Astrosat telescope sees first light

Image of PKS 2155-304, an X-ray source belonging to an enigmatic class of supermassive black holes in a galaxy 1.5 billion light years away.

The Soft X-ray focusing Telescope (SXT) onboard Astrosat, India’s first satellite dedicated to astronomical observations that was launched by the Indian Space Research Organisation (ISRO) on September 28, saw its first light from an astronomical source on October 26 after the camera door was opened at 06:30 UT 12.00 hrs IST). The telescope door covering the optics had been opened 10 days earlier.

According to a November 2 release issued by the Tata Institute of Fundamental Research (TIFR), Mumbai, for its first light, Astrosat was pointed at a target, PKS 2155-304, a special type of quasar known as BL Lac type object. The targeted object is an X-ray source, belonging to an enigmatic class of supermassive black holes in a galaxy very far away (1.5 billion light years away), which shoots out powerful jets of highly accelerated particles at speeds near the speed of light and pointing in a direction quite close towards the earth. The light from the jet overpowers the light from the galaxy and can be seen at almost all wavelengths from radio to very high-energy gamma-rays.

Astrosat is designed to carry out simultaneous observations in the UV, optical, low and high-energy X-ray emission from celestial objects and is expected to be a powerful tool for measuring the spectral energy distribution and time variability of compact sources such as white dwarfs, neutron stars and black holes, including the supermassive black holes at the centre of galaxies.

The SXT is India’s first X-ray telescope based on doubly reflecting grazing incidence optics, containing 320 mirrors assembled together in two sets of 40 co-axial shells. The golden mirrors and the precision structure for assembling them were built at TIFR. All the mirrors assembled at different radii from the central axis were aligned perfectly to image a single point, and the entire telescope was then further aligned with an X-ray camera. The camera was previously assembled independently at the University of Leicester, United Kingdom, and delivered to TIFR. The electronics to control the Charge Coupled Device (CCD) in the camera and its thermal control, and the commands to read data in different modes of operation were made at TIFR.

The data received at the ground station in Bengaluru and being analysed at TIFR show that the telescope and the camera are operating perfectly and data quality is excellent. “This is the moment we have been waiting for anxiously after the satellite was launched. It is good to see all these mirrors working in perfect harmony to create an X-ray image on a camera containing a very small CCD situated 2 metres away from the mirrors. The CCD is working at a controlled temperature of – 82o C, and we have been able to achieve the sensitivity and the spatial and spectral resolution as per our specifications”, says K.P. Singh, the project manager of the team that developed the SXT.

The satellite is undergoing a phase of calibration and performance verification over the next six months. It has already observed several interesting objects in the sky in this phase with the other three instruments built in TIFR. All the four instruments built at TIFR [Charge Particle Monitor (CPM), Cadmium Zinc Telluride Imager (CZTI), Large Area Xenon Proportional Counters (LAXPCs), and SXT] are now functioning well.

R. Ramachandran