Death from the Skies! (16 page)

The gamma-ray detectors of the time had poor eyesight: early missions simply couldn’t see the direction from which the gamma rays came.

Optical light—the kind we see—has a relatively low energy. Carefully aligned lenses or mirrors inside a telescope bend or reflect the light, bringing it to a focus. This can be used to very accurately measure the position of a source of optical light. Gamma rays, however, are more like bullets zipping around. Changing their paths is much harder, and even today focusing them is beyond our technology.

What this means is that while a gamma ray can be detected and counted, getting a direction from whence it came is very difficult. Only the crudest of directions could be obtained by the Vela satellites (it wasn’t much better than “somewhere over there”
³⁰
). But the direction is
critical
to understanding the object. If the gamma-ray source’s position is known, other telescopes can be trained at that spot on the sky to see what’s what. Then any visible source seen there can be compared to known sources like galaxies or stars listed in existing catalogs. But some degree of precision is required: if the position of the burst can only be nailed down to, say, an area on the sky the same size as the full Moon, there are still thousands or even millions of objects detectable by a big optical telescope.

Eventually, technology started catching up to the problem. In 1991, NASA launched the Compton Gamma Ray Observatory satellite, which had GRB detectors on it. Compton’s ability to get the positions of GRBs was still not great—it could only nail them down to an area on the sky the size of a quarter held at arm’s length—but it was a definite improvement. Over the course of the mission, it detected over 2,700 GRBs. And while the directions were not precise, just getting that sheer number of observations was a huge advance; after enough bursts were detected, patterns began to emerge.

For one thing, that large collection of bursts allowed scientists to determine that there appeared to be two kinds of GRBs: short ones, lasting in general less than two seconds; and long ones, which lasted more than two seconds. Some bursts were even found to emit gamma rays for several minutes. As more GRBs were observed, it was found that the shorter bursts tended to give off higher-energy (“harder”) gamma rays, and the longer bursts had lower-energy (“softer”) gamma rays. While it wasn’t understood why this might be, it was an important clue to their origins.

But the big scientific result from Compton’s observations was perhaps far more important in solving the riddle: it saw GRBs spread out evenly across the entire sky. At first glance this may not seem to help, but in fact it eliminates many possibilities for their origins.

Imagine standing in a field, and insects are buzzing around. If you’re in the center of the field, then you’d expect, on average, to see the same number of insects no matter what direction you look. But if you’re close to the eastern edge of the field, you will see far more insects to the west (looking out across the length of the field) than to the east (looking out over the edge). The number of bugs you see in a given direction tells you something about your placement in the bug swarm (assuming the swarm is relatively random and symmetric).

So the information from Compton—that GRBs were spread randomly across the sky—instantly tells us an important fact: we are in the center of the GRB distribution
in space.

If GRBs were inside our solar system, we’d expect to see more in one direction than another, because we are not in the center of the solar system—the Sun is. We’re offset from the center by nearly a hundred million miles, and you’d expect to see that reflected in the distribution of GRBs. But there is no offset, so they are not coming from objects in our solar system.

But this also means that GRBs are not coming from sources spread around inside our Milky Way Galaxy. Since the Earth is halfway to the edge of the galaxy, GRBs in that case would be seen preferentially toward the center of the galaxy as viewed from Earth. They aren’t, so they are not galactic in origin either.

That doesn’t leave too many options. They could come from stars
very
near the Sun, like only a few light-years away, but not from farther stars, say, more than a few hundred light-years away, because then we’d start seeing more toward the galactic center. The other choice is that GRBs are very,
very
far away, from well outside the galaxy, millions of light-years distant.

 

Neither of these options is terribly palatable either. Stars shouldn’t be able to make such high-energy bursts, and if they were really far away, the intrinsic energy emitted would be ridiculously high.

Still, astronomers staked their claims on either side of this issue, publishing papers furiously and arguing—sometimes also furiously—over it. They even staged a famous debate about it between two accomplished scientists who took different sides of the debate: one defending the idea that they were from nearby stars, the other saying they were coming from the distant reaches of the Universe. But even by the time the debate was held, preparations were under way to get the real answers.

The joint Dutch-Italian satellite BeppoSAX was launched in 1996. While it was not designed specifically to hunt for GRBs, it had that capability. More important, it had on board a revolution waiting to happen: detectors that could actually get a good direction for incoming X-rays (which, like their higher-energy brethren, gamma rays, are difficult to pin down). It also had a wide field of view, which increased the odds of detecting a randomly placed burst, even if the position was not well known at first.

In February 1997, a long GRB was detected by the BeppoSAX monitor. It also happened to lie within the field of view of the X-ray detectors. Observations were made, and then repeated a few days later. Breakthrough! The results were clear—a bright source of X-rays had faded considerably in the interval. Astronomers knew that must be from the fading afterglow of the burst. And better yet, the X-ray detectors were able to get a reasonably good position for the burst, now called GRB 970228 (for the gamma-ray burst seen in 1997 on February 28).

Within a month, the Hubble Space Telescope was pointed at the location of the GRB and the breakthrough got more momentum: a fading glow in
visible
light was detected, and it appeared to be right next to a dim, distant galaxy. This was too close to be a coincidence.

Then, finally, the clincher. In May of that same year, the mammoth ten-meter Keck telescope in Hawaii obtained spectra
³¹
of a GRB afterglow. This allowed astronomers to determine an accurate distance to GRB 970228, and they were astonished to see that it was located a numbing
nine billion light-years away.
That’s more than halfway across the Universe!

Finally, after thirty years, thousands of burst observations, and countless arguments, a major question was answered: bursts were not only far away, they were
very
far away. After this, no one doubted the vast distances to gamma-ray bursts. They were coming from well outside our Milky Way, and in fact close to the visible edge of the Universe.

But that left one problem, vast in its own right: what event could
possibly
generate such incredible energies?

No matter how you slice it, GRBs are, for a short time, the most luminous objects in the Universe, the best bangs since the Big One.

This is no small problem. Imagine a source of light in space: the light it emits will expand as a sphere with the source at the center. As the sphere grows, the light gets spread out, and will appear dimmer to an observer (that’s why lights get fainter with distance). When the distance to the object doubles, the area over which the light spreads out goes up by four times,
³²
so the brightness will dim by four times. If you increase your distance to 10 times farther away, the light will be only one-hundredth (1 percent) as bright, and so on. The brightness of an object therefore decreases
very
rapidly with distance. This presented a serious problem for GRB researchers: from a distance of billions of light-years, the explosion that formed the GRB must be
huge
to be able to be detected at all from Earth. When the numbers were crunched, it didn’t make sense. Even
converting an entire star into energy
using Einstein’s
E = mc
²
(see chapter 2) wouldn’t provide enough energy to fuel the burst, and that is literally the most energy you can get from a star (ignoring the inconvenient fact that there’s no known way to convert an entire star into energy, and certainly not in the span of a few seconds).

But there was still an out. What if the blast
wasn’t
symmetric, expanding equally in all directions? What if it was
beamed
?

If you take a small lightbulb and turn it on, it emits light in all directions, and its apparent brightness fades rapidly with distance. But if you put it in a flashlight, which collects its light and focuses it into a beam, the light appears brighter from farther away.

Astronomers could almost taste the answer to this piece of the GRB puzzle. Instead of a nearly impossibly energetic blast at a colossal distance expanding spherically and fading rapidly,
maybe the explosion was less energetic, but focused into beams.
Beaming would mean only a tiny fraction of the energy would be needed compared to a spherical blast.

The energy of the detonation would still have to be frighteningly huge to be seen clear across the Universe, but not impossibly so. In fact, the energy involved would be similar to that of a supernova. This gave astronomers hope that they could find the Holy Grail of GRB science: the engine that drove this phenomenon.

And of all the objects in the cosmic zoo that astronomers knew of, only one could possibly generate those kinds of forces.

Black holes are notorious for sucking down matter and energy, not spewing them out, so it might seem paradoxical that they could be at the heart of gamma-ray bursts, the brightest objects in the Universe.

But the key to this is gravity. And the key to
that
is how black holes form, so let’s take a step back (a good idea when dealing with black holes) and take a look at this singular event.

In chapter 3, we saw that massive stars explode when they run out of fuel to fuse in their core. The incredibly strong gravity of the core makes it collapse, which sets off a series of events that blows up the star. That description focused mostly on what happens to the outer layers in a supernova, but not what happens to the core itself. But it’s there that the power of the GRB lies.

As the iron core of the incipient supernova collapses, the electrons are rammed into the protons, making neutrons (and emitting neutrinos, the major trigger of the supernova explosion). In a flash the entire core of the star becomes a sea of neutrons with almost no normal matter left. What was once a ball of iron thousands of miles across is now an ultradense neutron star, perhaps ten miles across. It has a mass equal to the Sun, but a density magnified beyond belief: a spoonful of neutron star matter would weigh
a billion tons
! That is somewhat more than the combined mass of every single car in the United States—imagine 200 million cars crushed down to the size of a sugarcube and you’ll start to get an idea of how extreme neutron star matter is.

The neutron star’s incredible mass is supported by a weird quantum mechanical effect called
degeneracy
(see chapter 3). It is similar to electrostatic repulsion—the idea that like charges repel—but instead it’s a property of certain subatomic particles where they resist being squeezed too tightly together. Degeneracy will occur if you try to pack too many electrons together, but it also affects neutral particles like neutrons. It’s an astonishingly strong force, and is able to keep the vast bulk of the core from collapsing further. The collapsing core of the star slams to a halt, and a neutron star is born . . .

. . . most of the time. It turns out that if the mass of the collapsing stellar core exceeds about 2.8 times the Sun’s mass, even neutron degeneracy cannot hold it up. The core’s gravity is too strong, and the core collapse continues. This time there is no force in the Universe strong enough to stop it.

What happens next is so bizarre that it stretches the human mind to its limit to understand. As an object gets smaller, but retains its mass, its gravity gets stronger. As an easy example, if you were to somehow shrink the Earth to half its current diameter while still keeping its mass, the gravity you feel (and therefore your weight) would increase. The smaller the Earth gets, the stronger its gravity.