by S. Prasad Ganti

The cold war between the US and the Soviet Union gave some insights into some of the violent cosmic explosions in our Universe. In the 1960s, US launched the Vela satellite to detect any nuclear tests performed by other countries, notably the Soviet Union. Typically it involved looking for Gamma rays, which  is the radiation at the highest end of the spectrum of electromagnetic waves. 

Gamma rays are the most energetic of all the electromagnetic radiation. The energy of the radiation is proportional to the frequency. Gamma rays having the highest frequencies, have the most energy. 

In 1967, the Vela satellite detected an intense burst of Gamma rays lasting for a few seconds. Chaos prevailed in the Pentagon for a few days, not knowing where the radiation was coming from.Was there any indication that it came from any nuclear test on the earth or from some source in outer space. Whether to react or take a risk of letting go of a nuclear test by any country. 

Further investigations revealed that the burst came from outer space. Its terrestrial origins were ruled out. Several such bursts have been detected since then. All of them coming from outside of our galaxy Milky Way. Now it is known that Gamma Ray Burst indicates the birth of a black hole, either from a hyper nova resulting from the collapse of a rapidly spinning massive star,  or a binary pair of neutron stars collapsing into a black hole, or a highly magnetized neutron star called a Magnetar “falling” into a black hole. Something similar to conditions which lead to the generation of gravity waves. 

Gamma ray bursts are short lived. From a few seconds to a few days. But very intense, outshining the entire output of solar systems or even galaxies during these brief time intervals. Why were they not detected earlier than the cold war period ? Mainly because gamma rays cannot penetrate earth’s atmosphere. Of the electromagnetic radiation, earth’s atmosphere lets only radio waves and visible light reach the surface. Absent such a protective blanket, no life would have been possible on earth. Gamma rays being very energetic are very harmful for life on earth. Gamma rays can be detected in space easily. That is why we had to wait till the launch of a gamma ray detector to space against the backdrop of the cold war.

All the Gamma ray bursts detected so far have come from distant galaxies. None from the  Milky way. Such violent conditions are very rare for a given location. But since the Universe is so vast with billions of galaxies each with billions of stars, the probability of some event happening somewhere is high. It is theorized that a Gamma ray burst in our own Milky way, about 6000 light years away in one of its arms, was responsible for the first mass extinction of life experienced on earth. Around 430 million years ago, before plants and animals  moved to the land, much before the dinosaurs ruled, this extinction took place. Called Ordovician extinction, the unconfirmed cause depleted the ozone layer due to the Gamma ray burst and exposed the Ocean life to the deadly gamma rays. But no evidence exists to support this theory. Alternate theory is the glaciation and cooling down of oceans could have led to this event. It is just too much in the past to leave any direct concrete traces which last till the current times.

In the 1990s, there was the Compton Gamma ray observatory and now the Fermi Gamma ray telescope in space studying the gamma rays. On the ground, techniques have been developed to detect gamma rays. Whenever gamma rays strike the earth’s atmosphere, they produce electron-positron pairs from the air molecules. As these pairs streak through the earth’s atmosphere, they produce something known as Cherenkov radiation. This radiation can be studied to determine the direction and the source of  gamma rays.    

This is another example of astronomy branching out beyond the visible light.