A Franklin & Marshall physics professor is part of an international research team whose discovery of an unusual pulsar has led to an important breakthrough in how scientists understand dead-star collisions and the expansion of the universe.
F&M Professor Fronefield Crawford was part of the team, led by the University of East Anglia, that discovered the pulsar PSR J1913+1102, which was found to be part of a binary neutron star system – which means that it is locked in a fiercely tight orbit with another neutron star.
The discovery, published recently in the journal Nature, was made using the Arecibo radio telescope in Puerto Rico.
“This is quite an important discovery for our team,” Crawford said. “We’ve been finding new pulsar systems in our long-term pulsar survey of the Milky Way galaxy with Arecibo, and this discovery has the potential to significantly advance our understanding of these kinds of binary systems.”
Neutron stars are the dead stellar remnants of a supernova, made up of the densest matter known – packing hundreds of thousands of times the Earth’s mass into a sphere the size of a city. Pulsars are neutron stars that act as deep space’s magnetized spinning ‘lighthouses’ and emit highly focused radio waves from their magnetic poles.
In about half a billion years, the two neutron stars in the PSR J1913+1102 system will spiral inward in their orbit and merge, releasing astonishing amounts of energy in the form of gravitational waves and light.
But the newly discovered pulsar is unusual because the masses of its two neutron stars are quite different – with one far larger than the other.
This asymmetric system gives scientists confidence that double neutron star mergers will provide vital clues about unsolved mysteries in astrophysics – including a more accurate determination of the expansion rate of the universe, known as the Hubble constant.
Lead researcher Professor Robert Ferdman, from UEA’s School of Physics, said, “Back in 2017, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected the merger of two neutron stars. The event caused gravitational-wave ripples through the fabric of spacetime, as predicted by Albert Einstein over a century ago.”
Known as GW170817, this spectacular event also was seen with traditional telescopes at observatories around the world, which identified its location in a distant galaxy, 130 million light years from our own Milky Way.
Ferdman said, “It confirmed that the phenomenon of short gamma-ray bursts was due to the merger of two neutron stars. And these are now thought to be the factories that produce most of the heaviest elements in the universe, such as gold.”
The power released during the fraction of a second when two neutron stars merge is enormous – estimated to be tens of times larger than all stars in the universe combined.
While the GW170817 event was not surprising, the enormous amount of matter ejected from the merger and its brightness was an unexpected mystery.
“Most theories about this event assumed that neutron stars locked in binary systems are very similar in mass,” Ferdman said. “Our new discovery changes these assumptions. We have uncovered a binary system containing two neutron stars with very different masses.”
According to Ferdman, “the discovery highlights that there are many more of these systems out there – making up more than one in 10 merging double neutron star binaries. … this may also allow for a completely independent measurement of the Hubble constant – the rate at which the universe is expanding. The two main methods for doing this are currently at odds with each other, so this is a crucial way to break the deadlock and understand in more detail how the universe evolved.”
The research was led by UEA in collaboration with scientists at Max Planck Institute for Radio Astronomy in Bonn, the Arecibo Observatory in Puerto Rico, Columbia University, Cornell University, Franklin and Marshall College, the University of Amsterdam, McGill University, West Virginia University, the University of British Columbia, and the Netherlands Institute for Radio Astronomy (ASTRON).
Pulsar Discoveries at F&M
Franklin & Marshall College has a history of students discovering pulsars, some extremely rare, under Fronefield Crawford, professor of astronomy and director of the Grundy Observatory.
“F&M astronomy students have discovered pulsars located outside of our galaxy,” Crawford said.
Jack Madden ’14 and Tori Bonidie ’20 have found pulsars residing in the Large Magellanic Cloud, a nearby dwarf galaxy.
“These so-called extragalactic pulsars make up only about 1 percent of the currently known pulsar population, and are among the most distant pulsars known,” Crawford said.
More recently, Ben Nguyen ’18 and Issac Lin ’22 found new “millisecond pulsars” in the Arecibo survey data while visiting Japan’s Kumamoto University in 2019 with Crawford. They were training graduate students in pulsar search techniques.
“Millisecond pulsars spin hundreds of times per second, much faster than normal pulsars, and these discoveries are critical tools in the search for low-frequency gravitational waves that are thought to be emitted from distant supermassive black hole pairs,” Crawford said.
The search for gravitational waves is being conducted by the NANOGrav research consortium, of which F&M is a member.