ASTROSAT MISSION

I. The principle scientific objectives of ASTROSAT mission are:

Multi-wavelength studies of cosmic sources from ultraviolet band (1200-3600 A) using a UV imaging telescope (UVIT) system to soft and hard X-ray bands covering 0.3-100 keV energy range using 3 different instruments i.e., LAXPC, SXT and SSM (Details of payloads given below)

Low and medium resolution broad-band x-ray spectroscopic studies of different types of cosmic x-ray sources

Sky survey of intensity of known x-ray binaries, AGNs and other variable sources as well as new transient sources,

Ultraviolet imaging and spectroscopic studies of a variety of galactic and extra-galactic sources including stellar objects, star burst galaxies, AGNs etc.

II Basic features of instruments:

Energy Coverage

· Large Area Xenon Proportional Counter (LAXPC) for X-ray energies 2-100 keV
· Soft X-ray Imaging telescope (SXT) for X-ray energies 0.3-8 keV
· Scanning X-ray Sky Monitor (SSM) for X-ray energies 2-10 keV
· Ultraviolet Imaging Telescope (UVIT) for UV range 1200-3600A

Detector Systems

- LAXPC: A bank of three large area xenon-filled proportional counters with a field of view of 1^o x 1^o and total effective area of at least 6000 cm² for timing and low resolution wide spectral band X-ray detection in 2-100 keV band. A Cadmium-Zinc-Telluride (CZT) Array for X-ray imaging would be added for higher spectral resolution in the energy range.

- SXT: A soft x-ray imaging telescope based on use of conical foil mirrors and a charged couple device (CCD) at its focal plane for medium resolution x-ray spectroscopic studies in 0.3-8 keV band

- SSM: A scanning X-ray sky monitor for monitoring x-ray transients, x-ray binaries and other variable sources in 2-10 keV band

- UVIT: A 40 cm aperture twin ultraviolet imaging telescope (TBD) with photon counting detectors at its focal plane and a visible imaging channel.

III Details of Science Goals

To carry out multiwavelength observations covering spectral bands from radio, optical, IR, UV, X-ray and Gamma ray regions both for study of specific sources of interest and in survey mode. While radio, optical, IR observations would be co-ordinated through ground based telescopes, the high energy regions, i.e., UV, X-rays and Gamma rays would be covered by the dedicated satellite borne instrumentation of ASTROSAT.

To study near simultaneous mutiwavelength data from different variable sources. In a binary system for example regions near the compact object emit predominantly in X-rays, the accretion disc emitting most of its light in the UV/ optical waveband, and the mass donating star being brightest in the optical band.

Carry out (a) low to moderate resolution spectroscopy over wide energy band with the primary emphasis on studies of X-ray emitting objects, (b) Timing studies of periodic and aperiodic phenomenon in X-ray binaries, (c) Studies of pulsations in X-ray pulsars, (d) QPOs, flickering. Flaring and other variations in X-ray binaries, (e) short and long term intensity variations in AGNs, (f) time lag studies in low/hard X-rays and UV/optical radiation, (g) detection and study of x-ray transients.

IV. Requirements:

- The preferred orbit for the satellite will be equatorial (I=~20⁰) at an altitude of 500-

600 km. This would minimise satellites time over the SA anomaly and provide a

mission life of ~5 years

- Absolute pointing of the UV telescope will be ~3' arc, drift rate at 1" arc/sec and an

angular resolution of 1" arc.

V. Why "Multi-wavelength Astronomy"?

Simultaneous optical, ultraviolet (UV), radio and X-ray observations provide valuable data to infer the nature of the objects, their structure and environment. A few examples are:

The nature of the compact stars in X-ray binary sources and the role of gravitational energy released by accretion of matter onto the compact object in producing the high energy photons could be understood when the binary X-ray sources were identified with their optical counterparts.

Nature of transient X-ray sources could be understood when X-ray observations are
supplemented by radio and optical studies.

Radio and infrared studies revealed the presence of jets in superluminal sources GRS 1915+105 and GRO J1655-40 which brought out their similarity to quasars and they have been
termed as "micro-quasars".

The presence of black-hole in several X-ray transients has been inferred from determination of their mass function from the optical studies.

Simultaneous X-ray, radio and infrared observations of the black hole candidate GRS 1915+105 have revealed that the radio and infrared radiation is produced by synchrotron emission from a plasma cloud ejected from the inner accretion disc around the black-hole.

Recent discoveries of many transient X-ray sources and optical studies of the companion stars, have now provided more credible evidence for the existence of black holes in many X-ray binaries. The most notable black hole binaries discovered recently are GS 1124-68 (Nova Mus 1991), GRO J1655-40 (Nova Sco 1994) and GRO J0422+32 (Nova per 1992) for all of which the mass of the X-ray source is measured to be in the range of 3.6 to 5.4 M_¤, and therefore most likely to be black-holes.

The field of accretion physics developed rapidly after the discovery of accreting X-ray binaries. The most rapid brightness variations among all the wavelength bands down to a time scale of 0.1 ms, have been observed in X-ray binaries. The X-ray binaries have, therefore, been at the forefront of research in X-ray astronomy.