
Email: risaliti@arcetri.astro.it
Norbert Schartel
ESA XMM-Newton Project Scientist
Tel: +34 91 8131 184
Email: norbert.schartel@esa.int
