Hunt for pulsars gains traction as scientists look to test fundamental theories

April 2, 2021

Scientists are using pulsars to test Einstein's most famous theory. (Jurik Peter)

Astronomers searched dense clusters of stars in the Milky Way and found eight previously unknown millisecond pulsars, ultradense supernova remnants that appear to "pulse" in the sky and can complete a rotation in a tiny fraction of a second.

The newly found pulsars, documented in a paper published March 18 in Monthly Notices of the Royal Astronomical Society, could help scientists test theories of gravity and other cosmology because of their extreme density and unusual ability to act as precise "clocks."

The survey is the first of its kind to be conducted on MeerKAT, a South African radio telescope that is expected to usher in dozens of new discoveries in the near future.

Generally about 6 miles across but weighing more than the sun, neutron stars are the densest known stellar objects in the universe after black holes. They are created after large stars go supernova and shed their outer layers in a massive explosion, and by preserving most of the star's angular momentum, many neutron stars begin spinning very quickly.

If spinning fast enough, neutron stars emit beams of electromagnetic radiation from each of their magnetic poles, becoming pulsars. From Earth, they appear to blink or "pulse" at a steady pace when the radiation sweeps past the planet once per rotation, much like a lighthouse. 

These unusual stars can provide scientists with unique insight into how gravity works and the matter that inhabits the space between stars, said study author Tasha Gautam, a Ph.D. student at the Max Planck Institute for Radio Astronomy in Germany.

"Since they are so compact, they show extreme physics phenomena in such regions where spacetime itself is disturbed due to their compactness, due to the high densities inside them," Gautam said.

Surveying the Milky Way Galaxy, Gautam and astronomer Alessandro Ridolfi led an investigation into nine dense groups of stars known as globular clusters to look for millisecond pulsars, which spin at least 33 times per second. They usually gain their remarkable rotational speed by stealing matter and angular momentum from a companion star in a binary system, a process that has earned actively absorbing pulsars the classification of "spiders."

Using 44 of MeerKAT's 64 radio telescopes, the astronomers searched radio-wave frequencies and found eight previously unidentified pulsars in six globular clusters, bringing the total number of globular-cluster pulsars up to 221. Five were in binary systems orbiting a companion star, while the other three were isolated.

Of the new discoveries, the fastest-spinning pulsar has a period of 2.74 milliseconds — rotating 365 times per second — and is located about 15,300 light-years away in 47 Tucanae, one of the most massive globular clusters in the Milky Way Galaxy. The pulsar, named 47 Tuc ac, is not only a "spider" but also a "black widow" because its companion star is a small fraction of the size of the sun.

Another of the new pulsars, NGC 6624, has a highly elliptical orbit. Ridolfi, a postdoctoral researcher at Italy's Cagliari Observatory, said it allows for the mass of the pulsar and its companion star to be estimated using general relativity, which describes the bending of space and time in the presence of gravity.

The pulsar is roughly twice the size of the sun, so massive that the astronomers believe that its companion star is its second; the first was likely ejected from the system after being partially consumed. The scenario is rare among known pulsars. 

The researchers are organizing a follow-up program to confirm and expand their findings with MeerKAT and other observatories, possibly peering at the pulsars in different frequencies.

Ridolfi and Gautam are involved in an ongoing, much larger pulsar hunt using all 64 dishes of MeerKAT. Over the next three years, they will expand their search to 28 globular clusters and will be investigating farther into their outskirts than in the recently published study. The research's wider scope is why Ridolfi believes they will find dozens more pulsars in globular clusters and bring the total count up to as many as 300 in the near future.

"The full-fledged search of globular cluster pulsars is currently ongoing, taking advantage of full MeerKAT array, thus reaching higher sensitivity," Gautam said. "Definitely, it will lead to many more discoveries soon."

There are also plans to turn MeerKAT into part of the upcoming Square Kilometer Array in the next decade by adding 133 more dishes, which would triple its size and create the world's largest radio telescope.

Ridolfi said he and his collaborators hope to find undiscovered pulsar systems that would allow for new testing of extreme physics, such as two millisecond pulsars orbiting each other. Only one double-pulsar system is known, though only one of two stars is a millisecond pulsar, while the other rotates once every 2.8 seconds. 

But the "Holy Grail" of pulsar astrophysics, Ridolfi said, is a pulsar orbiting a star-sized black hole, which has yet to be found.

"Having a super precise clock orbiting the black hole, we would be able to test general relativity and other theories of gravity with unprecedented precision," he said.

The study, "Eight new millisecond pulsars from the first MeerKAT globular cluster census," published March 18 in Monthly Notices of the Royal Astronomical Society, was authored by Alessandro Ridolfi, Cagliari Observatory and Max Planck Institute for Radio Astronomy; Tasha Gautam, Paulo Freire, Vivek Venkatraman Krishnan, Federico Abbate, Ewan Barr, Prajwal Padmanabh, David Champion, Weiwei Chen, Ramesh Karuppusamy and Aditya Parthasarathy, Max Planck Institute for Radio Astronomy; Scott Ransom, National Radio Astronomy Observatory; Sarah Buchner, Fernando Camilo, Marisa Geyer and Maciej Serylak, South African Radio Astronomy Observatory; Matthew Bailes and Ryan Shannon, Swinburne University of Technology and ARC Centre of Excellence for Gravitational Wave Discovery; Michael Kramer, Max Planck Institute for Radio Astronomy and University of Manchester; Benjamin Stappers and Laila Vleeschower, University of Manchester; Marta Burgay and Alessandro Corongiu, Cagliari Observatory; Andrew Jameson and Daniel Reardon, Swinburne University of Technology; Aris Karastergiou, University of Oxford and Rhodes University; and Renée Spiewak, University of Manchester and Swinburne University of Technology.

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