Sketch of the double star system of Hercules X-1: A donor star transports matter through an accretion disk to the small neutron star at the center of the disk. The neutron star is shown enlarged in blue in front. The IXPE data made it possible to measure the angles between all relevant axes and revealed that, unexpectedly, the rotation axis is not aligned with the direction of the angular momentum of the binary star’s orbit.
An X-ray pulsar is a rotating magnetized neutron star. The first direct measurement of the polarization of an X-ray pulsar by the Imaging X-ray Polarimetry Explorer (IXPE) space telescope has challenged previous models of such systems. The degree of polarization of the X-ray pulsar Hercules X-1 was much lower than theoretically predicted, so astrophysicists must now reconsider their basic ideas about the geometry and structure of matter flows. According to the new findings, the neutron star’s spin axis and magnetic axis, as well as the direction of the angular momentum in this binary star system, are not aligned, as expected, so the ‘object wobbles like a pettola. . A large international team of researchers has published these curious findings in the journal Nature Astronomy. The lead authors are Dr. Victor Doroshenko of the Institute of Astronomy and Astrophysics at the University of Tübingen and Dr. Juri Poutanen of the University of Turku in Finland.
X-ray pulsars are only about ten kilometers in diameter, but they are heavier than our Sun and have a magnetic field that is several billion times stronger than any magnetic field we know on Earth. They form a binary star system with a normal star, allowing matter to overflow into the polar regions of the pulsar through a magnetic field, as if through a funnel. This releases immense energies and makes X-ray pulsars very bright in the X-ray sky.
Rethink previous models
Now, the IXPE mission, launched earlier this year, is providing a new perspective on these pulsars. IXPE is the first pioneering mission to measure polarized X-rays from celestial objects. “His X-1 was the first X-ray pulsar observed by IXPE. We were very surprised that only low polarization was observed. This threw off our theoretical predictions. We still don’t understand,” says Victor Doroshenko. The average degree of polarization of 8.6 percent, measured with high precision by IXPE, was much lower than the expected approximately 80 percent predicted by theoretical work, he says. “Such a large discrepancy indicates that we must reconsider fundamentally our previous models of radiative transfer in strongly magnetized plasmas accreting at the poles of neutron stars, as well as our insights into the geometry and structure of the emission region in Her X-1 – and probably other pulsars”, adds Yuri Poutanen.
“I have studied Her X-1 for most of my life and it continues to amaze me,” says one of the study’s co-authors, Professor Rüdiger Staubert of the Institute for Astronomy and Astrophysics in Tübingen. “It’s the first X-ray pulsar for which we’ve been able to directly measure the neutron star’s magnetic field. And it’s one of the most studied objects of its kind. But we’re still far from fully understanding it,” says Staubert .
Final test pending
Despite all the new puzzles, the research team is excited about the results. “For the first time since the discovery of X-ray pulsars five decades ago, it was possible to measure the angle between the rotation axis and the magnetic dipole axis by studying the changes in the polarization angle with the rotation phase appropriate. We need this information to model the emission of these objects,” explains Doroshenko. “We combined these X-ray polarimetric observations with previous optical polarimetric measurements. Thus, we were able to show that the pulsar’s rotation axis is not aligned with the orbital angular momentum. This suggests, as do other previous observations, that the neutron star wobbles like a slow-moving snowball.”
Definitive proof is expected later this year, when IXPE is scheduled to observe the X-ray pulsar Her X-1 in another phase of its 35-day cycle, according to Professor Andrea Santangelo of the Institute of Astronomy and Astrophysics of Tübingen. “IXPE has just begun the new observation window of X-ray polarimetry and paves the way for the next generation of X-ray polarimeters. This is just the beginning of a great adventure,” he adds.
The Imaging X-ray Polarimetry Explorer (IXPE) launched from Cape Canaveral earlier this year aboard a Falcon 9 rocket and is now orbiting 370 miles (600 kilometers) above the equator of the land The mission is a collaboration between NASA and the Italian Space Agency with partners and scientists from twelve countries. The space mission is controlled by Ball Aerospace, based in Broomfield, Colorado, USA.
NASA press release:
https://www.nasa.gov/mission_pages/ixpe/news/2022/powerful-neutron-star-s-behavior-surprises-ixpe-researchers.html
Publication:
Doroshenko, V., Poutanen, J., Tsygankov, SS et al. Determination of X-ray pulsar geometry with IXPE polarimetry. Nature Astronomy, 24 October 2022,
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