A Mysterious Light In Space Keeps Switching On and Off. Now, We Know Why.

Scientists have discovered the mysterious reason that a radiant object in our galaxy keeps switching on and off between high and low energy modes, meaning that it cycles between brighter and darker phases, reports a new study. 

A massive observational campaign involving 12 telescopes has revealed that these cosmic switches are flipped by energetic interactions between a jet and a disk shaped by a pulsar, a type of pulsing dead star. The results open a new window into the spectacular dynamics of pulsars, which flash in such precise clockwork patterns that scientists use them as cosmic timekeeping devices.

Pulsar flashes are so regular that scientists initially thought they might be signals of an alien civilization, leading them to playfully call the first known pulsar emission “LGM” for “little green men.” We now know that pulsars are the corpses of stars that died and collapsed into a class of hyper-dense spheres known as neutron stars. Pulsars are rapidly spinning neutron stars that shoot brilliant jets of light out of their poles, producing characteristic flashes like a lighthouse in space.

For years, scientists have been perplexed by the behavior of a pulsar called PSR J1023+0038, or J1023 for short, which is about 4,500 light years from Earth. J1023 is what’s known as a “transitional pulsar” that alternates between quiet and active states for reasons that have remained unexplained—until now.

Scientists co-led by Maria Cristina Baglio, a pulsar researcher at New York University Abu Dhabi, and Francesco Coti Zelati, a researcher at the Institute of Space Sciences in Barcelona, Spain, conducted the most extensive multi-wavelength observations of J1023 ever in June 2021. The results revealed that the modes switch on and off due to complex interactions between the pulsar and material it has pulled off a nearby star, providing a benchmark for “unraveling the nature of elusive objects” such as J1023, according to a study published on Wednesday in Astronomy & Astrophysics.

“We aimed to conclusively uncover the underlying physical mechanisms behind the unique behavior exhibited by the source across the electromagnetic spectrum,” Baglio and Coti Zelati said in an email to Motherboard. “Over the years, we have made significant advancements in our understanding of its properties, continuously refining our theoretical models in an effort to offer plausible explanations. However, a comprehensive picture had remained elusive.” 

“This recent study, which presents an extensive collection of data, allowed us to propose a conclusive scenario that consistently explains the observed properties of the system across the electromagnetic spectrum,” the researchers added. “It represents the culmination of years of dedicated research on this unique source.”

Baglio and Coti Zelati have been studying J1023 since they were both in graduate school nearly a decade ago, but the pulsar has proved to be a tough nut to crack. In an effort to explain its strange behavior once and for all, the researchers harnessed the observational prowess of a dozen telescopes on the ground and in space, including the European XMM-Newton satellite, NASA’s Hubble Space Telescope, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. 

Over the course of two nights in June, these sophisticated telescopes set their sights on J1023 and captured 280 switches between its low and high modes. The observations spanned a huge range of wavelengths, from radio waves to X-rays, allowing the researchers to hone in on the mechanics behind the changes.

The results show that the mode-switching behavior stems from J1023’s accretion disk, which is a disk of material that the pulsar has pulled from its companion star. As this material falls toward J1023, it fuels a jet of light and energy perpendicular to the disk that crashes into winds around the pulsar, switching the system to its high mode. The jet continues to expel the material in bursts known as“discrete mass ejections” until the system falls back into the low mode again.

“Although our team and others had previously hypothesized that discrete mass ejections could be responsible for the pulsar's mode-switching behavior, we had not yet found conclusive evidence to support this claim,” Baglio and Coti Zelati said. “Our breakthrough came when we observed brief flashes of microwave radiation in the data. That's when we realized we were witnessing abrupt ejections of material from the system, confirming that these ejections were indeed the driving force behind the mode switching behavior.”

The discovery provides a satisfying explanation for J1023’s frenetic variations, and could help to explain weird emission patterns seen in other compact objects. Even as the team celebrates the milestone though, they look forward to future observations of J1023, and similar objects with a new generation of telescopes, including the Extremely Large Telescope that is set to become operational in the late 2020s.

“The transitional behavior of J1023 serves as an invaluable case study for exploring different regimes of accretion, thereby enriching our understanding of the behavior of matter under extreme conditions,” Baglio and Coti Zelati said. “For instance, observing how the accretion disk appears and disappears can provide insights into disk stability and formation mechanisms.” 

“Furthermore, these systems can be studied across multiple electromagnetic wavelengths, enabling a more comprehensive understanding of the radiative processes involved,” they concluded.