2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

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CHAPTER 6 ASTROSAT: A major observatory under construction ASTROSAT is an Indian multiwavelength astronomy satellite planned for launch in early 2007 into a circular 600 km orbit with inclination of < 15 degree. The spacecraft and the launch vehicle are based on India’s successful IRS spacecraft and Polar Satellite Launch Vehicle (PSLV). Planned to carry five instruments, ASTROSAT will have a lifetime of about 5 years. energy response and simultaneous observations in the ultraviolet will help one to identify and quantify contributions of different components (e.g., thermal, non-thermal, black-body, thermal- Compton, spectral lines) and thus enable us to understand the nature of the sources and astrophysical processes in them. This unprecedented combination of simultaneous multi-wavelength and broad-band X-ray capability for spectral studies, and time-resolved spectroscopy of thermal as well as non-thermal plasmas in the universe will play a significant role in exploring the physics of AGNs of various types, X-ray binaries, Cataclysmic Variables, etc. A brief description of the 4 X-ray instruments and an outline of the science expected from them is given below. LAXPCs Deployed configuration of ASTROSAT The payloads being developed for the satellite are: • LAXPC (Large Area Xenon Proportional Counters), • CdZnTe Imager, • SXT (Soft X-ray Telescope), • SSM (Scanning Sky Monitor), and • UVIT (UV Imaging Telescopes) – this is discussed in the section dealing with optical, IR & UV astronomy. The first 3 are being made at TIFR/ISRO, the SSM at ISRO/RRI, and the UVIT at IIA/IUCAA/ISRO. With a broad spectral coverage in X-rays, from 0.3 to 100 keV, ASTROSAT will be a unique mission during the time frame of 2007–2010 A.D. Its wide-band X-ray coverage with overlapping Three Large Area X-ray Proportional Counters (LAXPCs), each with an effective area of at least 2,000 cm 2 and a total minimum effective area of 6,000 cm 2 , will be flown aboard the ASTROSAT. A large depth of 15 cm of the LAXPC Large Area Xenon Proportional Counter 77
HIGH ENERGY ASTRONOMY detectors, and high gas pressure of 2.5–3.0 atm, along with the large geometric area will make it a very sensitive instrument in a wide energy band of 2–80 keV. In the 20–80 keV energy band, the proposed LAXPC detector array will have an effective area that is about 5 times larger than that of the present largest instruments in this energy band: the PCA and HEXTE detectors onboard Rossi X-ray Timing Explorer (RXTE). Large effective area, high detection efficiency over a broad energy band and high time resolution (10 ms) will enable us to explore new phenomena in the hard X-ray band in a variety of astrophysical objects ranging from the nearby stellar-mass Galactic X-ray binaries to clusters of galaxies. To give a few examples, • Measurement of magnetic field strength of the X-ray pulsars, Anomalous X-ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs) through the detection of cyclotron absorption lines, • Discovery of new Accreting Millisecond X-ray Pulsars, • Accurate measurement of the spectrum of the Cosmic X-ray Background (CXB) in the hard X-ray band, • Study of nonthermal emission component in the cluster of galaxies etc. CdZnTe Imager Multiwavelength coverage of ASTROSAT A comparison of the effective area and spectral resolution of the LAXPC and CZT with other X-ray observatories Proportional counters and scintillating crystals are the commonly used X-ray detectors in astronomy for hard X-rays as these can be made into large areas relatively easily. Recently developed solid state detectors CdZnTe (CZT), however, offer improved energy resolution (3–5% at 60 keV) and better detection efficiency, particularly at higher energies. At TIFR, a 1000 cm 2 CZT detector array with a pixel size of 2.5 mm × 2.5 mm and thickness of 5 mm is planned to be built for the ASTROSAT. The pixellated detector plane will be used in conjunction with a Coded Aperture Mask made from 1mm thick Tungsten. The detectors would be used in a spectroscopic mode in the energy range of 5–100 keV, and above 100 keV the array would be operating as an all-sky gamma-ray detector with limited imaging capability. 78
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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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RECOMMENDATIONS • The committee s
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List of addresses Radio Observatori
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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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