Astronomers shed light on behavior of mysterious dead star

The pulsar called PSR J1023+0038, or J1023 for short, switches between two brightness modes on a regular and constant basis.
Chris Young
An artist's impression of pulsar PSR J1023+0038 and its companion star.
An artist's impression of pulsar PSR J1023+0038 and its companion star.

ESO / M. Kornmesser 

Astronomers used 12 ground and space observatories to uncover the mechanism behind the strange behavior of a pulsar.

The pulsar called PSR J1023+0038, or J1023 for short, switches between two brightness modes on a regular and constant basis.

Until now, astronomers didn't know why J1023 exhibited this peculiar behavior, according to a press statement from the European Southern Observatory (ESO).

Investigating a mysterious pulsar

Pulsars are a type of dead star that spins at extremely high speed — they can spin at speeds as high as 70,000 km per second.

As they spin, pulsars emit a beam of electromagnetic radiation into space. Astronomers are able to detect pulsars when this beam intersects the line of sight to Earth.

The astronomers behind the new investigation into the mysterious Pulsar J1023 detailed their findings in a new paper in the journal Astronomy & Astrophysics. They found that sudden ejections of matter from the pulsar over short time spans are the cause of the strange shift in brightness.

"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 celestial object rotating at incredibly high speeds," explained Maria Cristina Baglio, a researcher at New York University Abu Dhabi and the lead author of the new paper.

The pulsar J1023 closely orbits another star some 4,500 light-years away from Earth in the Sextans constellation. Over the last ten years, the pulsar has been pulling matter off its companion. This matter accumulates in a disc around the pulsar and then slowly falls into the dead star.

Since that process began, the beam of electromagnetic radiation emanating from J1023 virtually disappeared and the pulsar started switching between two modes: a 'high' mode in which it gives off bright X-rays, ultraviolet and visible light, and a 'low' mode that is dimmer in those frequencies and also emits more radio waves.

The pulsar stays in each of these modes for seconds or minutes at a time before quickly switching.

"Our unprecedented observing campaign to understand this pulsar’s behavior involved a dozen cutting-edge ground-based and space-borne telescopes," explained Francesco Coti Zelati, a researcher at the Institute of Space Sciences, Barcelona, Spain, and co-lead author of the paper.

That campaign involved ESO's Very Large Telescope (VLT) and New Technology Telescope (NTT), as well as the Atacama Large Millimeter/submillimeter Array (ALMA). Over two nights in June 2021, they observed the system as it made over 280 switches between its high and low modes.

Switching between a 'high' and 'low' mode

The astronomers discovered that the mode switching occurs because of interactions between "the pulsar wind, a flow of high-energy particles blowing away from the pulsar, and matter flowing towards the pulsar," Zelati explained in the statement.

When the pulsar is in its low mode, matter is expelled in a narrow jet perpendicular to the disc. This begins to accumulate closer and closer to the pulsar. Eventually, it is hit by wind blowing from the pulsating star, causing the matter to heat up. This dramatic rise in temperature triggers the switch to the pulsar's high mode. The pulsar shifts back to the low mode when hot matter is expelled by the pulsar's jet.

In the future, ESO's under-construction Extremely Large Telescope (ELT) will have the capacity to shed even more light on the peculiar mechanism of this dead star.

As Sergio Campana, another co-author on the study, puts it, "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."

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