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Plejaran A's House of Science:

Gamma-Ray Bursts Of doom!!!

  Light on science, heavy on special effects, the movie industry seems to have gone off the deep end with depictions of natural disasters, from tornados and volcanos, to cosmic doom  in the guise of asteroid impacts. But Hollywood has ignored one potential natural disaster that may have all the others beat — high-energy radiation from a near by gamma-ray burster.

The good news, relatively speaking, is that the gamma radiation from one of these awesome events would merely wipe out the ozone layer and possibly darken the sky, devastating world ecology and food production. The bad news, according to at least one recent model of how bursters work, is that the radiation pulse might be followed by a month-long blast of extremely energetic cosmic-ray particles. These would be lethal to humans and most everything else and could render much of the planet's surface radioactive for thousands of years.

The Burster Mystery

  Cosmic gamma-ray bursts have been one of astronomy's outstanding mysteries since they were first detected with the Vela nuclear-surveillance satellites more than 30 years ago. A breakthrough in the mystery came last year, and it has only made these events seem more fantastic.

   About two or three times a day, a flare of gamma rays arrives from a random point on the sky. The burst can last from a fraction of a second to several minuets, and it can show rapid flickering on a time scale of milliseconds.

   The mystery deepened following the launch of NASA's Compton Gamma Ray Observatory in 1991. The satellite's sensitive Burst And Transient Source Experiment (BATSE) has recorded some 2,000 bursts, enough to demonstrate that they happen uniformly across the sky. They show no tendency whatever to group toward the plane of the Milky Way, as would be expected if the sources were within our galaxy.

   Moreover, there are fewer faint bursts that would be expected if the sources were distributed uniformly throughout space. Apparently we are located very close to the centre of a spherical arrangement of bursters. One possibility is that they lie in a large, spherical halo centred on the Milky Way and extending more than 300,000 light-years out. But that idea is becoming less and less tenable.

   Alternatively, the bursters could lie at "cosmological" distances — scattered across billions of light-years throughout the cosmos. In this case, the apparent falloff with distance would result naturally from the expansion of the universe and the curvature of space over great distances (Sky & Telescope: September 1996, pg. 32).

   If this explanation is correct, the faintest , farthest bursts should appear stretched out in time and red-shifted by the expansion of the universe. This is, indeed, what some analyses of the time histories and spectra of the bursts have found. But others have not.

   The worst stumbling block in resolving the gamma-ray burster debate was — until February, 1997 — their total invisibility at every other wavelength aside from X-Rays. The discovery that a few bursts have faint, lingering visible-light and radio counterparts was among the biggest astronomical news last year (Sky & Telescope, July ‘97, pg. 19, August pg. 17). Gamma-Ray burst researchers are jumping for joy as they see their exotic and relatively obscure field at last attracting the attention of mainstream astronomers.

   The optical and radio detections are not without controversies of their own. But they bring strong evidence for cosmological distances. In particular, red shifted spectral lines in the probable optical counterpart of the burst recorded on May 8th imply a distance of 4 to 8 billion light-years. The delay of several days in the radio emission also matches what had been predicted for bursters at extreme distances. Now that astronomers have refined the process of localizing gamma-ray bursts and following up with radio and optical observations, we can foresee the distance question soon being settled for good.

 Merging Pairs Of Neutron Stars

   If gamma-ray bursters do lie at cosmological distances, they must be fantastically powerful. Moreover, they must very small in size to show large variations on millisecond time scales. Where does their energy come from?

   The most popular models involve the merger of 2 neutron stars — extremely dense objects that are slightly more massive than the Sun but only about 20 kilometres in diameter. When a neutron-star pair finally spirals together to produce a black hole, it is more luminous in its final second that a billion galaxies like the Milky Way.

   The consensus among theorists is that to account for a spectrum that peaks at gamma-ray wavelengths, the merging pair must eject material at speeds up to 99.9995 or 99.99995 percent the speed of light. A better way to express such velocities i that the "Lorentz factor" of the out-flying material must be between 100 and 1,000. This means that it's moving so close to the speed of light that the mass of each particle is boosted by a factor of 100 to 1,000 according to the special theory of relativity. Only about a hundred-thousandth of a solar mass needs to gain this speed.

   One variant has been proposed by Nir Shaviv and Arnon Dar of the Israel Institute of Technology. They suggest that the Black Hole formed by the merging pair of neutron stars is briefly surrounded by a disk that emits powerful jets of matter from its poles with a Lorentz factor of 1,000.This material is so energetic that when it collides with starlight itself, it kicks the photons of light forward to gamma-ray energies. We observer these gamma-ray photons if the jet is aimed towards Earth.

   Obviously this model works best when the neutron-star merger happens near a luminous third star or in a dense star cluster. If there is not much light around, no gamma-ray burst would be kicked forward by the jet. This scenario can explain the diverse light curves of the bursts, from brief to long, and from smooth to highly choppy, as a consequence of the different starlight environments near each source, and the structure of the jets themselves.

   The Shaviv and Dar model has a nasty side effect. The ultra relativistic heavy nuclei in the jet are high-energy cosmic rays that would punch right through the local interstellar medium. They would yield a prolonged blast of cosmic rays following closely behind the gamma-ray burst.

Bursts in the ‘Hood

It would be catastrophic to life on Earth if a typical cosmological burst 3 billion light-years away happened in our region of the Milky Way at a distance of, say, 3,000 light-years. We shall call this a local gamma-ray burst, or, if you like, a "burst in the ‘hood."

   Indeed, if bursts result from mergers of neutron stars, it is only a matter of time before a local on occurs. There are three known, and two probable neutron-star pairs (see table). All of them will eventually spiral together and merge, due to their loss of orbital energy by gravitational radiation. Estimates suggest that there are hundreds more pairs in the galaxy remaining to be discovered.

Pairs Of Neutron Stars In Our Galaxy
Distance (Light Years)
Orbital Period (Hours)
Oribital Eccentricity
Time till mereger (millions of years)
PSR B134+12
PSR B1913+16
PSR B2127+11C
PSR B2303+46
PSR J1518+4904


1. The  Hulse-Taylor Pulsar, in which the first evidence for gravitational radiation was found.

2. In the globular cluster M15.

3. Suspected to contain neutron stars; precise somponent masses are not yet established.

   Shaviv, Dar, and fellow Israeli Ari Laor estimate that, on average, one of these pairs will merge every 2 or 3 million years in our own galaxy. The galactic magnetic field may not shield us from a cosmic-ray jet starting as close as 3,000 light years from the Sun. The average rate of mergers within this distance works out to one every hundred million years, which is alarmingly similar to the mean time between the largest mass extinctions in Earth's geologic record.

   What would happen if a burst occurred at that distance? The first effect would be an extraordinarily bright gamma-ray bath, briefly outshining the energy we receive from the Sun. Most of this, of course, would be invisible. We would see a patch of the sky glowing eerie blue, the Cherenkov radiation resulting from interactions when the gamma-rays hit the upper atmosphere. This patch might appear as bright as a full Moon and a little larger in angular size. The gamma rays themselves would be stopped before they reached the lower stratosphere, and we might breath a sigh of relief.

   But as Stephen E. Thorsett (Princeton University) pointed out in the May 1st, 1995 Astrophysical Journal Letters, the blast of gamma rays would break apart air molecules in the upper atmosphere, triggering chemical reactions that would produce enormous amounts of nitric oxides in the half of the atmosphere facing the burst. Strong absorbers of visible light, nitric oxides are familiar components of smog that colour polluted city air reddish-brown. Within seconds, the daytime sky might darken on the side of the globe facing the burst, depending on the amount of pollution created.

   These compounds are catalysts for the destruction of the ozone. They would wipe out the entire ozone layer as they were carried worldwide by upper-atmosphere circulation. Drastically increased Ultraviolet radiation from the Sun would reach the ground. Humans can protect themselves from Sunlight, but plants and animals cannot. Ecosystems would be devastated plants fundamental to the food chain were altered or wiped out. The planet's temperature and climate might also be affected. The nitrogen oxides would take decades to settle out of the Stratosphere.

   But that was the good news!!! The bad news is the horrific damage caused by the subsequent cosmic-ray bath. It arrives a few days after the gamma-ray burst and lasts perhaps a month. At this stage, Earth turning on it's axis could be portrayed as a chicken roasting on a splint. Each trillion-electron-volt particle that hits the atmosphere would produce, by a cascade of sub-atomic interactions, a shower of energetic muons that wold reach and enter the ground. The total sea-level dose of muons would be roughly 100 times the dose lethal to humans. The muons would penetrate hundreds of metres underground and underwater to kill all but the most well-protected or radiation-resistant species.

   In addition, the high-energy particles would break up nuclei, some of which are radioactive, and take many years to decay. Global winds would spread the airborne component of this radioactive pollution everywhere.

   Shaviv, Dar, and Laor point out that such an event can explain sudden mass extinctions, any preferential survival of deep-water organisms, and the sudden rise of new species by increased mutation.

   The month-long cosmic-ray blast will contain as much as 10 million years worth of normal cosmic ray bombardment, and it will be in the form of more lethal, higher energy particles. One signature it would leave is excess of the long-lived radioactive nuclei iodine-129, samarium-146, lead-205, and plutonium-244, which have half-lives ringing from 15 million to 146 million years. However, identifying such enhancements in the geologic record will not be as easy as one might think. The ordinary cosmic-ray background during the 100 million years between nearby bursts also produces these isotopes in rocks near the Earth's surface. Careful mass spectroscopy should be able to test the hypothesis, however. Another test would be to look for a sudden change in the density of cosmic-ray tracks in rocks that formed just before, and just after a mass extinction.

Other Cataclysms

But wait...don't asteroid or comet impacts cause mass extinctions? The best candidate for a global extinction caused by a giant impact is the one that coincided with the demise of the dinosaurs at the end of the Cretaceous era some 65 million years ago. A 180-km crater buried under sediments in the Yucatan has been dated at 64.98 ± 0.05 million years, and the impact event would certainly have been catastrophic. But whether this alone killed off the dinosaurs is still under debate.

   Consider the possibility that there is more than one way to wipe out large numbers of species on Earth. Some of the mass extinctions in the geological record could have resulted from terrestrial phenomena such as tectonic plate motions and volcanic out-gassing resulting in climate change, and others from various kinds of cosmic disasters.

   In fact, as astronomers explore the Universe, the list of possibilities has grown. A scenario not unlike a local gamma-ray burst would be a really close supernova (within 30 light-years). This too would produce a heavy radiation dose. Another theory is that the centre of the Milky Way undergoes recurrent outbursts, becoming an especially nasty galactic nucleus every 100 million years. If so, the increased cosmic-ray flux could adversely affect life on Earth.

What Can We Do?

When the subject of asteroid impacts comes up, the discussion quickly turns to the fact that we can almost solve the problem now. We could inventory all the small bodies in our solar system and push a threatening one off course using missile technology hardly beyond what we already have. But what could we possibly do about the pending merger of two neutron stars thousands of light-yeas away?

   The best chance of minimising the overall damage would be if the burst occurred near one of the celestial poles. The Earth, itself, would shield the opposite hemisphere from the direct  rays. A burst near the celestial equator would leave only a small shaded region near the North or South Pole. A few sheltered spots might also exist behind mountains, or in deep canyons at high latitudes. But in any case, the deadly effects carried by the winds would be global.

   There is a silver lining to the picture. Unlike marauding asteroids, the merger of a binary neutron star is predictable many millions of years in advance. Its slow spiralling together, due to emission of gravitational radiation, can be forecast from the binary's component masses, orbital eccentricity, and period. The table lists time until merger of the five neutron-star binaries currently known or suspected in the Milky Way galaxy. Astronomers of the future should be able to inventory the galaxy for the several hundred more that ought to exist. Once their merger dates are calculated, their locations, orbital orientations, and space motions will reveal whether they will pose any threat to us when they go off.

   The next crisis would be, typically, 50 million years away, which leaves plenty of time for action. Suppose no one did anything until the last 2% of the countdown. In that final million years, human civilization could dig itself into comfortable quarters half a kilometre underground, along with a Noah's Ark worth of the biosphere, to ride out the storm. Depending on how spacious the quarters were to be, it might take an earth-moving effort equivalent to one shovelful of dirt, per person, per year. Planned in advance, it would be easy.

   A better solution would be to create a shield to protect the entire Earth, using material from the moon, or an asteroid. The asteroid Ceres contains more than enough material to produce a disk a wide as the Earth, and 1 km thick, easily blocking the gamma and cosmic-ray bath. The trick would be to construct such a shield and place it into an orbit where it would occult the burster for about a month. Such orbits do exist. A construction project of this size might be quite manageable, considering the time available.

   Of course, a million years is 200 time the length of recorded history, and it's vastly longer than any society has been able to plan it's future. Gamma-ray bursters not only stretch the limits of the mind with their awesome power, they may someday stretch the timetable on which the human race, or it's descendants, plans ahead.

Out Now!!!

Catching Tigers By The Tail: The Search for counterparts to gamma-ray bursts

Coming soon...

Do Bursters Cause ripples in Space Time?

It's that time again!!!

Now, If you've reached this after reading all that, how hard would it be to send a pitiful soul a little e-mail?

Go on! It wouldn't hurt!

That's right! Mail me! It's not hard! I wouldn't mind!!!