“Nobody knew what a pulsar was until I discovered the first two,” says Jocelyn Bell Burnell in a 2021 documentary. As a PhD student with Antony Hewish at Cambridge University, she helped build a radio telescope at the Mullard Radio Astronomy Observatory. One day in 1967, looking through the telescope’s recording charts, she saw regularly spaced peaks in its tracks that she couldn’t explain. These strings of signals, 1.3 seconds apart, came from a specific region in the sky and led Burnell to the first pulsar to be ever discovered.
Chart recording with the radio signal from the first pulsar to be discovered by Jocelyn Bell Burnell in 1967. Image by Billthom shared under a CC by 4.0 license.
A pulsar is a neutron star, one of the final stages in a star’s life cycle, where a star moderately heavier than the sun collapses upon itself into a volume tens of kilometres across. It spins really fast, sending out a strong beam of electromagnetic radiation along its magnetic axis. This becomes visible when the radiation points towards us as it sweeps across space in the course of the pulsar’s rotation. This is like being illuminated by a lighthouse beacon when it rotates to face us again and again. Similarly, the electromagnetic beam emanating from a pulsar is seen at regular intervals by a telescope in its line of view, keeping time of the pulsar’s rotation. The first pulsar that Burnell observed, named CP 1919, made a rotation every 1.3 seconds.
A pulsar is a neutron star, which emits beams of electromagnetic radiation from its magnetic axis, which is different from its axis of rotation. This beam of radiation becomes visible when it sweeps across earth in the course of the pulsar’s rotation. Image shared from source under a CC by 4.0 license.
“It’s incredibly hard to get people to believe you’ve discovered something amazing if you’ve got only one. I knew that finding more would be the clincher,” Burnell says. Her discovery of the second pulsar later in the same year laid to rest apprehensions that the pulses came from background noise, instrumental anomalies, or more alarmingly, little green men from another civilization. In 1974, Hewish – Burnell’s advisor, won the Nobel Prize in Physics for the discovery of pulsars, along with Martin Ryle. In 2018, Burnell won the Breakthrough Prize in Fundamental Physics for her discovery of pulsars and donated the prize money to the Institute of Physics UK to support minority students.
The dance
In the same year as Hewish and Ryle won the Nobel Prize, Joseph H Taylor from the University of Massachusetts, Amherst and his PhD student, Russell A Hulse, were puzzled by irregularities in the ticks of another pulsar. It was recorded at the Arecibo Observatory in Puerto Rico, where a massive dish captured radiowaves, reflecting it to receivers which amplified these signals from the cosmos. Based on the Doppler shift in the signal, they deduced that the pulsar was spinning once in every 59 milliseconds, it had an invisible partner, and the pair danced around each other in elliptical orbits. This was the discovery of the first binary pulsar, PSR 1913 + 16, where the visible pulsar travels at 300 km per second around its silent companion, a neutron star (a non-pulsar whose electromagnetic beam is not falling on earth).
An aerial view of the Arecibo Observatory in Puerto Rico showing the reflector dish in a natural sinkhole. It was built in 1963 and was one of the largest single-aperture telescope for many decades before collapsing in 2020. Image by H. Schweiker/WIYN and NOAO/AURA/NSF shared under a CC by 4.0 license.
Six decades before this discovery, Einstein’s general theory of relativity predicted that massive accelerating bodies emit gravitational waves. These ripples of energy arise from the source and travel through space at the speed of light distorting space-time along the way. Binary pulsars with their enormous mass and velocity lose energy as gravitational waves. This shrinks their orbit over time, bringing them closer as they spiral faster and faster into each other. These changes in the orbital period are reflected in changes in the time between pulses detected by a radio telescope. Taylor and colleagues observed the binary pulsar over four years and found that its orbital period is decaying at a rate predicted by theory, providing indirect evidence for the presence of gravitational waves.
In a binary pulsar, two pulsars orbit around their center of mass emitting energy in the form of gravitational waves. As they lose energy, their orbits become smaller over time and they accelerate to spiral into each other. Image from source shared under a CC by 4.0 license.
Taylor and Hulse won a Nobel Prize in 1993 for their discovery of the binary pulsar, which had served as a natural laboratory for studying gravitational waves. Two decades later, direct evidence for gravitational waves was detected from a violent collision of two black holes over a billion light years away. In a collaborative project involving researchers across countries, the Laser Interferometer Gravitational-wave Observatory (LIGO) in the USA captured the feeble whispers of this massive cosmic event. In 2017, Rainer Weiss, Barry C Barish and Kip S Thorne from LIGO/Virgo (gravitational wave detector of the European Gravitational Observatory) won a Nobel Prize for building the detector and finding evidence of gravitational waves. However, this was just the tip of the iceberg - gravitational waves in other frequencies remained elusive.
The array
In 1982, Shrinivas Kulkarni from the University of California, Berkeley, was another graduate student peering at the skies at the Arecibo Observatory. This time around, the blips that he recorded were more frenzied than ever before, pointing to a new pulsar that spun around its axis at an incredible 641 times per second (~ 0.0016 seconds per rotation). Kulkarni, along with his advisor Donald Backer’s research group at Berkeley, had discovered the first millisecond pulsar. Later work showed that the millisecond pulsar evolves when a regular pulsar draws matter from a nearby companion giving an enormous boost to its rotation. These super-spinners are precision timekeepers, making them excellent probes of ripples in space-time.
When an old pulsar (right, with electromagnetic beams) draws matter from a nearby star (left), it resurrects into a millisecond pulsar, which spins hundreds of times per second. It keeps time very precisely, making it an ideal instrument to detect gravitational waves. Image by Dana Berry (Skyworks Digital) from source.
When a gravitational wave passes through, it perturbs space-time affecting the time at which a millisecond pulsar’s signal arrives on earth. Observing a constellation of millisecond pulsars spread across the sky, it is possible to measure very small deviations in their signals over background noise. Since these millisecond pulsars are separated by thousands of light years, they act as enormous receivers of low-frequency gravitational waves arising from the merger of supermassive blackholes.
“There are many pairs of supermassive blackholes in the universe that are dancing around each other on their way to merger and emitting gravitational waves,” says Manjari Bagchi, an astrophysicist at the Institute of Mathematical Sciences, Chennai. “The superposition of gravitational waves emitted by individual pairs results in the existence of these ripples in every direction, known as stochastic or background gravitational waves. Stochastic gravitational waves can also be created by the cosmological properties of the very early universe,” she adds.
Cosmic-scale experiments to measure gravitational waves need researchers from across the globe to come together in massive collaborative efforts. The International Pulsar Timing Array is a consortium of groups from Europe, North America, India, Japan and Australia, that has its eyes on a hundred millisecond pulsars whose signals may potentially surf gravitational waves. Within this, the Indian Pulsar Timing Array (InPTA) is a Indo-Japanese collaboration that has been observing around 25 millisecond pulsars over the last few years. It uses the upgraded Giant Metrewave Radio Telescope (uGMRT) at Narayangaon near Pune.
Last year, InPTA along with the European Pulsar Timing Array (EPTA) detected evidence for nano-Hertz stochastic gravitational waves, which was also reported by the North American and Australian groups.
The upgraded Giant Metrewave Radio Telescope in Narayangaon near Pune is being used to observe an array of millisecond pulsars to detect gravitational waves. Image by Aditya Laghate shared under a CC by 4.0 license.The road ahead
“Pulsar Timing Array experiments have not reached the end of their journey yet. Scientists are now aiming to detect gravitational waves from mergers of individual pairs of blackholes,” says Manjari. Scientists also want to find out what fraction of the stochastic gravitational waves arise from the origin of the universe and the fraction of astrophysical origin, she adds. There is an ongoing merger of gravitational wave data across global observatories to better detect signatures of gravitational waves. “The Indian group is aiming to search for gravitational waves using Indian data only. The results and the data will be made public soon!,” Manjari says.
