Brighter than a
IN 1973, SCIENTISTS AT LOS ALAMOS ANNOUNCED THE DETECION OF BRIEF, intense bursts of cosmic gamma rays that go off like unimaginably huge flash bulbs. Ever since then, the mystery of these flashes has tantalized, frustrated and fascinated space gurus, proving to be one of space's most difficult mysteries to decipher. Untold millions of dollars have been poured into efforts to figure out the what and where of these strangest of space's denizens. At the very tail end of the twentieth century a breakthrough finally occurred that is helping to solve part of the enduring puzzle.
In one of life's typical little twists, the breakthrough came from a most unexpected source — a small and unheralded Italian-Dutch satellite called BeppoSAX. In February, 1997, by means of a gamma-ray detector bolted on almost as an afterthought, BeppoSAX made a vital chance discovery. Another, more highly-acclaimed detector, the Compton Gamma Ray Observatory, had provided vital data. But it was data that only seemed to deepen the mystery.
In this chapter we'll tell the story of these enigmatic flashes of energy known as gamma ray bursts, and the contribution made by BeppoSAX toward understanding them. Many mysteries still remain, declaring yet again the genius of God.
What is so special about the strange phenomenon of gamma ray bursts, abbreviated as GRBs? Two things, primarily. First, until BeppoSAX's find in 1997, nobody knew whether these blasts of radiation were coming from virtually next door, possibly even from our own solar system, or whether they were coming from the furthest reaches of the universe. Second, the mechanism that might be responsible for producing such flashes of radiation, whether coming from near or far, defies analysis. Although the first puzzle has almost certainly been resolved, the solution of the second remains stubbornly slippery, in spite of the best efforts of space detectives to nail it down.
An answer to the first puzzle is critical to figuring out how powerful these bursts of energy are. A supermarket-bought bar radiator will warm you from a few meters away. What sized radiator would be needed to warm you from the other side of town? Until scientists could determine where the GRBs were coming from, they could not be sure whether they were dealing with a local molehill or a distant mountain, little local upheavals or the biggest noises in creation. They did know that if it was a distant mountain, it was unimaginably energetic.
A profile of bursts—cosmic snowflakes
An outstanding feature of GRBs is their variability. The difference between the tallest and shortest, thinnest and fattest, smartest and dumbest human beings, nowhere near matches the differences between bursts. Bursts are like snowflakes elevated to the cosmic level — no two are anywhere near identical. That fact itself presents severe difficulties in interpreting the phenomenon. Nevertheless, certain features identify GRBs, distinguishing them from any other heavenly phenomenon.
Typically, the signal comes in a flash, or series of flashes, lasting from one to ten seconds, then suddenly stops. Some last for only one hundredth of a second. The longest goes for about sixteen minutes, but about eighty seconds is the "usual" maximum. Thus, the longest burst can be 100,000 times longer than the shortest. This huge variation makes the task of explaining the phenomenon really hard. What kind of generator varies so much?
Bursts that last less than two seconds (called "short" bursts) have relatively more high-energy gamma rays than longer bursts do.
Bursts show great variation in pattern of pulses. Some rise quickly and drop off equally suddenly, with no following flashes. Others flash on and off. Some are bright, some are dim; some vary wildly in brightness, some wax and wane smoothly. None experiences a periodic pulse, though. (A periodic pulse would be one that throbs evenly at a regular tempo, like the bass drum in the marching band.)
Amazingly, bursts show a lower limit of signal strength below which none has been detected, a bit like human beings having a minimum adult size. To grasp the possible significance of this fact, consider how this phenomenon contrasts with signals of visible light coming from distant galaxies. Distant galaxies show no similar, sudden cut-off in degree of brightness. They just appear steadily dimmer the further away they are. The gradient of brightness tapers off gradually with distance. What would you conclude if space doctors were to tell you that the further away galaxies are located the dimmer they become, until suddenly, below some given degree of brightness, no more galaxies were to be seen even though telescopes were powerful enough to pick up fainter images? What can it mean?
GRBs are sprinkled evenly throughout space in all directions from us. That bland statement belies the significance of the fact. When the Compton observatory proved beyond any shadow of a doubt that these bursts come randomly from any direction, most astronomers were shocked because they were sure bursts originated in our own galaxy; uniformly scattered bursts proved otherwise. How? Almost all the radiant matter in our galaxy is located in a flattened disc. Thus, the vast majority of bursts should come from a narrow band of sky. But they came from every possible direction. Even after Compton proved the multi-directionality of bursts, many astronomers found it difficult to accept an origin outside the Milky Way.
The size of the object generating the radiation is very small. Based on the short time in which bursts change their intensity, experts have determined that the source of bursts is often less than forty miles in diameter! Yes, that's kilometers, not light-years.
Those are the facts; theories vastly outnumber the facts. In the mid 1980s there were over forty. (Things began to change in 1997.) Most of them saw neutron stars as the main culprit. The variations in theory centered around the mechanisms that might cause them to blast off huge amounts of high-energy radiation. Other suggestions have included such exotic ideas as exploding black holes, dust grains moving close to the speed of light and the fission of superheavy elements. The richness of proposed models led M.I.T.'s Paul Joss, a researcher of the phenomenon, to declare, "Nature is extraordinarily clever. Certainly she is far more clever than astrophysicists" (Time 15 Aug. 1983). Nature is not clever; her Inventor is.
The vast majority of the theories were based on the assumption that the bursts were occurring close by, probably within our own galaxy. To make any progress at all in figuring out what causes the bursts, it is essential to know for sure whether they are local or distant events. Herein lies a giant headache.
It's a conspiracy!
A couple of important factors have conspired to hinder pinpointing the distance of the source of GRBs. The first has to do with red shift. Red shift is a vital tool in helping to determine the distance of objects in deep space. We need to learn a couple of points about red shift now in order to understand the problem space scientists have faced in determining where GRBs are located, whether near or far.
The first point is that red shift occurs with radiation of all wavelengths, not just with visible light. Thus, radio waves and X rays can be red shifted. So can gamma rays. In these cases, the end result is not red light, but radiation with a longer wavelength than it started with. When you use a flash camera at an outdoor wedding, the light from that bulb will course out into space and churn on for eternity, being red shifted by the expansion of space as it goes. Your wedding flash could eventually become so redshifted that it would be picked up as a radio signal untold billions of years from now.
You may have heard of the very famous background microwave radiation. Scientists have detected a microwave signal throbbing throughout space from all directions. It's everywhere and going in all directions.
If scientists are to figure out the degree of red shifting that has occurred in radiation, the spectrum has to have telltale emission lines in it rather than being a smoothly smeared continuous spectrum. These lines correspond to the particular interactions taking place at the atomic level, each reaction emitting, remember, photons of a given and discrete wavelength. Even light coming from the sun does not give a perfectly continuous and smooth spectrum, but peaks at certain wavelengths corresponding to the most frequent atomic reactions and the resulting concentrations of photons. The degree to which these lines are shifted towards the red end indicates the degree of red shift of, say, a distant galaxy.
Gamma radiation from bursts is smooth, lacking such lines. Why it should be smooth is a puzzle. Thus, the degree of red shift cannot be worked out. The distance problem has to be solved by other means. Which leads us now to the second problem.
Whatever mechanism produces GRBs undoubtedly produces radiation in other wavelengths as well. Rarely does an energy-emitting celestial object produce pure radiation, that is, radiation of a very narrow band of the spectrum. One can fairly safely assume that whatever produces the gamma rays also produces visible light, which would almost certainly show the telltale spectral lines. Now if investigators could see the gamma-ray-radiating object in such visible light, progress could be made. If they could determine exactly where in the sky the gamma rays are coming from, they could train the Hubble telescope onto the spot, and see if there are any signs of a strangely-behaving visible object there. Once the object is pinpointed, then data obtained at other wavelengths would go a long way to solving the riddles.
The problem is, gamma rays reflect poorly, and cannot be focused to produce an image. The very best that a gamma ray detector can do is to give a rough indication of direction from which the rays are coming, making it extremely difficult to precisely pinpoint their source in space. A spot of plain old good luck was essential if scientists were ever to pinpoint a GRB's position in the sky precisely.
When Beppo's gamma ray detector spotted a burst in February, 1997, technicians were able to train its X-ray detectors in the same general direction in time to catch an X-ray afterglow. X-ray telescopes can home in on the position of an emitting object. Having found the location with fair accuracy, a team led by Jan van Paradijs of the University of Amsterdam was able to aim a big optical telescope in the same direction and, wonder of wonders, spotted a fading patch of light. (As the mechanism that generates gamma rays "dies down" the afterglow degrades first into X-rays then into less potent radiation such as light and radio waves.) After all these years the breakthrough had come. The same thing happened in May, at which time the light was bright enough for a team from the California Institute of Technology to dissect and analyze.
Their announcement was unambiguous and spectacular. This light had traveled through a cloud of interstellar gas several billion light-years from earth. Gamma-ray bursts come from afar, the majority from billions of light-years away, not from within our own galaxy! (Though one probably occurs in our galaxy once every few hundred million years.) At last the main problem had been solved.
Crash go the theories
This fact sounded the death knell for almost all the proposed models, for the simple reason that the discovery took bursts right out of our own galaxy, and placed them firmly in the distant reaches of space. It also meant that the amount of energy involved is staggeringly high — far higher than the theories could account for.
The breakthrough is a limited breakthrough. It has forced experts to discard most theories. But it has still not provided any clues to the mechanism involved. The mystery remains a mystery, wrapped in an enigma, shrouded in wonder. Experts are hard-pressed to imagine what could produce such unimaginably powerful outbursts. A fireball produced by merging neutron stars, in turn producing awesome amounts of radiant energy, is the favorite, with a black hole consuming a neutron star coming a close second.
Bursts vary so much in major details that one wonders if any one mechanism can possibly explain them all. For instance, after the discovery of an optical afterglow from a couple of bursts in 1997 helped track down the location of these bursts, other bursts were found that did not glow at all in visible light, remaining pitch dark. What is going on? Such differences are impossible to explain yet.
Just what sort of energy are we talking about? Lots. Though we are situated 93 million miles from it, energy from our sun is enough to fuel every living organism, power every storm and tornado, warm our atmosphere, and give us light to see by. Untold billions times more energy is simply going to waste, radiating out into the vastness of the universe, adding to the ever-growing sum of radiation doomed to a life of eternal wandering.
Our sun is one star in our galaxy. Approximately one hundred billion others similar to it exist. Try to imagine putting all those suns together. You can't. But try anyway. But you still don't have anywhere near enough radiation to match one powerful GRB. Put a million similar galaxies together, and then you have an idea of what sort of energy we are talking about! Incomprehensible. Putting it another way, in a few minutes an average burst generates and radiates more energy than many suns will generate and radiate in their entire lifetime of billions of years. Adding to the miraculousness of the phenomenon, these staggering amounts of energy come from an object only forty miles across! What an inconceivable thought.
Strict logic prohibits one from trying to prove that God has infinite power by appealing to a finite, created object. But one could be forgiven for suggesting that gamma ray bursts demonstrate God's limitless power better than any number of words can do, especially when one considers the words of Psalm 113:4:
The Lord is high above all nations, His glory above the heavens.
When you consider that one gamma ray burst radiates as much energy for a few moments as an entire galaxy, and that every galaxy contains numerous potential “bursters”, you cannot help but be driven almost senseless at the thought of so much energy locked up in the heavens. Yet, if we can accept the testimony of the Psalmist, God's glory is greater than all the “glory” — the flashing energy and swirling marvels — of the universe put together. And to think that one day believers will get to see His glory “in the raw” (Rev. 22:4). How can such a thing possibly be?
Update, December 2004
Though many questions remain, the level of debate among astronomers over the nature of GRBs is subsiding, with general consensus as to the fundamental generating mechanism having been reached:
Over the past five years, observations have revealed that bursts are the birth throes of black holes. Most of the holes are probably created when a massive star collapses, releasing a pulse of radiation that can be seen billions of light-years away (Gehrels & others 2002, p. 54).
Let's briefly survey the newer understanding of the fascinating world of GRBs.
Even stars grow old and die. As they use up their fuel supply, some first expand into a red giant. After perhaps a billion years they puff off their outer layers then shrink to a white dwarf under the relentless force of gravity. Their end is to quietly peter out, cooling to become space clinker. Other stars go out with a bang, producing what is called a supernova. In a violent outburst, the outer material of the star is flung off at near-light speeds, producing an expanding ring, or shell, or fireball, of matter and photons of electromagnetic radiation, only a tiny percentage of which takes the form of visible light.
Astronomers recognize two quite different ways in which the same general result occurs. (They are really quite different in their details). The first kind, or type 1a supernovae, occur when a star about the size of the sun, in gravitational contact with another star nearby (a binary star system), goes through the red giant stage then turns into a white dwarf. The companion star sometimes starts "unraveling", its material spiraling down onto the compact white dwarf. When the now-growing white dwarf reaches a mass 1.4 times the size of our sun, it undergoes a gigantic thermonuclear explosion as the new fuel ignites, producing the effect described above. The other kind, or type II supernova (SN II), occurs when a huge star eight or more times more massive than the sun finds "a different way to explode" (Kirshner 2002, p. 34) — instead of exploding through thermonuclear fusion processes, it explodes by collapsing. This collapse occurs when the outward pressure generated by the core's colliding gas particles can no longer hold back the inward force of gravity of the star's huge mass. The implosion produces a similar effect as described above. "Normal" SN II events leave behind as their legacy a rapidly spinning neutron star (pulsar) at the centre and an expanding shell of gas.
Theorists believe that when an SN II is produced by an exceedingly massive star, 20 or more times the mass of the sun, the resulting "hypernova" produces an outer expanding fireball while the core of the star collapses into a black hole with a surrounding disk of material. Under these circumstances, strange things happen. The intensity of the explosion produces high energy gamma rays; however, they are not immediately emitted as the enormous amount of radiation coming from such a small point packs the photons so tightly together they can't escape (Gehrels 2002, p. 56). Not until the fireball reaches a diameter of about 100 billion kilometers does the density drop sufficiently for the rays to escape.
This model of gamma ray generation includes an explanation for the sharpness of bursts. Let Gehrels and others explain:
Because the fireball is expanding so close to the speed of light, the timescale witnessed by an external observer is vastly compressed, according to the principles of relativity. So the observer sees a burst of gamma rays that lasts only a few seconds, even if it took a day to produce (p. 57).
As the fireball expands, it crashes into the external medium, producing a shockwave which generates the radiation of changing wavelengths — X-rays, visible light, radio waves — picked up by our instruments as afterglow.
The forces involved and the energy generated defy imagination. A GRB studied in 1999 (GRB990123) had a luminosity, even assuming the radiation was spurting out in narrow jets rather than uniformly, as investigators believe actually happens, of 1043 watts, which is 1019 times as bright as our sun and one thousand times brighter than famously brilliant quasars.
What human being can really grasp the idea of just how much energy can be locked up in one star? So much that, when much of that fuel is turned into energy in one humungous explosion, it outshines galaxies. We can be very grateful that such events occur in our own galaxy only about once every 100 million years. Of course, we don't have to be concerned that it will happen while the divine plan of salvation is in progress. Methinks the Great Astronomer has it all worked out.
In 2006, discoveries were made which call into question the brief outline above about the mechanism that generates gamma ray bursts. See "Cosmic supermegaexplosion".
This article is excerpted from the Dawn to Dusk book "How Great Thou Art ". Please click here if you would like more information about this book.
Gehrels, N., Piro, L., and Leonard, P. J. T. 2002, The Brightest Explosions in the Universe, Scientific American December
Kirshner, Robert P. 2002, The Extravagant Universe, Princeton University Press, Princeton and Oxford
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