2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

HIGH ENERGY ASTRONOMY
Gamma Ray Astronomy
Gamma ray astronomy covers a very wide region
of the electromagnetic spectrum - roughly 12
decades in energy from 100 keV to 100 PeV and
beyond. Almost all the cosmic gamma rays at
energies less than about 10 GeV impinging on the
Earth are absorbed in the terrestrial atmosphere
before reaching the ground and are accessible for
observation only through balloon and satellite
borne instruments. The higher energy gamma rays
(>10 GeV) could be studied with great sensitivity
from the ground using the atmospheric Cherenkov
technique. In this technique, the gamma rays are
detected through the Cherenkov radiation emitted
by the charged particle secondaries in the electromagnetic
cascade shower initiated by the primary
gamma ray in the upper atmosphere.
There have been pioneering efforts in India in
high energy gamma ray astronomy. The early
efforts date back to the late 1960s.Soon after the
discovery of a pulsar in the Crab Nebula, it was
realized that Crab could be emitting very high
energy gamma rays at a flux level that could be
detected. Two groups, one led by B.V. Sreekantan
and P.V. Ramanamurthy of the Tata Institute of
Fundamental Research and the other led by
The TeV gamma ray experiment at Ooty
H. Razdan of Nuclear Research Laboratory (NRL)
of Bhabha Atomic Research Centre (BARC) set up
experiments to detect TeV energy gamma rays
from celestial objects.
The TIFR group started with just two search light
mirrors mounted on an orienting platform at Ooty
in 1969. This set up was used till 1985 by steadily
increasing the number of mirrors and improving
the detection methods. A systematic search was
made for pulsed emission from a number of
pulsars, including the Crab and Vela pulsars. In the
year 1986, the set up was shifted to Pachmarhi in
Madhya Pradesh since it had better night sky
conditions.
The wavefront sampling Cherenkov array at
Pachmarhi
The BARC group started its high energy gamma ray
astronomy observations in 1970 at the High
Altitude Research Laboratory in Gulmarg. This
commenced with the setting up of an atmospheric
scintillation experiment for detecting prompt
gamma ray emission from supernova explosions
and primordial black-hole evaporation. The
experiment was also used to detect atmospheric
Cherenkov pulses initiated by primary cosmic ray
hadrons and cosmic gamma rays of energy ~ 0.5 PeV.
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