Outreach

Pulsars

Often when we hear about astronomy, beautiful pictures of nebulae, star fields and galaxies comes up in front of the eyes, Some imagine about being using a backyard telescope but astronomy is more about what you can't directly see. With the dawn of multi-wavelength astronomy since the last century, we are nowcapable to see over all of the electromagnetic spectrum. Specifically in the field of Radio astronomy, comes the extraordinary Pulsars. Which people dubbed as the cosmic lighthouses "seen" through radio telescopes.

The story starts at Cambridge, UK in 1967. Radio astronomer Antony Hewish and his colleagues at Cambridge University has already designed the interplanetary scintillation array for conducting a survey of the scintillations, or 'twinkling', caused when radio waves from cosmic sources passes through the Sun's winds. When the radio waves are viewed through the wind-driven material, they blink on and off, varying on time-scales of a few tenths of a second - in much the same way that stars twinkle when seen through the Earth's varying atmosphere. Repeated observations of several scintillating radio sources at different angles in relation to the Sun provide information about the properties of the solar wind and the angular structure of the radio sources. The fluctuations are greatest for the smaller emitters, just as stars twinkle more than the Moon or planets, which have larger angular extents. On 28th of November, His PhD student, Jocelyn Bell found a strong fluctuating signal in the middle of the night, when the array was pointed away from the Sun and the effects of the solar wind should have been small. She has detected a periodic signal. This later turned out to be the first pulsar discovered. It is because Hewish specifically designed a new type of radio telescope for a study of the rapidly changing solar wind effects that the radio pulsars were discovered accidentally. This discovery was announced by Hewish and his research group in February 1968 and later recognized by the Nobel Prize in Physics in 1974. The detection was a periodic signal with time period of precisely 1.3372795s second and 0.04 second pulse-width, Since the signal was coming from a point like object following sidereal time, This Newly observed Radio Star was named PULSAR (Pulsating Star). We now know that the term pulsar is misleading for the compact stars do not pulsate - they rotate - but the name has stuck. It designates repeating pulses of radio emission rather than a pulsating star. Although initially it was jokingly called "LGM-1“ by Bell's team, as a Extra-Terrestrial signal from Intelligent Beings but this idea was discarded very soon with more similar observations of other pulsars from other parts of the sky.

The First pulsar was thus named CP1919 and later became PSR B1919+21. Why these weird names? Well these numbers are the right-ascension and declination (equitorial coordinates) of a pulsar in the sky. Over 1600 pulsars have been found since their discovery, Nowadays the observation procedures are quite developed, If the pulsar is known, A Radio telescope is needed to record observations for some time duration covering the available bandwidth. India has two major radio telescope facilities named as GMRT and ORT. The data is processed in order to do any scientific study with it. Now in order to discover new pulsars, a very complex algorithm is to be followed, Radio Telescopes do a survey of sky and characterize some specific kinds of peaks in data as sign of pulsars. Sources for further reading can be found here. The images below, on left is the pulse-profile of Vela Pulsar and on the right is a schematic of Neutron stars.





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