His out-of-this-world discovery: Signals from billions of light years away

Dacheng Lin, research associate professor of physics.
“This is about the fundamental physics of our universe,” Dacheng Lin says. Photo by Matthew Modoono/Northeastern University

Dacheng Lin, a Northeastern research associate professor of physics, has identified a new class of high-energy signals that were created billions of light years from Earth.

The signals are known as fast X-ray transients (FXTs), a burst of X-rays that lasts for hours. They appear to be generated by a collision of two neutron stars that results in the creation of a magnetar, which is an even heavier neutron star with an extraordinary magnetic field. 

Lin’s finding, detailed in The Astrophysical Journal, offers new hope for locating the gravitational waves caused by such rare cosmic events.

Dacheng Lin, research associate professor of physics. Photo by Matthew Modoono/Northeastern University

“This is about the fundamental physics of our universe,” Lin says. “Neutron stars are extremely dense—equivalent to the sun compacted and squeezed into a body the size of New York City.”

There used to be limited opportunities for discovering magnetars. The violent union of two neutron stars creates gravitational waves—“ripples in spacetime,” according to Lin—that are accompanied by jets of radiation that are launched in opposite directions. 

A new vein of exploration was created in 2015 when an FXT was detected by NASA’s Chandra X-ray Observatory, a space telescope that was launched by the space shuttle in 1999. The FXT had been emitted 6.6 billion light years from Earth by the spinning energy and blinding radiation of a magnetar.

The discovery of the X-rays “makes another strong case that nature’s fecundity repeatedly transcends human imagination,” Niel Brandt, a principal investigator of the phenomenon, said in an official 2019 announcement.

Lin’s analysis of terabytes of data collected by Chandra over the past two decades led to his discovery of three similar FXTs. 

“These FXTs have unique features matching those expected for a newly-born magnetar formed in a neutron star-neutron star merger,” Lin says. “Our discovery dramatically expanded the sample and firmly established the existence of a new class of signals from neutron star-neutron star mergers.

“The signals are short-lived,” Lin adds. “But they are very, very important.”

The relationship between FXTs and magnetars offer two promising paths of research. One is to pinpoint the locations of gravitational wave events. The other is especially profound.

“One main mystery of astronomy is: What is inside a neutron star?” Lin says. “People have tried to study its interior, which is very compact and filled with neutrons. This signal, when combined with gravitational waves, provides a new way to study the interior.”

The development of neutron stars and other faraway mysteries has intrigued Lin for as long as he can remember.

“Normal stars are quite boring to me,” Lin says with a smile. “But black holes, neutron stars, and supermassive black holes are highly interesting. 

For media inquiries, please contact Ed Gavaghan at e.gavaghan@northeastern.edu or 617-373-5718.