Why is Gamma-Ray important to astronomy – by Albert Lim (2000)

Gamma-Ray astronomy is important because of many factors :

1. New Dimension - Prior to the space age, most astronomical observations are conducted through the ‘optical’ or ‘radio’ windows from the earth. Gamma-rays do not penetrate the Earth’s atmosphere and so gamma-ray observations are recent and necessarily conducted from space. Absorption by the Earth’s atmosphere is responsible for the late development of Gamma- ray astronomy as a new branch of astrophysics. Gamma-ray astronomy therefore provides a new dimension for astronomers to rediscover important information about known celestial objects previously not discoverable at other wavelengths.
    
2. Multiwavelength Synergy - Knowledge from intensive studies of celestial objects in other wavelengths has greatly complimented and assist the analysis and interpretation of gamma-ray observations. This multiwavelength synergy also help fuel the growth and importance of gamma-ray astronomy. Gamma-ray observations have been proven to be valuable beyond doubt in multiwavelength approach since objects such as high energy Quasars for example, emit large portions of their power at gamma energy levels. Multiwavelength studies provides information on celestial objects at different given energies. Because different physical phenomena produce different signature radiation, such studies provide crucial information on the physical conditions existing in the objects.
   
   

   Fig 1 : The sky in gamma-rays based on data above 100MeV from the EGRET instrument. The central bar is the MilkyWay

   
    
3. Detect ‘unseen’ objects - Gamma-ray astronomy is also important in enabling astronomers to detect celestial objects otherwise undetectable at other wavelengths. The study of black holes is a good example. The immense gravitational force of black holes prevents light from escaping from it. This makes it impossible for astronomers to observe black holes directly. Fortunately, black holes do emit gamma-rays. These gamma-rays are produced at extremely high temperatures by dynamic friction when materials are swept around an accretion disk before plunging into the black hole. Gamma-ray astronomy therefore not only allow astronomers to detect black holes but also to learn a great deal about them. In addition, gamma-ray astronomy is also continuously providing valuable information in the studies of many other objects such as pulsars, neutron stars and distant galaxies etc.
    
4. Diffuse Gamma-Ray Background – Gamma-ray observations of Seyfert galaxies are extremely important for understanding the diffuse gamma-ray background. Studies of individual seyfert galaxies can be combined with models of how such objects are distributed in the Universe and then compared to the diffuse gamma-ray background. This enables astronomers not only to understand phenomena such as Active Galactic Nuclei (AGN) and Blazars but also the grander scale nature of the Universe as a whole.
    
5. Access Edge of Universe – Gamma-rays constitute the most energetic form of electromagnetic radiation. Because of their high energies, Gamma-rays can retain their energies and travel far greater distances than lower energy photons. This enable astronomers to look very much deeper into space. Such high energy observations are important in helping astronomers understand how matter and radiation interact with each other; particularly under extreme conditions in and around the galactic centre. Studying Gamma-Ray Bursts (GRBs) at the extreme edge of the observable universe might also one day provide important answers to some of the most fundamental questions on the origin of the Universe.