PAUL VREESWIJK鈥檚 computer screen shows two photographs of the sky. One was
taken in 1993, the other comes from an observatory in South Africa and was taken
just a few seconds ago. Both show hundreds of points of light, but I immediately
spot the new star in the South African image. 鈥淭hat must be it, right?鈥� I say.
At first, Vreeswijk and his colleague Titus Galama are cautious, as the source
is unexpectedly bright. But soon they are convinced: it is a giant fireball on
the other side of the Universe. And I am the first to have spotted it.
For the next ten days, Galama and Vreeswijk鈥檚 cramped and stuffy
graduate-student cubicle in Amsterdam becomes a cosmic crisis centre controlling
giant telescopes on the other side of the world. It is here that the last-minute
decisions on observing strategies are taken. And it is the Dutch researchers鈥�
computers that store the photographs, spectra and other measurements that come
flooding in.
Astronomy used to be a slow science. Generations of patient observers watched
the largely unchanging heavens, pinning down positions, making sketches,
measuring distances. Those days are gone. We have learnt that the Universe is
shaken by titanic but brief explosions called gamma-ray bursts, and to catch
these explosions in the act modern astronomers have to move quickly.
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Gamma-ray bursts were first detected in the 1960s. American spy satellites
were on the lookout for illicit nuclear tests, but what they saw instead were
mysterious flashes of gamma rays from space. These fleeting sources turned out
to be extremely reluctant to yield their secrets. Only in the past couple of
years have astronomers discovered that the gamma rays originate billions of
light years away, and that bursts shine a million times as brightly as the
average galaxy. Most astrophysicists think that bursts are huge fireballs
exploding outwards at nearly the speed of light, created by the birth of a black
hole. But there is a lot missing from this picture, and it still has problems
explaining their tremendous power.
So how do you study something that goes off unexpectedly, releases more
energy than the Sun will in its entire lifetime, and vanishes again within
seconds? The trick is not to focus on the brief flash of gamma rays, but to look
instead at the telltale afterglow emitted by the rapidly cooling fireball.
Afterglows are like footprints that can be studied for days or weeks after the
burglar has left the scene of the crime.
What has made this possible is the X-ray cameras aboard an Italian-Dutch
satellite called BeppoSAX, which can pin down the position of gamma-ray bursts.
Using these pointers from BeppoSAX, Galama and Vreeswijk have been hunting
gamma-ray bursts for over two years. 鈥淥n 28 February 1997, our team spotted the
first optical afterglow,鈥� says Galama
(快猫短视频, 31 May 1997, p 28).
Though weak and fading, it was visible for over a week with the 4.2-metre
William Herschel Telescope on La Palma in the Canary Islands. For the first
time, astronomers learnt that optical afterglows could be seen.
But you have to move fast. So team leader Jan van Paradijs, who is also
affiliated with the University of Alabama in Huntsville, wrote a pile of
鈥渙verride proposals鈥� for telescopes all over the world. The idea was that as
soon as a gamma-ray burst was spotted, one phone call would be enough to make
each telescope stop what it was doing and point at the site of the
explosion.
Most of the proposals were accepted. 鈥淲e have arrangements with telescopes in
Australia, New Zealand, South Africa, Chile, La Palma, Hawaii, Israel, Greece
and even at Kitt Peak in Arizona,鈥� says Galama. 鈥淚t鈥檚 an awful lot.鈥� He doesn鈥檛
move an inch without his mobile phone and a list of the observatory phone
numbers. Day and night, Galama is ready to respond to a gamma-ray burst
alert鈥攎uch to the annoyance of his girlfriend Anna Bellomo, who has
complained that the bursts seem to prefer evenings or weekends. It鈥檚 an
undeniable fact, which Galama has described at conferences as the 鈥淎nna
肠辞谤谤别濒补迟颈辞苍鈥�.
On the morning of Monday 10 May, an extremely powerful burst went off, going
against that trend. GRB 990510 appeared at 0849 Universal Time (or GMT), almost
exactly above the South Pole, in the constellation Chamaeleon. This meant that
it was out of sight for the big telescopes in the northern hemisphere, including
the twin Keck telescopes on Mauna Kea, Hawaii, with their huge 10-metre mirrors.
With access to southern hemisphere telescopes, the Amsterdam group seemed to
have a clear field.
鈥淪hortly after I arrived at the institute on Monday morning, we received the
BeppoSAX alert,鈥� says Vreeswijk. 鈥淯nfortunately, at that time it was cloudy at
Mount Stromlo in Australia, so we had to wait for nightfall in South Africa.鈥�
After a call from Vreeswijk, a small 1-metre instrument at the South African
Astronomical Observatory in Sutherland photographed the burst position. Galama
called me, to invite me to watch the chase.
So, on the evening of 10 May I find myself in Galama鈥檚 cramped quarters.
鈥淒on鈥檛 mind the mess,鈥� he says when he shows me around. The room already looks
as if the astronomers haven鈥檛 left it for weeks. Empty coffee mugs and food
trays are everywhere. Desks are littered with books and preprints. On the wall
is that big, blue Nirvana poster showing a baby swimming underwater. Vreeswijk,
wearing shorts and hiking shoes, offers me a cup of tea. 鈥淭here might even be
some leftover burritos somewhere,鈥� he says.
I鈥檓 lucky. If Australia had had clear skies, the burst鈥檚 optical afterglow
might have been spotted before I arrived. But I鈥檓 there as the Sutherland images
come in, and I identify the afterglow.
鈥淭hat was suspiciously easy,鈥� says Galama. 鈥淲e really have to make sure this
is it before claiming the discovery.鈥� Confirmation comes later that night when
new images arrive from South Africa and from the 2.2-metre telescope at the
European Southern Observatory (ESO) on Cerro La Silla in Chile. By 2.30 am, the
new 鈥渟tar鈥� has faded to a mere 40 per cent of its original brightness, a sure
sign that this is indeed the afterglow of the burst.
That night, Galama and Vreeswijk arrange for observations with Antu, the
first of four 8.2-metre instruments that will comprise ESO鈥檚 Very Large
Telescope (VLT) at Cerro Paranal, also in Chile. The brand-new Antu鈥攊t was
completed only in April鈥攊s ordered to measure the polarisation of the
afterglow鈥檚 light. It will also take the object鈥檚 faint spectrum, from which the
minimum possible distance from Earth to the burst can be calculated. During
their trip through space, light waves are shifted to longer (redder) wavelengths
by the expansion of the Universe. A large red shift implies a long travel time
and therefore a large distance.
鈥淭his is the very first time that our group will obtain a spectrum and a
distance,鈥� says Galama. Reason enough to skip some nights of sleep, and to
survive on peanut butter sandwiches, bananas and herb tea.
It takes two whole days and nights to receive, calibrate and analyse the VLT
data. On 13 May, Galama, Vreeswijk and their colleague Evert Rol publish a red
shift for GRB 990510. It implies a distance of some nine billion light
years.
Combined with the observed brightness of the burst, this allows astronomers
to calculate the energy of the explosion. And like other distances for gamma-ray
bursts derived over the past two years, this leaves astrophysicists with a
problem. 鈥淎ssuming that the energy was emitted equally in all directions, we
arrive at 1.4 脳 1053 ergs,鈥� says Galama. That鈥檚 as much energy as you would
generate by taking a star like the Sun and converting a tenth of its mass
instantaneously into radiation. There鈥檚 no known way that can happen.
Instead, theorists have had to assume that gamma-ray bursts emit their
radiation in two opposite beams or jets. Then the total amount of energy needed
to explain the observed brightness is much smaller. It鈥檚 like rowing on the
Thames and listening to a coach on the river bank: if you didn鈥檛 know there was
a megaphone pointing in your direction, you鈥檇 think your coach was shouting.
Fortunately, the new observations support this theoretical fix: a few days
after the burst, the afterglow suddenly starts to fade more rapidly than before.
That would not happen if the emitting fireball were a sphere. At first the
fireball is expanding at nearly the speed of light, and according to the theory
of relativity this means we can only see the small part of the sphere that is
moving almost directly towards us. As the fireball cools, it becomes dimmer, but
it also slows down, and because of the lower speed more of the sphere can be
seen. So the apparent brightness falls only slowly.
Star beams
If the observed radiation is instead coming from a jet pointed towards Earth,
the same relativistic effects are at play. But after a while, the expansion
speed is low enough for us to see the entire jet. Further cooling and slowing
doesn鈥檛 bring more emitting matter into view, and the apparent brightness starts
to decrease much more rapidly.
So if bursts are beamed, what does that tell us about their origins? Stan
Woosley of the University of California at Santa Cruz believes that bursts
happen when a very massive star suddenly turns into a black hole. And according
to computer simulations by Woosley and his colleague Andrew MacFayden, this
鈥渃ollapsar鈥� should make a beamed explosion: neutrinos emitted by the collapsing
material blast out two cones above the star鈥檚 poles, and then squirt out jets of
high-speed material. Another popular theory could also produce beaming. A binary
pair of neutron stars will gradually spiral together, losing their orbital
energy as they emit gravitational waves. Just before they finally merge into a
black hole, they should form a flattened, rotating spheroid, allowing jets of
ultrahot matter to escape at the poles.
鈥淲oosley鈥檚 collapsar model seems to be favoured by most people,鈥� says Galama.
His own discovery, in April 1998, of an unusual supernova that coincided with a
run-of-the-mill gamma-ray burst, fits nicely into this picture. A nearby
collapsar should look like a supernova with a weird spectrum. But the resulting
beams of radiation seem not to have been directed exactly towards Earth,
otherwise the accompanying gamma-ray burst would have been much brighter.
Even if the collapsar theory becomes established, the merging neutron star
scenario need not be written off. There might be two unrelated populations of
gamma-ray bursts: the short ones, lasting less than a second, resulting from
merging neutron stars, and the long ones (lasting up to a few minutes) from
collapsars. Unfortunately, BeppoSAX can鈥檛 see the short bursts, so astronomers
don鈥檛 know where to look for the optical afterglows.
A week after the May burst, Galama and Vreeswijk are exhausted. 鈥淲e鈥檝e been
working like mad,鈥� says Galama. 鈥淲e鈥檙e drowning in data.鈥� Computers have
crashed, satellite connections to La Silla have been down, and analysing the
avalanche of images from South Africa and Chile is taking much longer than
expected. Worse still, there鈥檚 a deadline looming for Galama. 鈥淎nna and I are
going to marry on Friday. I鈥檓 afraid Wednesday will be my last day here.鈥�
Meanwhile the competition is catching up. A group at Caltech that has
published the red shift of every other burst, and an Italian team from the
University of Milan, are issuing e-mail circulars about their measurements. The
source of the polarisation measurements published by the Milan astronomers is
familiar, and takes Galama by surprise. 鈥淲e didn鈥檛 know they also had an
override arrangement with the VLT,鈥� he says.
It turns out that the Milan and Amsterdam teams arrive at the same
polarisation of a few per cent in the light of GRB 990510. According to Galama
and Vreeswijk, this is the first time that polarisation has been detected in the
afterglow of a gamma-ray burst. The polarisation must be caused by a magnetic
field, and future observations might reveal the magnetic properties of these
mysterious explosions.
Early next year, NASA plans to launch the High Energy Transient Explorer
(HETE) satellite. Astronomers should then get a couple of gamma-ray burst alerts
a week, ten times as many as BeppoSAX provides. HETE will catch the short,
sub-second bursts too. Meanwhile, Danish and American astronomers have proposed
two special-purpose satellites, called Ballerina and Swift, armed with gamma-ray
detectors, X-ray instruments and optical telescopes, to scrutinise gamma-ray
bursts as they happen.
Small, robotic telescopes might also help to solve the gamma-ray burst
mystery. In January, an automated instrument at Los Alamos National Laboratory
in New Mexico called ROTSE (Robotic Optical Transient Search Experiment) snapped
the bright pulse of visible light that accompanied the gamma flash of GRB
990123. Similar systems might find visible-light afterglows originating from
bursts whose tight beams of gamma rays don鈥檛 happen to be pointing our way. That
would tell us how common bursts really are, and how tightly they are beamed.
Since my visit in May, Galama has moved to California to take up a three-year
Fairchild fellowship at Caltech, where he is joining forces with his old
competitors. But he still has to complete the analysis of the GRB 990510
observations, help to write another paper on the burst and finish his doctoral
thesis. 鈥淭here鈥檚 an awful lot of work to do,鈥� he says. Vreeswijk agrees:
鈥淪ecretly, we hope there won鈥檛 be a new gamma-ray burst for a while to
肠辞尘别.鈥�
Even so, on Galama鈥檚 wedding day, Vreeswijk kept his mobile phone switched on
for as long as possible. You never know.