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Two years ago the Gamma Ray Observatory (GRO), the second of NASA’s
four planned orbiting observatories, was launched aboard the space shuttle
Atlantis. At 17 tonnes, it remains the largest civilian satellite ever
launched by the shuttle. The observatory was intended to give astronomers
a view of the Universe in a part of the electromagnetic spectrum that is
particularly difficult to observe. The view is turning out to be a panorama
that takes in solar flares, nova and supernova explosions, white dwarfs,
neutron stars and black holes, quasars, the cores of active galaxies and
the interstellar medium itself.

Gamma rays are at the most energetic end of the electromagnetic spectrum.
Even the least energetic gamma ray photons have an energy of 100 000 electronvolts,
about 100 000 times the energy of a visible photon, and from there the energies
spread upwards to at least a million million electronvolts (1 teraelectronvolt).
We know very little about the origin of cosmic gamma rays. They come from
a variety of astronomical sources which produce charged cosmic rays (energetic
electrons and protons). For example, high energy protons in cosmic rays
interact to form pi mesons, which decay to gamma rays. So the search for
cosmic gamma rays is also the search for the origin of cosmic rays.

Cosmic gamma rays do not penetrate the Earth’s atmosphere, and at the
most energetic wavelengths they are also extremely rare. This makes measuring
them a particularly tough task. But cosmic gamma rays are the only direct
source of information about the most energetic processes in the Universe.

Astronomers began searching for gamma rays from space in the late 1950s,
using ground-based instruments. But distinguishing signals from cosmic gamma
rays from those caused by more abundant charged cosmic rays proved difficult,
and the first cosmic gamma rays were not identified until 1967, when detectors
on board a satellite were able to make observations unhindered by the Earth’s
atmosphere. During the 1970s, the American SAS-2 and European COS-B satellites
revealed that our Galaxy, the Milky Way, glows bright with gamma rays.
They detected several point sources too, including the Crab and Vela pulsars
and a mysterious gamma-ray emitter that was named Geminga. COS-B also discovered
the first source of gamma rays outside our Galaxy, the bright quasar 3C₂73.
But it is the GRO, with detectors ten times as sensitive as any previous
ones, that is enabling astronomers to explore the gamma ray Universe in
depth.

The Compton Gamma Ray Observatory, renamed after the American physicist
Arthur Compton, carries four complementary experiments, each covering part
of the gamma-ray spectrum. These are the burst and transient source experiment,
BATSE, which covers 20 to 600 kiloelectronvolts (keV); the oriented scintillation
spectrometer, OSSE, covering 100 keV to 10 MeV); the imaging Compton telescope,
COMPTEL, covering 1 MeV to 30 MeV; and at the top end the energetic gamma
ray experiment, EGRET, which runs from 20 MeV to 30 gigaelectronvolts (GeV).
These instruments can detect when gamma rays arrive from space, their direction
and their energy. The energy range covered by each detector overlaps with
the next, so the observatory gives a complete picture of cosmic gamma radiation
across a large part of the sky. Last November, the instruments completed
their first routine survey of the entire sky and began to look at some of
the known sources of gamma rays in more detail. The observations turned
up many surprises, and answered a few questions as well.

Mystery bursts

Perhaps the most pressing problem that astronomers hoped the Compton
observatory might solve was the origin of the mysterious bursts of gamma
rays that were first seen a quarter of a century ago, from the early gamma
ray detectors aboard satellites. These sporadic, sharp increases in gamma
ray intensity come from unpredictable directions in the sky. No one knows
how they are produced, or where they originate. One possible explanation
is that the bursts are produced nearby, within the Solar System, although
no one has suggested a plausible explanation for how this might happen.
Another is that they might be produced somewhere in the disc or halo of
our galaxy, at distances of between 100 000 and 300 000 light years. Or
they could originate in the most distant parts of the Universe, thousands
of millions of light years away. The last theory seems to be the most popular
at present, although there is no firm evidence to support it.

The energy of the gamma rays in a burst can be anything from a few tens
of kiloelectronvolts to tens of megaelectronvolts and the burst can last
for as little as a fraction of a second or as long as several minutes. The
‘switch on’ time of the burst can be very sharp – less than a millisecond
– after which the intensity may die away smoothly or follow a complicated
pattern of peaks and troughs.

Catch it if you can

Looking for gamma-ray bursts is rather like using an optical telescope
to look for people in space who are known to be operating bright flash
guns, but without knowing when they will do it or where they will be. There
are several gamma ray bursts every day, and detectors must scan the sky
for them. BATSE was designed to do this, and to measure characteristics
of the bursts it detects. When BATSE observes a burst – about once a day,
on average – it ‘triggers’ the other telescopes on the observatory, so that
they can add information on the burst if it was in their field of view.

By late 1991, BATSE had detected 177 bursts (see New ÐÓ°ÉÔ­´´, Science,
5 October 1991). The direction they came from suggested that bursts were
distributed uniformly across the sky, and by the middle of last month the
tally had increased to 566 bursts. By then it was certain that the bursts
were isotropic on the sky – that is, uniformly distributed in two dimensions
– but they were not spread evenly in space. Weaker bursts, which we would
expect to arrive from the most distant sources, were not detected. Were
the bursts detected by BATSE from a source beyond our galaxy, with an enormous
energy output, or sources of modest power, closer to us?

A key piece of missing information is the identification of a burst
with an object a known distance away. But this involves careful measurements
in a different part of the spectrum, perhaps X-ray or optical wavelengths.
Unfortunately, the chances of a burst happening while such a measurement
is being made are small. The most recent all-sky survey carried out by BATSE
included new details of the duration and structure of bursts, over a range
of brightness. But frustratingly, no hint of the distance to any gamma ray
burst, nor of an identification of any burst with a known source, has yet
been found.

The one feature which offered some prospect for understanding the bursts
was the presence of ‘lines’ – enhanced intensity at particular angles –
in their gamma ray spectra. Such lines, which were first detected in the
spectra of various gamma ray sources in the 1970s, are the ‘fingerprints’
of specific particle interactions, and so are of great interest to astronomers
and astrophysicists. Gamma rays at an energy of 511 keV arise when electrons
and positrons collide and annihilate each other; radiation at 1.8 MeV comes
from the decay of radioactive aluminium-26 to magnesium-25, which one of
the isotopic decay chains that occurs in the process of nucleosynthesis
in stars; and gamma rays at 122 keV from the decay of cobalt-56 have been
detected from the supernova SN 1987a in the Large Magellanic Cloud, adjacent
to the Milky Way.

Any similar lines visible in spectra from gamma ray bursts might hold
some clue to the processes taking place where the bursts originated. In
the late 1980s, researchers examining data from Japan’s Ginga spacecraft
reported lines in gamma ray bursts, but none of the 566 bursts recorded
anything similar. In all, the Compton Observatory should clock up 1500 or
so bursts in its complete five-year mission. But given that more than 500
bursts have already been observed, there does not seem much hope of a breakthrough
in our understanding coming from either the sky map of burst positions
or from the brightness distribution. The origin of the bursts remains a
mystery – perhaps the astronomical mystery of the century. Some other result
must, it seems, provide the first hint of a solution.

As well as monitoring the bursts, BATSE has detected gamma rays from
pulsars. These include pulsars in X-ray binaries where the energy comes
from accretion of matter from a companion onto a neutron star, and isolated
pulsars where a strong magnetic field is the source of the gamma ray power.
In both types of pulsar the frequency of the pulses of gamma radiation reveals
the speed of rotation of the neutron star. For almost a year, BATSE has
been continuously monitoring the 4.8-second period of the spin of a neutron
star in the X-ray binary Centaurus X-3. This has revealed day-to-day variations
in the spin frequency that did not show up in the occasional X-ray measurements
done previously. These variations suggest that accretion of matter by the
pulsar is not uniform. BATSE has also discovered a new gamma ray pulsar
– only the fifth ever to be discovered – with a spin period of 150 milliseconds,
that has been identified with the pulsed radio source PSR 1509-58.

Meanwhile, the observatory’s other instruments have been investigating
different energy ranges. COMPTEL has mapped gamma rays from the plane of
the Galaxy, and studied the Crab and Vela pulsars, as well as some possible
black holes. OSSE detected gamma ray lines that could be a clue to what
lies at the heart of the Galaxy. It recorded the 511-keV line, produced
when electrons and protons annihilate each other, coming from the direction
of the galactic centre, though it is not yet clear whether the line is coming
from a point source or is part of a more diffuse emission.

Quasars with something to say

EGRET’s task has been to track down sources of high-energy gamma rays.
Some such sources had already been identified with extragalactic objects,
and among the 400 selected as likely candidates was a class of extragalactic
objects known as ‘radio loud’ quasars. Many of these objects are thought
to contain jets of cosmic rays which act as cosmic particle accelerators,
producing beams of gamma rays and neutrinos.

The first 18 months’ results from EGRET outstripped expectation. High-energy
gamma rays of around 1 GeV were detected from 24 extragalactic sources and
while observing the known gamma ray source 3C₂73 the EGRET team
also discovered a very strong source only a few degrees away. Initially
they thought the telescope was not being pointed accurately, but these gamma
rays turned out to come from another radio source, 3C₂79. Later
observations revealed that its intensity varied, by up to a factor of five
over only a few weeks. The short timescale of this variation suggests that
the source of the gamma rays must be small – much smaller than had previously
been thought, as the variations cannot happen on a timescale shorter than
the time it takes a gamma ray to go from one side of an object to another.
EGRET’s measurements of the gamma rays from the Large and Small Magellanic
Clouds adjacent to our galaxy, reported earlier this year, also provide
evidence that cosmic rays originate in our Galaxy. Astronomers know how
gamma rays are produced when the energetic charged particles that make up
cosmic rays collide with interstellar gas, and they know the amount of interstellar
gas from radioastronomers’ measurements of its density. From this they can
calculate what the flux of cosmic rays should be. If cosmic rays come from
all over the Universe, the cosmic ray flux should be the same everywhere.
But the intensity of the gamma rays from the Magellanic Clouds was lower
than that predicted from calculations based on the cosmic ray flux near
the Earth. This suggests that cosmic rays do not come from all over the
Universe, but are produced within galaxies.

Cygnus signals

Another unsolved puzzle of the 1970s was whether or not Cygnus X-3 produced
gamma rays. Cygnus X-3 is an X-ray source on the edge of our Galaxy which
was first observed to burst out violently as a radio source in 1972. Since
then it has done this two or three times a year, and astronomers are intrigued
by the possibility that Cygnus X-3 might be a source of the very highest
energy gamma rays, with energies of 1000 teraelectronvolts. If so, only
a few objects like Cygnus X-3 would account for the energy of all the cosmic
rays in the galaxy.

The SAS-2 project found evidence for gamma rays of lower energy from
Cygnus X-3, but COS-B later found none. The initial results from EGRET’s
observations of Cygnus X-3 are tantalising. Some gamma rays with energy
greater than 100 MeV were detected from the direction of Cygnus X-3. But
they do not show the characteristic 4.8-hour modulation of the rays seen
by SAS-2. Astronomers are eagerly awaiting further observations later this
year, but at the moment it seems that this gamma ray emission varies in
intensity. This and many other results from the Compton Observatory seem
to suggest that sporadic and variable emission is a feature of the gamma
ray sky, something that astronomers will have to remember when observations
cannot be reproduced.

One of the best-known sources of gamma rays, and one which puzzled astronomers
for over a decade, is the strong gamma ray source called Geminga. Its identity
had eluded astronomers for about 20 years, but after a number of false
starts they decided that the best bet was a source of soft X-rays known
as 1E0630+178. EGRET detected gamma rays from Geminga in 1991. Then in
mid-1992, researchers with the German-American ROSAT X-ray project reported
a 237-millisecond periodicity in the X-rays from the X-ray source. The
EGRET team soon confirmed that these pulsations were also present in their
high-energy gamma ray data, confirming that a pulsar spinning with a period
of 237 milliseconds, between 100 and 300 light years away, was the source
of the X-rays and the gamma rays. The same periodicity has since been recognised
in the early SAS-2 and COS-B gamma ray data. There are even suggestions
from two independent ground-based experiments that very high-energy gamma
rays, with an energy of 1 TeV, have been detected with the telltale 237-millisecond
timing, indicating that they too come from the same source.

Last month Giovanni Bignami and his colleagues at the Universities
of Milan and Cassino reported that they had identified Geminga’s optical
counterpart – a very faint object – and succeeded in recording its movement
across the sky. This allowed Neil Gehrels and Wan Chen of the NASA Goddard
Space Flight Center, who knew the age of the pulsar to be 300 000 years,
to work out the likely position of the supernova which gave birth to it.
It seems that the supernova which produced the Geminga pulsar may also have
been the cause of the Local Bubble – a cavity of hot, low density X-ray
emitting gas embedded in the interstellar medium, which has the Solar System
in its edge.

The Compton Observatory should carry on recording gamma rays for at
least another three years. After that, the next major space mission dedicated
to gamma ray astronomy is the International Gamma Ray Astrophysics Laboratory,
INTEGRAL, which is planned for launch in the year 2000.

Meanwhile, improved detectors have revitalised comparatively cheap ground-based
studies of gamma rays. For the first time, the overwhelming difficulties
of the background seem to have been solved. A team at the Whipple Observatory
in Arizona has used the atmospheric Cerenkov light technique to make observations
at the very highest gamma ray energies, which are of great astrophysical
interest. By increasing the size of ground-based gamma ray telescopes, astronomers
in several countries hope to detect lower energy gamma rays more like those
being studied by the Compton observatory. They will undoubtedly add even
more colour to the emerging picture of the gamma ray sky.

Ted Turver is professor of physics at the University of Durham.

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GAMMA RAY ASTRONOMY FROM THE GROUND

Gamma ray astronomy has its beginnings in the early 1950s, when the
British physicists John Jelley and Bill Galbraith used a sensitive light
detector to measure the Cerenkov radiation produced by cascades of charged
cosmic ray particles passing through the atmosphere. Cerenkov radiation,
which appears as faint blue light, is emitted in a narrow cone when fast
charged particles pass through a transparent medium. The original Cerenkov
light receiver consisted of a mirror and a photomultiplier, placed in a
dustbin.

The first attempts at gamma ray astronomy also used this technique.
When gamma rays encounter the particles in the atmosphere, they generate
a cascade of electrons, which in turn produce Cerenkov radiation. But this
meant that while the Cerenkov light from gamma rays could be easily detected
on clear and moonless nights, the technique suffered from the high background
of similar flashes produced by charged cosmic rays.

Then in the late 1980s, a group at the Whipple Observatory in Arizona
developed a method for separating the Cerenkov flashes produced by gamma
rays from those produced by charged cosmic rays. This involves measuring
the shape of the flash of Cerenkov light, using a large electronic camera
made up of many small photomultipliers that receives light collected by
a mirror 10 metres in diameter.

During the 1980s, groups at Durham and elsewhere began to suspect that
several sources, including pulsars and X-ray binary systems, were emitting
gamma rays of teraelectronvolt energies. Many of these sources were sporadic,
so pinpointing them was difficult. Since then, the Whipple group has detected
the emission of teraelectronvolt gamma rays from the Crab nebula and, even
more fascinating, from the closest of the quasars detected as gamma ray
sources by EGRET. This source, Markarian 421, is a giant elliptical galaxy
which includes an active galactic nucleus. It is the nearest of a class
of active galactic nuclei called the blazars, which are around 300 million
light years away. Its relative closeness may explain why it is the only
one of the EGRET quasar sources which has been detected at the highest energies.
Nevertheless, a teraelectronvolt gamma ray source at such a distance implies
an energy output in gamma rays of 1043 ergs per second – equivalent to the
energy of all the visible starlight in the galaxy.

This detection of a nearby quasar producing such high energy gamma rays
may be very important, not only for astronomy but for cosmological studies.
Detectors on the ground are capable of measuring the highest energy gamma
rays, but few have been detected so far. A combination of ground and space
based measurements should reveal whether there is a sudden cut off in the
gamma ray spectrum as the highest energies are approached. If so, this
suggests that high-energy gamma rays do exist, but that something is stopping
them arriving. A strong possibility is that as the distance to the source
increases, gamma rays with an energy of about 1000 GeV interact with the
infrared background radiation that permeates intergalactic space.

This interaction is thought to ‘degrade’ them to lower energies. So
the highest energy gamma rays from more distant sources would not be detected.
This very failure might be useful in increasing our understanding of the
infrared starlight.