2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

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CHAPTER 6 These first set of observations provided corroborative evidence for the reported ultra high energy gamma ray emission from the X-ray binary Cygnus X-3. These early studies were consolidated with the commissioning of the multi-element atmospheric Cherenkov telescope in Gulmarg in 1985. The Gulmarg γ-ray telescope was operated almost continuously during the period 1985–1990 and produced a number of important results. The early TeV gamma ray experiment in Gulmarg In 1992 the BARC group shifted its activities to a new site at Mount Abu in Rajasthan. Here, they embarked on a new phase of consolidation of the gamma ray activities by initiating the project GRACE (Gamma Ray Astrophysics through Coordinated Experiments) which will consist of 4 independent gamma ray telescope systems to cover the entire gamma ray energy band of ~ 100 GeV to 100 TeV. The TACTIC array (TeV Atmospheric Cherenkov Telescope with Imaging Camera), the first imaging Cherenkov telescope in the country, was successfully commissioned in April 1997, with a prototype 81-pixel PMT-based camera. During its maiden run, the imaging telescope successfully detected TeV gamma ray flaring activity from the active galaxy Mkn 501 which suddenly became ‘active’ in 1997 after being dormant for several years. Gamma Ray Astronomy at lower energies Although gamma ray astronomy in India began at very high energies, there has been a significant SROSS-C2 satellite launched by India’s ASLV rocket on May 4, 1994 effort at lower energies. In the energy range below 1 GeV, the Gamma Ray Burst experiment on the SROSS-series of Indian satellites (the most successful one being SROSS-C2, launched in 1994; re-entry 2001) pioneered by the scientists at the ISRO Satellite Centre has produced some interesting results. The GRB experiment on SROSS-C2 worked well throughout the mission, yielding 56 bursts and provided an independent sample of events to address the overall properties of the GRB population and cross-verify the findings from the BATSE detector on the Compton Gamma Ray Observatory. Some of the important results obtained from this experiment include the independent establishment of the bimodal distribution of bursts with respect to duration, determination of the V/Vmax for the burst distribution indicating a non-homogeneous Gamma Ray Burst detector designed and built by the ISRO Satellite Centre launched onboard the SROSS-C2 satellite 81
HIGH ENERGY ASTRONOMY A gamma ray burst light curve detected by SROSS-C2 distribution of bursts and establishing the correlation between spectral hardness and burst intensity. In the MeV–GeV range, a significant contribution is the detection of spectral hardening in gamma ray blazars and its implications for the extragalactic gamma ray background. Currently existing facilities • Pachmarhi Array of Cherenkov Telescopes This is an array of 25 telescopes called PACT. It consists of a total of 175 large area reflectors spread around an area of 8000 m 2 . and uses wave-front sampling technique for enhancing the Signal-to-Noise ratio. The present sensitivity of this array for a [5 σ] detection at a threshold energy ~ 1 TeV has been estimated to be 2.2 × 10 –12 photons/cm 2 /s for an on-source exposure of 50 hours. • The Mt. Abu TACTIC Imaging Cherenkov Telescope The imaging camera of the TACTIC system has been upgraded to its full size of 349-pixels. Comprehensive Monte Carlo simulations of the system performance have been carried out to optimize the γ-proton segregation algorithms for improving the γ-ray detection sensitivity of the system. The sensitivity of TACTIC for a [5 σ ] signal in ~ 50 hours on source is ~ 5 × 10 –12 photons/ cm 2 /s at E > 0.5 TeV. • The balloon launch facility in Hyderabad Some significant results in gamma ray astronomy The field of Ultra High Energy Gamma Ray astronomy is still in its infancy. Statistically significant detections have been rather few till recently. And so it has been with our efforts in 82
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ASTRONOMY & ASTROPHYSICS A Decadal
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This publication of the Academy on
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Preface The Indian Academy of Scien
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The Academy Committee for Astronomy
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h e P a n e l s Theoretical Astroph
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C H A P T E R 1 Executive Summary
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EXECUTIVE SUMMARY The recommendatio
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EXECUTIVE SUMMARY feasibility study
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C H A P T E R 2 The Present Revolut
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THE PRESENT REVOLUTION IN ASTRONOMY
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A large eruptive prominence in HeII
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SOLAR PHYSICS • Detailed study of
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Page 47 and 48: C H A P T E R 4 Optical, Infrared &
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Page 105 and 106: C H A P T E R Theoretical Astrophys
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Page 113 and 114: C H A P T E R Gravitation and Parti
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S.N. Bose National Centre for Basic
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Indian Academy of Sciences C. V. Ra
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2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

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