快猫短视频

THE PAST few years have seen the discovery of an entirely new kind of beast
living right here in our Galaxy. These objects flicker wildly, sometimes shining
so brightly that they are the most brilliant source of X-rays in the sky. The
radiation they give off shows that within them, matter and antimatter are
colliding. And they emit jets of electrons that appear to be speeding outwards
faster than light. Welcome to the world of the microquasar.

In many ways, microquasars mimic their big cousins, quasars. Quasars proper
are the brightest things in the Universe. They look like brilliant blue stars,
and emit colossal jets of electrons that stretch out into space for millions of
light years. The current theory is that at the heart of every quasar is a black
hole heavier than 10 million Suns. As gas from the surrounding galaxy falls
inwards, it forms a hot disc that swirls around the hole in grand slow motion.
But this is still a crude picture. One of the puzzles we have still to unravel
is why quasars emit those vast jets.

In human terms, quasars are not only very big, they鈥檙e very slow. A lifetime
is too short see a full-size quasar chewing up stars and spitting them out
again. So microquasars aren鈥檛 just interesting in their own right. A microquasar
does all the exciting things a quasar does鈥攂ut it鈥檚 a million times
smaller and a million times faster.

The first microquasar was tracked down because astronomers thought it was a
real quasar. Somewhere near the heart of the Milky Way is an object emitting
gamma rays with precisely the energy produced when an electron and its
antiparticle, a positron, annihilate each other. This is a type of emission that
astronomers expect quasars to produce. So was it coming from a giant black hole
at the centre of our Galaxy鈥攐ur very own, if rather feeble, quasar?

It turned out to be something stranger. In 1992, Felix Mirabel of the Saclay
Research Centre near Paris discovered that this object, which he named the Great
Annihilator, was over 350 light years from the heart of the Milky Way鈥攖oo
far to be a massive black hole. Then, with Luis Rodriguez from the National
Autonomous University of Mexico, Mirabel checked out the object鈥檚 radio
emission. It was straddled by a pair of jets which looked just like those of a
quasar鈥攂ut were a millionth of the size.

One annihilator doesn鈥檛 make a whole new class of astronomical objects. Our
Galaxy is littered with peculiar one-off beasts that defy conventional
astronomical taxonomy. But the story changed a couple of years later, when
Mirabel and Rodriguez tuned in to the radio emissions from another gamma-ray
source, called GRS 1915+105. They saw radio jets erupting from the source at a
fantastic rate, fast enough to travel across the Solar System in just six
hours鈥攁 journey that takes light nearly eight hours. The jets appear to be
travelling 25 per cent faster than light, breaking Einstein鈥檚 speed limit.

But the astronomers had an escape clause. 鈥淭his superluminal motion is an
illusion, caused by the ejecta moving so fast that it nearly catches up with its
own radiation,鈥� Mirabel explains. If the jets are going at nearly light speed,
and we see them almost end-on, then they appear to be moving faster than light.
To make the illusion work, Mirabel and Rodriguez discovered that the jets had to
be moving at 92 per cent of the speed of light. Before this, such ultra-fast
jets had only been seen in quasars.

The combination of a powerful gamma-ray source surrounding a black hole and
high-speed jets made these sources appear like miniature quasars, hence the name
鈥渕icroquasar鈥�. The sources in our Galaxy have companion stars, whose orbits
allow astronomers to weigh the black hole. It typically comes out as around 10
Suns. That鈥檚 the size of black hole that could be created in the explosion of a
very massive star.

Old Faithful

鈥淏lack holes of different masses behave in qualitatively the same way,鈥�
explains Martin Rees, England鈥檚 Astronomer Royal. 鈥淎lmost everything about them
simply scales up or down with the black hole鈥檚 mass.鈥� So if we can understand
how a microquasar works, we can scale it up to comprehend quasars.

The second microquasar to be discovered, GRS 1915+105, has been the most
revealing. In 1996, Steve Eikenberry from MIT discovered smaller outbursts from
this source, repeating every 30 to 40 minutes. The object emits X-rays that
flicker violently for several minutes, then as the X-ray emission suddenly
drops, the source emits a powerful burst of infrared. This behaviour has led to
GRS 1915+105 being given the more memorable name of Old Faithful.

Eikenberry believes that Old Faithful鈥檚 black hole is surrounded by a disc of
hot gas pulled from a luckless companion star, as in all microquasars. The gas
whirls ever closer to the cosmic plughole, heating up until it reaches the
searing temperatures required to produce X-rays. In the black hole鈥檚 intense
gravity this swirling gas is unstable, and the X-rays may flicker because shock
waves rip through the disc.

鈥淭hen the black hole somehow grabs the inner part of the accretion disc,鈥�
Eikenberry continues, 鈥渁nd ejects some of it at 92 per cent of the speed of
light.鈥� In this explosion, electrons are accelerated up to even higher
velocities, and they emit infrared radiation as they whirl around in the
magnetic field.

With the inner accretion disc blown away, the X-ray output drops. But gas
trickles in from the cooler outer disc to set the whole process in motion again.
Half an hour later, Old Faithful erupts once more.

Are these smaller blowouts watered-down versions of the huge eruption that
Mirabel and Rodriguez saw in 1994? 鈥淎t first, I thought they might be just `baby
jets鈥�,鈥� says Eikenberry, 鈥渂ut I鈥檝e turned against the idea now.鈥� He says the
half-hourly outbursts fade much too quickly. Instead, they may be failed jets.
The gas ejected from the disc is not squeezed into a narrow jet, but blows up in
all directions and rapidly dissipates its energy.

To add further spice to the cake, Eikenberry has now reported a third kind of
outburst from Old Faithful. These are small blowouts that occur before there is
any disruption to the accretion disc. His best bet is that they are shock
fronts, either in the accretion disc or moving through the gas in an existing
jet.

鈥淭his is a Rosetta Stone for understanding accretion discs and the formation
of jets,鈥� Eikenberry enthuses, 鈥渂ut we haven鈥檛 decoded the languages yet.鈥� Many
astronomers have tried to develop theories that explain every aspect of Old
Faithful, he adds, but they鈥檝e all been shot down before they鈥檝e even been
submitted for publication. 鈥淚鈥檓 just working on a new model,鈥� he adds, 鈥渨hich is
now in the shooting range stage: I鈥檝e just handed my collaborators the
谤颈蹿濒别!鈥�

And it鈥檚 not only his collaborators who will be making life difficult. A few
months ago, the latest and weirdest microquasar decided to announce its
presence.

On 15 September, Ron Stubbings, an Australian amateur astronomer, discovered
that the star V4641 Sgr had flared to six times its brightness on the previous
night. He e-mailed the news around the world. Astronomers running a NASA
satellite, the Rossi X-ray Timing Explorer, found that the star was producing
X-ray fireworks too. Bob Hjellming of the US National Radio Astronomy
Observatory quickly checked out V4641 at radio wavelengths. 鈥淲e could
immediately see it had structure鈥攊t was big,鈥� he recalls.

The jet was already three times as long as the distance from the Sun to Pluto
and was apparently speeding along 50 per cent faster than light. Familiar with
the superluminal mirage, Hjellming could work out the jet鈥檚 true speed, about 90
per cent of light speed. So V4641 Sgr was a true microquasar. But that鈥檚 not
all.

鈥淭here鈥檚 something fundamentally different about this one,鈥� says Hjellming.
When other microquasars, such as Old Faithful, suffer a major outburst, they
take weeks to fade away. V4641 Sgr faded in just a few hours. And during the
outburst its X-ray emission flickered wildly. Donald Smith from MIT was
astonished to see its X-ray brightness changing by a factor of 500 in a matter
of minutes. 鈥淲e saw the most dramatic rapid X-ray intensity changes ever seen
from one star,鈥� Smith recalls. 鈥淭his behaviour is new. We鈥檝e never seen anything
like it.鈥�

Smith鈥檚 colleague Ron Remillard speculates that the gas in V4641 may be
flowing into the black hole without forming a large disc first. 鈥淥r perhaps the
black hole itself is significantly different from other microquasars, perhaps in
its mass, its spin or its electric charge,鈥� he says.

V4641 Sgr is in a prime position to have its secrets cracked wide open. It is
the nearest known black hole to the Sun, just 1600 light years away. 鈥淏ecause
this system happens to be so close to us,鈥� says Smith, 鈥渋t鈥檚 likely there are
many more objects like V4641 Sgr waiting to be discovered. The rapidly flaring
systems in our Galaxy may have been too faint and too fast for us to notice
迟丑别尘.鈥�

Whipping up a storm

The great ambition of those working on microquasars is to discover how the
jets are produced: what kind of natural particle accelerator can whip electrons
up to a speed approaching that of light, and then confine them in a narrow jet?
Theorists are unanimous in invoking the power of magnetism, but there the
consensus ends.

Eikenberry makes no claim to be a theorist, so he can only watch from the
sidelines. 鈥淭he magnetic field originates in the gases of the accretion disc,
but the question is how it is amplified,鈥� he observes. 鈥淚s there a powerful
dynamo in the disc itself, or does the magnetism thread the black hole and get
its energy from the black hole鈥檚 rotation?鈥�

David Meier of NASA鈥檚 Jet Propulsion Laboratory in California believes that
the disc alone is responsible. He has created the first realistic computer
models of an accretion disc with magnetism built in. When the magnetic field is
low, it gently pushes charged particles away from the disc. But the swirling
disc constantly winds up the magnetic field, as though it is twisting a bunch of
rubber bands. Meier鈥檚 key discovery is that when the field reaches a critical
level, the gentle flow of particles suddenly becomes a gale.

Not everyone agrees with this model. Roger Blandford, just down the road at
Caltech, finds it difficult to see how to extract that much energy from the
gaseous disc.

On the other hand, the black hole has plenty of rotational energy. 鈥淪ome 29
per cent of the energy of a rotating black hole is there for the taking,鈥� says
Blandford, 鈥渁nd can be extracted by a magnetic field threading the region around
the event horizon.鈥� Again, the field is twisted like a set of rubber bands, but
in the extreme environment very close to the black hole, it builds up to far
higher field strengths. The concentrated energy can generate pairs of electrons
and positrons from the vacuum, along with powerful waves of electromagnetic
energy, and feed them all into the jet.

To match observations of microquasar outbursts, Blandford鈥檚 model has to
eject gas from the inner part of the accretion disc. 鈥淭his could be done if the
inner edge of the disc rotates at a different rate to the black hole,鈥� he
explains. 鈥淭hen you get a large EMF [voltage] between them which leads to the
gas being expelled.鈥�

If Blandford is right, then astronomers may be able to measure the spin of
the black holes in microquasars. Theory tells us that we can only ever learn
three things about a black hole: its mass, its spin and its electric charge. No
other property survives the black hole鈥檚 collapse, giving rise to the peculiar
expression 鈥渁 black hole has no hair鈥�. Astronomers can already work out a black
hole鈥檚 mass from its gravitational effects, and all but the smallest black holes
(see 鈥淗oles in One鈥�)
probably have little electric charge. Measuring spin would tie down
the last of the trio.

All this makes microquasars tantalising objects. The three different kinds of
outburst from Old Faithful, all unpredicted, indicate just how little
astronomers understand of the connection between outbursts and jets, let alone
how they relate to the two theories of how to generate the jets. 鈥淪o far we
really haven鈥檛 learned much about these systems,鈥� Rees says. 鈥淲e need to get a
feel for how the X-rays and the radio jets correlate.鈥�

That will require a coordinated campaign to pick up outbursts as they happen,
instead of relying solely on the efforts of dedicated amateurs. NASA scientists
are discussing the possibility of a satellite that would watch the whole sky for
X-ray outbursts from microquasars and alert us as soon as one is detected.

We also need to observe the spectrum of X-rays from the inner part of the
accretion disc as it moves in the black hole鈥檚 gravity. This might show us how
the orbital energy of the gas disc is extracted to produce the brilliant
outbursts we see both in microquasars and in quasars proper. Fortunately, two
new satellites are perfectly placed to fulfil this need: NASA鈥檚 Chandra X-ray
Observatory and the European XMM-Newton mission.

In a few years we will get to the heart of microquasars, and through them
their big cousins. 鈥淲e are now in the position,鈥� says Eikenberry, 鈥渨here we can
see the car, see the exhaust coming out, and hear the engine running. What we鈥檙e
hoping to do next is to get right under the hood.鈥�