Astronomers solve the puzzle of an unusual pulsing dead star

Jun 22, 2023

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An international team of astronomers has unraveled new insights about an object at the center of a cosmic mystery — a pulsar that appears to constantly change in brightness. Now, the scientists think they know what’s behind it.

Rapidly rotating dead stars, pulsars are so named because as they spin, they release beams of electromagnetic radiation across space that appear to pulse like celestial lighthouses.

But a pulsar called PSR J1023+0038, located about 4,500 light-years away from Earth in the Sextans constellation, is even more unusual in that some of the pulses are brighter than others, almost like it’s switching between different modes.

The spinning dead star and the companion star that it closely orbits were first discovered in 2007. But new observations made with multiple telescopes show that strange things happen as the pulsar peels material away from the other star, and this activity has persisted for the last decade.

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A study on the new observations was published Wednesday in the journal Astronomy & Astrophysics.

“We have witnessed extraordinary cosmic events where enormous amounts of matter, similar to cosmic cannonballs, are launched into space within a very brief time span of tens of seconds from a small, dense object rotating at incredibly high speeds,” said lead study author Maria Cristina Baglio, a researcher at New York University Abu Dhabi, which is affiliated with the Italian National Institute for Astrophysics, in a statement.

Once the pulsar began siphoning off material from the companion star, its characteristic pulsing beam disappeared. Instead, the pulsar entered a constant, alternating cycle of operating in what astronomers call “high” mode and “low” mode.

During high mode, the pulsar releases X-rays, ultraviolet and visible light wavelengths. Once it’s in low mode, the pulsar becomes dimmer, sharing more radio waves than other wavelengths of light. The two modes can each last for seconds or minutes before switching to the other.

Astronomers needed a diverse set of observatories that could detect different kinds of light to solve the celestial puzzle.

“Our unprecedented observing campaign to understand this pulsar’s behaviour involved a dozen cutting-edge ground-based and space-borne telescopes,” said study coauthor Francesco Coti Zelati, a researcher at the Institute of Space Sciences, Barcelona, Spain, in a statement.

In June 2021, the pulsar made more than 280 switches between high and low mode. The various telescope observations gathered the details astronomers needed to understand what occurred during both modes.

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They discovered that an exchange of matter between the pulsar and its companion star triggers the unusual behavior of the pulsar.

As the pulsar tugs at its companion star, gas releases from the companion and forms a disk around the pulsar before slowly falling toward it.

Eventually, in low mode, that same material is released from the pulsar in a narrow jet. The expelled matter is hit by wind blowing off the pulsar. The wind heats up the stellar matter, causing it to glow in different light wavelengths, which activates high mode.

The process repeats as the jet continues to push more material away from the star, which moves some of the hotter, glowing material out of the way and reinstates low mode.

“We have discovered that the mode switching stems from an intricate interplay between the pulsar wind, a flow of high-energy particles blowing away from the pulsar, and matter flowing towards the pulsar,” Coti Zelati said.Astronomers don’t know if there are other similar pulsar systems, or if this one is unique.

Astronomers will continue to observe the unusual system and have plans to use future observatories, like the European Southern Observatory’s Extremely Large Telescope that is currently being constructed in Chile, to glean more details. The Extremely Large Telescope, or ELT, will begin observations in 2028.

“The ELT will allow us to gain key insights into how the abundance, distribution, dynamics, and energetics of the inflowing matter around the pulsar are affected by the mode switching behavior,” said study coauthor Sergio Campana, research director at the Italian National Institute for Astrophysics’ Brera Observatory, in a statement.