NASA’s IXPE imager reveals mysteries of uncommon pulsar

An international team of astronomers, led by the Italian National Institute for Astrophysics’ (INAF) Brera Observatory and researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC), has uncovered new evidence to explain how pulsing remnants of exploded stars interact with surrounding matter deep in the cosmos, using observations from NASA’s IXPE (Imaging X-ray Polarimetry Explorer) and other telescopes. The findings have been published in The Astrophysical Journal Letters.

The team set their sight on a mysterious cosmic duo called PSR J1023+0038, or J1023 for short. J1023 consists of a rapidly rotating neutron star feeding off of its low-mass companion star, which has created an accretion disk around the neutron star. This neutron star is also a pulsar, emitting powerful twin beams of light from its opposing magnetic poles as it rotates like a lighthouse beacon.

J1023 is rare and valuable to study because it transitions clearly between its active state, in which it feeds off its companion star, and a more dormant state, when it emits detectable pulsations as radio waves. This makes it a “transitional millisecond pulsar.” 

“Transitional millisecond pulsars are cosmic laboratories, helping us understand how neutron stars evolve in binary systems,” said researcher Maria Cristina Baglio of the Italian National Institute of Astrophysics (INAF) Brera Observatory in Merate, Italy, and lead author of the paper in The Astrophysical Journal Letters illustrating the new findings. 

NASA’s NICER (Neutron star Interior Composition Explorer) and Neil Gehrels Swift Observatory provided valuable observations of the system in high-energy light. Other telescopes contributing data included the European Southern Observatory’s Very Large Telescope in Chile, and the Karl G. Jansky Very Large Array in Magdalena, New Mexico.

The big question for scientists about this pulsar system was: Where do the X-rays originate? The answer would inform broader theories about particle acceleration, accretion physics, and the environments surrounding neutron stars across the universe.

The source surprised them: The X-rays came from the pulsar wind, a chaotic stew of gases, shock waves, magnetic fields, and particles accelerated near the speed of light, that hits the accretion disk.  

To determine this, astronomers needed to measure the angle of polarization in both X-ray and optical light. Polarization is a measure of how organized light waves are. They looked at X-ray polarization with IXPE, the only telescope capable of making this measurement in space, and compared it with optical polarization from other telescopes. IXPE was launched in December 2021 and has made many observations of pulsars, but J1023 was the first system of its kind that it explored. The result: scientists found the same angle of polarization in across the different wavelengths.

“That finding is compelling evidence that a single, coherent physical mechanism underpins all of the light we observe,” said Francesco Coti Zelati, IEEC researcher at the ICE-CSIC, co-lead author of the findings. Diego F. Torres and Nanda Rea, also ICE-CSIC and IEEC members, played an important role in the study contributing to the interpretation of the complex observational data.

This interpretation challenges the conventional wisdom about neutron star emissions of radiation in binary systems, the researchers said. Previous models had indicated that the X-rays come from the accretion disk, but this new study shows they originate with the pulsar wind. 

Astronomers continue to study transitional millisecond pulsars, to assess how observed physical mechanisms compare with those of other pulsars and pulsar wind nebulae. Insights from these observations could help refine theoretical models describing how pulsar winds generate radiation – and bring researchers one step closer, Baglio and Coti Zelati agreed, to fully understanding the physical mechanisms at work in these extraordinary cosmic systems.

Press release prepared in collaboration with the Institute of Space Sciences (ICE-CSIC).