Unlike the steady hissing sound of quasars, bursts of radio noise like Morse code telegraph signals are heard from some points in the sky, if one observes it through a radio telescope. The source of such radio pulses has been traced to a completely new kind of celestial objects, called pulsars. They were first noticed in 1967 by a research student in Cambridge (UK), Ms Jocelyn Bell, on the new radio telescope constructed by Prof. Anthony Hewish. Resembling flashes from a lighthouse, four sources were detected by the time the new class of objects was announced in 1968. At first, some people imagined the flashes to be signals from an alien civilization. The objects were described as Little Green Men!
Astronomers hastened to explain it in terms of nuclear physics. They said pulsars are rotating neutron stars. A neutron star is one which has exhausted its inner energy and has acquired such extraordinary density that its protons and electrons end their separate existence and become neutrons. A typical neutron star may be only 10 km across but with an unbelievable amount of matter (some ten million, million g/cu cm) where a pin would weigh a tonne! Such density explains how they remain in one piece despite their rotation. If an ordinary star rotated even once a second, it will disintegrate. But pulsars rotate rapidly, ranging from once every few seconds to several times a second.
When is most remarkable is the regularity of the pulse period for a given pulsar, though its intensity varies unpredictably. A single pulse, for instance will last 33 milliseconds, if the pulsar rotates 30 times a second. The pulsing period is fantastically constant to within a few tens of nanoseconds (billionth of a second) per annum. The pulsars seem to be Nature’s most accurate clocks in the sky.
The giant radio telescope at parkes in Australia with its 13 beams operating simultaneously discovered the 1000th pulsar within our Galaxy. About 1800 pulsars have so far been discovered.
Astronomers maintain that hundreds of thousands of pulsars must exist in the Milky Way galaxy. Pulsars seem to gravitate towards the galactic equator and occupy a disc-shaped region which is estimated at about 2,000 light years in depth and 40,000 light years in radius. The Sun, which is believed to be 30,000 light years away from the galactic center, seems to be near the outer boundary of the pulsars.
The Rossi X-ray Timing Explorer located the first millisecond pulsar that emits X-rays. It is accreting material and hence spinning up and not slowing down like the regular radio pulsars. The fastest millisecond pulsar (1.55 millisecond) has also been discovered.
Russian astronomers have claimed that one of the brightest pulsars has two planets orbiting it. The pulsar (PSR 0329 + 54) is in a faint constellation close to Ursa Major and Cassiopeia. Based on the bursts emitted by the pulsar, the orbital periods of the planets are believed to be 140 and 1,110 days, respectively. The fist, PSR 1257 + 12 in the constellation Virgo, was also reported to have two planets, one circling every 67 days and the other, every 98 days. The planets evolved from the debris of the explosion of a star, as pulsars themselves are believed to be relics of supernova explosions.
An extragalactic pulsar has been optically identified in the Magellanic Cloud, a satellite galaxy of our galaxy. The region has cloud of dust and gas, the remains of a supernova explosion. The Einstein satellite showed that pulsed X-rays were emitted by the pulsar. In sharp contrast, the least luminous-ever pulsar has also been recorded, only 280 light years away. More such dim pulsars are expected. In 1992 astronomers reported another nearby pulsar named ‘Geminga’ (450 light years away) which is extremely faint.
The Crab Nebula has one of the fastest and youngest pulsars. Initially, pulsars could be identified only at radio frequencies (100-1,000 MHz), but other types of pulsars were discovered which put out X-rays and gamma rays too. A peculiar phenomenon was noticed when a pulsar-like object was revealed only by its X-rays and not by its radio frequencies. It is thought that the radio waves are swamped by X-rays arising from the transfer of matter from a neutron star to its companions.
Astronomers who had thought that pulsars are generally solitary objects were surprised, through they could not observe the twins. However, before long, some pulsars revealed their companions and their mutual relations. The first of three such pulsars, code-named PSR 1913 + 16, was found in 1974, with a companion star which was a compact objects. As predicted by the general theory of relativity, close binaries should lose energy by gravitational waves over a period. As the rapidly spinning star radiated its energy, it was thought that the pulsar would slow down. Accordingly, the orbit of this pulsar, which had 1.4 times the mass of the Sun, was supposed to diminish by about 1.2 seconds after five years. When observations were made, astronomers were delighted to find their prediction becoming true. Einstein was against proved right. Perhaps this is the fist (and so far only) experimental evidence to prove the existence of gravitational waves in space.
The first pulsar discovered by GMRT has a rotational period of 4.99 milliseconds and an orbital period of 18.8 days GMRT also discovered the first known pulsar in the globular cluster NGC 1851 and a binary millisecond pulsar. GMRT discovered a new pulsar in a supernova remnant, whose image was taken by Chandra X-ray telescope. The pulsar, second only to the one in Crab Nebula in luminosity has a rotational period of 61.96 milliseconds.
In 2004, an international team including Indian astronomers announced the discovery of the fist known double pulsar. The pair orbited each other every 2.4 hours. One pulsar rotated faster than the other, which wobbled.
Astronomers D.C. Backer, Shrinivas R. Kulkarni and their colleagues in the University of California found that a pulsar, PSR 1937 + 214, was not typical as it rotated at an astonishing rate of 642 times a second, each pulse lasting exactly 1.5578 millisecond. This is far more rapid than the pulsar in the Crab Nebula that rotates only at 30 times a second. The astronomers suspect that it could be a new class of pulsars.
In June 2008, Arecibo telescope found a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric 95-day orbit around a solar mass companion spinning rapidly in a circular orbit.
A new class of pulsars that ‘Blinks’ only in gamma rays has been discovered (2008).
In February 1987 a star in the Large Magellanic Cloud, a satellite galaxy of the Milky Way—1, 69,000 light years away exploded. Termed Supernova- 1987A, it was the nearest and brightest supernova since Kepler saw one in 1604. It was also the first bright star explosion since the invention of the telescope.
There was also a brief period of radio emission following the explosion. Then there was silence. A satellite (International Ultraviolet Explorer) observed a shell of gas around the supernova. Three months later, as predicted by astronomers, there were broadcasts, heard first on Australian radio telescopes. It looked as if there was a pulsar up there, radiating massive amounts of radio energy into space. But no pulsar has yet been found, despite intensive searches.
Astronomers have reported interactions between the stellar material ejected before the explosion and the surrounding ring-shaped nebulae formed after the flare-up.
Possible Use of MST Radar facility
About 32 km from Tirupati in Andhra Pradesh is a radar for atmospheric research. Called the Mesosphere, Stratosphere, Troposphere (MST) Radar facility, it has indigenously designed, developed and commissioned under a project funded by a number of government agencies. The MST radar provides continuous data on atmospheric winds and turbulence with high resolution.
In 2002, scientists examined the scope for using the radar occasionally for astronomical observations at 50 MHz with a portable pulsar receiver, developed by the Raman Research Institute. It was found that the results were encouraging and that the observations were more sensitive than those done at Gauribidanur radio array of the Raman Institute. It is also suggested that the MST Radar, the radio array at Gauribidanur and the GMRT may together conduct astronomical observations in an integrated manner.
In fact, GMRT and the arrays at Gauribidanur and Ootacamund together observe transient phenomena such as pulsars.