The most massive known
black hole in the universe has been discovered, weighing in with the mass of 18 billion Suns. Observing the orbit
of a smaller black hole around this monster
has allowed astronomers to test Einstein's theory of general relativity with stronger gravitational fields than ever
before.
The black hole is about six
times as massive as the previous record holder and in fact weighs as much as a small galaxy. It lurks 3.5
billion light years away, and forms the
heart of a quasar called OJ287. A quasar is an extremely bright object in which matter spiralling into a giant black
hole emits copious amounts of
radiation.
But rather than hosting
just a single colossal black hole, the quasar appears to harbour two - a setup that has allowed astronomers to
accurately 'weigh' the larger
one.
The smaller black hole, which weighs about 100 million
Suns, orbits the larger one on an
oval-shaped path every 12 years. It comes close enough to punch
through the disc of matter
surrounding the larger black hole twice each orbit, causing a pair of outbursts that make OJ287 suddenly
brighten.
General relativity predicts
that the smaller hole's orbit itself should rotate, or precess, over time, so that the point at which it
comes nearest its neighbour
moves around in space - an
effect seen in Mercury's orbit around the Sun, albeit on a smaller scale.Bright outbursts
In the case of OJ287, the
tremendous gravitational field of the larger black hole causes the smaller black hole's orbit to precess at
an incredible 39° each orbit. The
precession changes where and when the smaller hole crashes through the disc surrounding its larger
sibling.
About a dozen of the
resulting bright outbursts have been observed to date, and astronomers led by Mauri Valtonen of Tuorla Observatory
in Finland have analysed them to measure
the precession rate of the smaller hole's orbit. That, along with the period of the orbit, suggests the larger black
hole weighs a record 18 billion
Suns.
A couple of other black
holes have been estimated to be as massive, but their masses are less certain, says Valtonen. That's because
the estimates were based on the speed of
gas clouds around the black holes, and it is not clear whether the clouds are simply passing by the black holes or
actually orbiting them.
But Tod Strohmayer of
NASA's Goddard Space Flight Center in Maryland, US, says he is not convinced that Valtonen's team has really
measured the mass of the large black hole
in OJ287 accurately.
That's because only a
handful of the outbursts have been measured with high precision, making it difficult to determine if the
precession scenario is responsible for the
outbursts. "Obviously, if subsequent timings continue to agree with the model, then that would provide further
support," he told me No limit Just how big can black holes get? Craig Wheeler of the
University of Texas in Austin, US, says it
depends only on how long a black hole has been around and
how fast it has swallowed
matter in order to grow. "There is no theoretical upper limit," he says.
The new research also
tested another prediction of general relativity - that the black holes should spiral towards each other as they
radiate energy away in the form of
gravitational waves, or ripples in space. This radiation affects the
timing of the disc crossings and their
accompanying outbursts.
The most recent outburst
occurred on 13 September 2007, as predicted by general
relativity. "If there was
no orbital decay, the outburst would have been 20 days later than when it actually happened," Valtonen told New
Scientist, adding that
the black holes are on
track to merge within 10,000 years.
Wheeler says the
observations of the outbursts fit closely with the expectations from general relativity. "The fact that you can fit
Einstein's theory ... is telling you that
that's working," he says.
The research was presented
on Wednesday at a meeting of the American Astronomical Society in Austin, Texas,
US.
If you would like to reuse
any content from New Scientist, either in print or online, please contact the syndication department first
for permission. New Scientist does not own
rights to photos, but there are a variety of licensing
options available for use
of articles and graphics we own the copyright to.
