Astronomy Principles and Practice Fourth Edition.pdf

γ -ray astronomy 389
Figure 23.9. A sectional view of the COS B γ -ray detector equipment with a spark chamber (SC), a Čerenkov
detector (C) and other photomultipliers (P) attached to scintillation systems. The enclosing dome (A) provides an
anti-coincidence detection system.
Several early satellites have carried γ -ray detectors, notably Explorer II, OSO III, SAS II and
COS B. The detection system on board SAS II, which was operational for six months in 1972/73 was
centred on the 100 MeV region. Some 8000 photons were recorded during its operation. The COS B
satellite was launched in August 1975 and the system was operational for about six and a half years
before the gas supply ran out. A cross section of the COS B γ -ray detector is depicted in figure 23.9.
Overall, some 200 000 γ -ray photons were detected with an angular precision of a few degrees. It is of
interest to note that in many optical measurements the same number of photons might be recorded in
one second! COS B detected about 25 sources, some of which have been identified optically.
A giant step forward occurred in April 1991 with the launch of the Compton γ -ray observatory.
Unlike earlier missions, Compton covered a broader band of energies from 30 keV to 3×10 4 MeV with
10 times higher sensitivity and greatly improved angular resolutions and timing capabilities. One of the
on-board packages, BATSE, designed to investigate the γ -ray burster detected a new source virtually
every day, the 2000th event being recorded by the end of 1997, revealing a random distribution over
the sky.
One of the aims of such telescopes is to be able to obtain a fix of any burster with sufficient
accuracy to allow immediate follow-up of observations in other wavebands. In 1997, the BeppoSAX
telescope detected a γ -ray burst that was identified optically within hours with a spectrum obtained by
the KECK telescope suggesting that the event involved the most energetic phenomena at the limiting
edges of the Universe.
23.3.2 γ -ray spectral lines
γ -rays may be generated by a variety of mechanisms such as matter–antimatter annihilation,
radioactive decay, energetic particle collisions, synchrotron radiation (high-energy electrons being
deviated by interstellar particles), Bremsstrahlung (high-energy electrons being deviated by a magnetic
field) and by hot plasmas. Some other mechanisms involve the production of γ -rays with discrete
energies so allowing positive identification of the process.
The most famous γ -ray line emission is that at 0.511 MeV resulting from an e − e + collision at