The study of the cosmos in the ultraviolet (UV) has also gained importance in recent times, as several celestial objects emit radiation in this region of the spectrum. Moreover, simultaneous monitoring of both X-ray and ultraviolet regions is considered desirable. The inclusion of a UV detector in ASTROSAT is timely as all foreign UV satellite probes are nearing the end of their design life. UV does not usually produce spectacular images, as it comes from diffuse clouds of gas in the vast void between galaxies, but the gas becomes eventually the stuff of the stars. UV is useful in probing a wide range of physical conditions of matter from the very cold gas (30K) to the hot gas in supernova remnants and corona of stars (10 million K).
The Indian Institute of Astrophysics (IIA), Bangalore, is in charge of the design and fabrication of the ultraviolet imaging telescopes of ASTROSAT. Ultraviolet Imaging Telescope (UVIT) provides the coverage of ultraviolet and visible wavelengths for multi-wavelength observations with ASTROSAT. Ultraviolet radiation is emitted in large quantities by very hot matter, say much hotter than surface of Sun which is about 5500 deg Celsius. In addition to the many stellar objects which emit a lot of X-rays, there is a large variety of objects which emit a lot of ultraviolet rays. For example, star-forming regions of galaxies have young hot stars and emit lots of ultraviolet radiation, and observation of this radiation is an important tool to understand the physical conditions leading to formation of stars.
The Ultraviolet Imaging Telescope consists of two telescopes, each of about 40-cm aperture and it observes simultaneously in three channels: Visible (350 nm to 550 nm), Near Ultraviolet (180 nm to 300nm), and Far Ultraviolet (130 nm to 180nm). The main observation mode is making images of about one half (0.5) degree of sky, simultaneously in the three channels; within any of the channel, a band can be selected by a suitable filter. One of the most important features of this telescope is a relatively high spatial resolution of about 1.8” over a field of about 0.5 degrees. This resolution of ~1.8” can be compared with a resolution of ~5” obtained by Galex, a recent ultraviolet mission, over a field of about 1.2 degrees. A higher angular resolution of UVIT provides an opportunity to see objects with greater clarity.
The total mass of this instrument is about 200 kg, and its size is roughly 80 cm in diameter and 300 cm long. It is being developed as a multi-institutional collaboration. Involving several institutes from India as well as a collaboration with the Canadian Space Agency. The full instrument would be assembled and tested at the Indian Institute of Astrophysics, Bangalore before being sent to ISRO Satellite Centre for further tests, and integration with ASTROSAT spacecraft.
During its life, the UVIT would observe X-ray astronomical sources in coordination with the X-ray telescopes on ASTROSAT, and a variety of other astronomical objects which emit large amounts of ultraviolet.
X-ray Polarimeter Satellite
Photons from celestial objects can provide information on their direction, image, energy (spectroscopy) and the variation in their arrival time on Earth. However, there is a fourth dimension viz. their polarity, which had not been exploited until recently. The real nature of many cosmic objects (e.g. the emission pattern of a pulsar) cannot be known until the polarity of their X-radiation is measured. With a view to filling the gap in the detection of polarity, RRI has made a proposal to ISRO for a small satellite with a X-ray polarimeter on board, to be orbited at 600 km near the Earth.
RRI scientists have done considerable research on the subject and have set up a special laboratory for making the X-ray detector for polarization measurements to be flown in space. Though polarization in optical and radio telescopes is routine, its introduction in a satellite would be watched with keen interest. However, ASTROSAT is not designed for measuring polarity of celestial objects. When flown, the X-ray polarimeter satellite would be the first of its kind in India.