The Study Of Gamma Rays

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Unlike ordinary stars that emit mostly low energy photons as light, many exotic astronomical sources generate photons of various energies which reach us as radio waves (nano electron volts or nanoeV) to X-rays (kiloeV) and gamma rays (billion eV). Gamma ray sources include pulsars quasars, x-ray binaries and active galactic nuclei. Gamma rays will go through mirrors and hence special detectors are made to catch them.

Gamma rays are at the most energetic end of the electromagnetic spectrum. They are produced not by thermal process but by spectacular events such as stars exploding, matter falling into black holes and celestial objects colliding. Astronomers want to see how these events shape the Universe. When individual stars blow themselves up into pieces, some chemical elements are created and gamma rays can detect them.

Another mysterious phenomenon is the Gamma Ray Bursts (GRBs), which occur at random. Probably they arise from the collision of neutron stars or a merger of a neutron star into a black hole. Gamma rays will allow astronomers study bizarre objects such as black holes, and Neutron stars that are so dense that a teaspoonful of matter would weight millions of tones on Earth.

In the mid-1980s, the Tata Institute of Fundamental Research (TIFR) Mumbai conducted pioneering experiments in catching gamma rays at Oootacamund, while the Bhabha Atomic Research Centre (BARC) did some studies at Gulmarg in Kashmir. Later, the detectors were moved to Pachmarhi in Madhya Pradesh and Mt. Abu in Rajasthan. The arrays detected gamma rays in pulsars and X-ray binaries.

The Indian Space Research Organisation designed, made and launched two experimental satellites in the early 1990s to study gamma ray bursts. SROSS-C and –C2 were launched in 1992 and 1994, respectively. The mission life of SROSS-C was 55 days, while the other satellite was designed for five years. The satellites monitored GRBs in the energy range of 20 Ke V to 3 Me V as well as their intensity variations over a short period. SCROSS payloads were not designed to participate in any rapid response campaign or conduct multi-wavelength observations.

Gamma Ray Bursts

A gamma ray burst occurs only once in a million years in a galaxy. Since there are millions of galaxies other than our own, as many as three a day can be observed across the world with an all-sky detector. GRBs remain one of the biggest mysteries in modern astronomy, since their discovery in the 1960s. They have more energy than what the Sun could produce in 80 billion years! Yet the bursts disappear within seconds or at times last only a few minutes. The so-called long-term blast lasts more than 2 seconds and short-term ones stay on only for milliseconds. The short bursts especially remain a complete mystery. They are away at more than 10 billion light years from Earth. The flow is believed to have a velocity greater than 99.99 per cent of the speed of light. One theory holds that expansion is driven by its internal energy. Another theory states that the outflow is due to electromagnetic energy.

Where Seconds Count!

Here is a field of astronomy where seconds really matter. The satellite, Swift Gamma Ray Burst Explorer, launched in 2004, is good at catching instantaneously such fleeting phenomena at almost any point in the sky. Once it detects a GRB, it can turn on its X-ray and ultraviolet optical telescope and pinpoint the explosion within 20 seconds. For the first time, swift caught a star exploding into a supernova in real time in January, 2008.

Astronomers are puzzled by the duration of the bursts. Even before Swift, the long GRBs were fairly well understood. The afterglows continued for days and ground-based telescopes could identify the sources. Massive stars—with some 40 times the mass of our Sun—die, throwing up the energy as magnetically driven polar jets at almost the speed of light. The explosions are ten times more energetic than a typical supernova (star burst).

Short GRBs still pose a puzzle. The theory is even stranger! After millions of years, a neutron star and a black hole merge to spawn a short GRB; or perhaps when two neutron stars collide with each other. Short GRBs represent the ‘last gasp of energy’ released in such mergers. They orbit for millions or billions of years and when they merge, a single black hole is formed in a few hundredth of a second!

On May 9, 2005, a 0.03 second GRB left an afterglow for five minutes, when ground-based telescopes traced its location in an elliptical galaxy 2.7 billion light years away. More short-burst afterglows were soon detected. For the first time, NASA detected 50 millisecond gamma ray bursts, sometimes described as the birth cries of black holes. In September 2005, swift observed the farthest GRB ever seen; the event was considered to have occurred only about 900 million years after the black hole was formed. Only a few quasars have been found at these distances. The discovery proves that massive primitive stars coexisted with quasars—super massive black holes—in the era when stars and galaxies first formed.

Long duration GRBs are supposed to arise, when the first stars—giants made of almost pure hydrogen and helium—collapse as supernova. Swift is expected to detect even older bursts, taking us to the time when the first stars were formed. Nearly half the gamma ray bursts were followed by several X-ray explosions, as matter falling into new black hole releases energy.

The European Space Agency’s Integral (International Gamma-ray Astrophysics Laboratory), launched in October 2002, plays a key role in gamma ray astronomy. This is followed by GLAST (Gamma Ray Large Area Space Telescope), launched by NASA in June 2008 GLAST has neither lenses nor mirrors; it uses the technology adapted from ground-based particle accelerators.

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