Gamma-Ray Astronomy

I

Introduction

Gamma-Ray Astronomy, field of astronomy involving the observation of gamma rays from space. A gamma ray is a very-high-energy form of electromagnetic radiation with a wavelength even shorter than that of X-rays. The boundary between hard X-rays and gamma rays is rather arbitrary, but gamma-ray photons are generally considered to be those with energies above 100 keV (100,000 electronvolts); for comparison, typical hospital X-ray machines operate at 20 to 30 keV.

Gamma rays are produced by changes within atomic nuclei and are also decay products of collisions between cosmic rays and interstellar matter. One source of gamma rays is solar flares, and their study aids in understanding the high-energy processes occurring in outer space, such as those associated with neutron stars (see Pulsar), quasars, and black holes.

A positron (the antimatter counterpart of an electron) produces gamma rays when it collides with an electron and the two particles are annihilated. The mass of the two particles is completely converted into energy, and each of the two gamma photons produced has an energy of 511 keV. Gamma-ray observations at this energy thus serve as a means of detecting the presence of antimatter in space. Both this “annihilation line” (in spectroscopy a “line” is a sharply defined wavelength) and a line at 1.8 MeV (1.8 million electronvolts) owing to the radioactive decay of aluminium-26 have been seen from the direction of the centre of the Milky Way galaxy. The aluminium is almost certainly produced in supernova explosions involving massive stars.

II

Detecting and Locating Gamma Rays

Although highly energetic, most gamma rays are absorbed by the Earth’s atmosphere. When this happens the highest energy gamma rays give rise to Cherenkov radiation, a faint glow which can be detected by specially designed optical telescopes at ground level. However, the major part of gamma-ray astronomy has to be carried out above the atmosphere, and the field really began with the advent of satellite-borne detecting devices. The difficulty in obtaining gamma-ray images of the sky is that gamma rays cannot be reflected and focused in a telescope using mirrors (in the way that X-rays and lower-energy photons can). Instead, gamma-ray detectors must either have their view of the sky restricted (for instance, with heavy collimating material such as lead), or derive directional information from the way in which the gamma rays are detected (a gamma ray will produce an electron-positron pair in a spark chamber, and the path of each particle points to the direction of the incoming gamma-ray photon).

III

Compton Gamma-Ray Observatory

In 1991 NASA launched the 17-tonne Compton Gamma-Ray Observatory (CGRO) into high Earth orbit. It carried four instruments designed to observe gamma rays over a very wide energy range (30 keV to 30 GeV, or 30 thousand to 30 billion electronvolts). These operated perfectly for nine years until June 2000, when NASA terminated the mission with a deliberate de-orbit of the spacecraft into the Pacific Ocean. Adding to the list of known gamma-ray sources, such as the pulsar in the Crab Nebula and the binary star system Cygnus X-3, CGRO identified over 400 gamma-ray sources, ten times more than were previously known. These included many different types of objects. For example, the CGRO detected dozens of gamma-ray quasars. Many of these, such as the quasar known as 3C279, are extremely powerful, but highly variable. Active galaxies, powered by supermassive black holes in their centres, were found to represent the largest class of gamma-ray emitters. Young, rapidly spinning neutron stars (pulsars) are also gamma-ray sources. Seven of these were confirmed by the CGRO, including the Crab and Vela pulsars.

IV

Gamma-Ray Bursts

One of the CGRO instruments, BATSE (the Burst and Transient Source Experiment), was designed to study gamma-ray “bursts” (GRBs). These intense bursts of radiation were first detected by the US Vela satellites (originally designed for surveillance of atmospheric nuclear weapons tests) in 1969, but their origin remained a mystery for more than 20 years. A single gamma-ray burst releases more energy than the Sun will release during its entire 4.5 billion-year life cycle. However, as their name implies, these flashes of gamma-rays are very short-lived , and last from a few seconds to a few minutes. On average, at least one GRB is recorded each day. Prior to CGRO, these GRBs were positioned on the sky by the simple method of triangulation (exploiting the finite speed of light) involving their observation by an interplanetary network of satellites (such as the Pioneer Venus Orbiter) operating throughout the solar system. The time at which each spacecraft sees the GRB depends on the direction from which the gamma-rays are coming. BATSE actually consisted of eight detectors, one in each corner of CGRO. They were oriented so that any gamma-ray source would be seen by four of the eight detectors, and the relative intensity detected by each of them enabled BATSE to determine the location of the GRB to within a few degrees. By the end of the CGRO mission, BATSE had detected 2,702 GRBs, whose distribution across the sky was found to be remarkably uniform. Such a distribution (showing no structure associated with our galaxy) could only be explained if these GRBs are either very close to us or very far (in distant galaxies).

The first major breakthrough in our understanding of GRBs occurred in 1997 with the observation of a GRB at optical wavelengths using the 4.2 m William Herschel Telescope at the Roque de los Muchachos Observatory on La Palma. Using the Italian-Dutch X-ray satellite BeppoSAX to observe the X-ray afterglow within a few hours of a gamma-ray burst enabled an accurate position to be quickly obtained and then transmitted to optical astronomers. For GRB 970228 (the serial number indicating its date of discovery, February 28, 1997) in the direction of Orion, a fading object (already below 20th magnitude) was imaged—the first time an object visible at optical wavelengths had been identified with the source of a GRB. Hubble Space Telescope observations of its further decay showed the GRB to be embedded in a “fuzzy patch”, which was interpreted as the GRB’s host galaxy. Spectroscopic observations of another GRB in May 1997, made with the Keck telescope at the Mauna Kea Observatory, provided further evidence that GRBs are at cosmological distances.

The optical fading of GRBs is so rapid that automated (that is, robotic) observing techniques are required for their study. Such an instrument allowed GRB 990123 to be recorded almost from start to finish, allowing astronomers to see the GRB brighten to its peak of almost 8th magnitude within 50 seconds and then start dimming, reaching 14th magnitude (that is, more than a hundred times fainter) within only 10 min. It therefore seems that, to be detectable at such distances, GRBs must represent sources of energy so intense as to exceed by many times the total energy of an exploding supernova (although these energy requirements can be relaxed if the emission is relativistically beamed).

The nature of the event which causes GRBs is still the subject of great debate. One explanation is that they arise when a pair of orbiting neutron stars spiral together and merge, forming a black hole. At least two such systems are already known in our own galaxy and their stars will eventually merge (losing orbital energy by gravitational radiation) in 200 million-300 million years’ time. In early 2003, the 6.5-m (256-in) Multiple-Mirror Telescope (MMT), in Arizona, was used to observe GRB 030329, a burst 2 billion light years away in the constellation of Leo. The MMT provided direct evidence that the afterglow of a burst exhibits the same patterns as light from a supernova, thus further suggesting that at least some bursts originate from supernovae.

In recent years, several spacecraft have been launched to observe the gamma-ray sky, and gamma-ray bursts in particular. These include INTEGRAL (the International Gamma-Ray Astrophysics Laboratory), which was launched by the European Space Agency in October 2002, and SWIFT, a joint NASA-United Kingdom mission launched in 2004. True gamma-ray imaging of the sky became achievable with INTEGRAL, which employs “coded masks”, produced by patterns of heavy absorbing material, that cast a shadow on the gamma-ray detector. The signature of this shadow allows much higher angular resolutions than those previously attainable. INTEGRAL also has X-ray and optical monitors that give it wide wavelength capability. SWIFT can rotate very quickly to point toward the source of the gamma-ray signal. This enables its gamma-ray, X-ray, and optical telescopes to carry out almost immediate follow-up observations of most gamma-ray bursts. As a result of SWIFT’s discoveries and follow-up observations by other observatories, most scientists now agree that gamma-ray bursts can arise from the explosion of a massive star, or from a collision between a neutron star and a black hole, or a collision between two neutron stars.

In June 2008, INTEGRAL and SWIFT were joined by NASA’s GLAST (the Gamma-ray Large Area Space Telescope). One of its instruments, the GLAST Burst Monitor, is able to detect GRBs and, within seconds, identify their location. This information is sent to scientists on the ground, and if the burst is exceptionally strong, the spacecraft can turn so that its extremely sensitive Large Area Telescope can also observe the burst.