India’s first space observatory, successfully launched last Sept. 28, captured the Crab Nebula, the brightest hard X-ray source found in the sky.
Using the Cadmium Zinc Telluride Imager (CZTI) instrument, ASTROSAT detected the Crab Nebula – remnant of the Supernova identified by Chinese astronomers in the year 1054 – last Oct. 9. Both the operation centers at Peenya in Bengaluru and Inter-University Centre for Astronomy and Astrophysics IUCAA in Pune made the discovery, with the Indian Space Research Organization showing the stunning blue image on its Twitter account.
In its first week of operation, CZTI continually stared at the Crab Nebula, which is often used by researchers for calibrating hard X-ray detectors. The black hole source Cygnus X-1, too, was observed for two days.
India is the fourth nation to launch a space observatory satellite, following the United States, Russia, and Japan.
Prior to detecting the Crab Nebula, scientists experienced a “nerve wracking period” that seemed to span an eternity but actually was only three days short of the celebrated discovery.
According to Dr. Varun Bhalerao, post-doctoral fellow at the IUCAA, they would get a number of background photons if they were hardly looking at the source. The background photons, however, “were a lot more than anticipated,” he recalled.
Dr. Bhalerao explained that when a cosmic ray hits a particular matter, such as the telescope, it produces a lot of photons because of cosmic ray interaction. What would then be seen as a single photon turns into 10 – a shower, so to speak. The signal that the Crab Nebula was emitting was hence filled with noise and therefore the scientists failed to spot it in the first orbit.
The ASTROSAT soon afterwards made a passage through the South Atlantic Anomaly (SAA) region while the Crab Nebula was within field of view. SAA is the magnetic field anomaly area of Earth, thus plenty of noise every time a satellite journeys through the region.
The SAA region could also be damaging to the instruments, which was why all instruments had been turned off when the ASTROSAT passed through it.
A couple more orbits and they finally saw the Crab, by which time the experts had been able to “suppress the noise” and spot the signal coming from the Crab. “[W]e can now see the Crab from parts of all orbits. In hindsight, it becomes easier,” said Dr. Bhalerao.
ASTROSAT is poised to observe black holes such as GRS 1915+105 and Cygnus X-3 in November, after which it will take a peek at other celestial bodies. In the next few weeks, its other X-ray instruments will also become operational – in a month, CZTI, LAXPC, and SXT instruments will be geared to look “at interesting stars.”
