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An international team of astronomers 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.
Scientists based in the U.S., Italy, and Spain, set their sights on a mysterious cosmic duo called PSR J1023+0038, or J1023 for short. The J1023 system is comprised 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, spinning like a lighthouse beacon.
The J1023 system is rare and valuable to study because the pulsar 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 a paper in The Astrophysical Journal Letters illustrating the new findings.
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 comparing it with optical polarization from the European Southern Observatory's Very Large Telescope in Chile. IXPE launched in Dec. 2021 and has made many observations of pulsars, but J1023 was the first system of its kind that it explored.
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 Karl G. Jansky Very Large Array in Magdalena, New Mexico.
The result: scientists found the same angle of polarization across the different wavelengths.
"That finding is compelling evidence that a single, coherent physical mechanism underpins the light we observe," said Francesco Coti Zelati of the Institute of Space Sciences in Barcelona, Spain, co-lead author of the findings.
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.
"IXPE has observed many isolated pulsars and found that the pulsar wind powers the X-rays," said NASA Marshall astrophysicist Philip Kaaret, principal investigator for IXPE at NASA's Marshall Space Flight Center in Huntsville, Alabama. "These new observations show that the pulsar wind powers most of the energy output of the system."
Astronomers continue to study transitional millisecond pulsars, assessing 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.
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