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
Name
Distance (Light Years)
Orbital Period (Hours)
Oribital Eccentricity
Time till mereger (millions of years)
Notes
PSR B134+12
1,600
10.4
0.274
2,730
PSR B1913+16
24,000
7.75
0.617
300
1
PSR B2127+11C
35,000
8.05
0.681
220
2
PSR B2303+46
7,500
296
0.658
4,000,000
3
PSR J1518+4904
2,300
207
0.249
2,400,000
3
Notes:
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.
