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University of Tübingen: Like a tumbling toy top- surprising news from the X-ray pulsar Her X-1

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The very first direct measurement of the polarization of an X-ray pulsar, a rotating magnetized neutron star, by the IXPE space telescope ( Imaging X-ray Polarimetry Explorer ) called previous models of such systems into question. The degree of polarization of the X-ray pulsar Hercules X-1 was much lower than theoretically predicted, so astrophysicists’ ideas about the geometry and structure of the matter flows have to be fundamentally reconsidered. According to the new findings, the axis of rotation and the magnetic axis of the neutron star, as well as the direction of the angular momentum in this binary star system, are not in a line, so that the object tumbles like a spinning top. In the journal Nature Astronomya study by a large international team has been published. Main authors are Dr. Victor Doroshenko from the Institute for Astronomy and Astrophysics at the University of Tübingen and Dr. Juri Poutanen from the Finnish University of Turku.

X-ray pulsars are only about ten kilometers across, but they are heavier than the Sun and have a magnetic field several billion times stronger than any magnetic field known on Earth. They form a binary star system with a normal star funneling matter over a magnetic field into the pulsar’s polar regions. This releases immense energies and makes X-ray pulsars very bright sources in the X-ray sky.

Rethink previous models
Now the IXPE mission, which launched earlier this year, is providing a new perspective on these objects. IXPE is the first pioneering mission to measure polarized X-rays from celestial objects. “Her X-1 was the first X-ray pulsar observed by IXPE. We were very surprised that only low polarization was observed, throwing our theoretical predictions out of whack. We haven’t understood that yet,” says Victor Doroshenko. The average degree of polarization of approximately 8.6 percent measured by IXPE with high accuracy is said to be much lower than the expected approximately 80 percent predicted by theoretical work. “Such a large discrepancy implies that previous models of radiative transfer in strongly magnetized plasmas,

“I’ve studied Her X-1 almost all my life, and it never ceases to amaze me,” says Professor Rüdiger Staubert from the Tübingen Institute for Astronomy and Astrophysics and one of the co-authors of the study. “It is the first X-ray pulsar in which we have been able to directly measure the magnetic field of the neutron star. And it’s one of the most studied objects of its kind. But we’re still a long way from fully understanding it,” says Staubert.

Ultimate proof is still pending
Despite all the new puzzles, the research team considers the new results to be fundamental findings. “For the first time since the discovery of X-ray pulsars five decades ago, it has been possible to measure the angle between the spin axis and the magnetic dipole axis by studying the changes in polarization angle with spin phase. We need this information to model the emission from such objects,” explains Doroshenko. “We combined these X-ray polarimetric observations with earlier optical polarimetric measurements. In this way, we were able to prove that the spin axis of the pulsar is not in line with the orbital angular momentum. This, as well as other previous observations, suggests that the neutron star is tumbling like a toy spinning top that is leaking out.”

The ultimate proof of this is expected later this year when IXPE is scheduled to observe the X-ray pulsar Her X-1 in a different phase of its 35-day cycle, reports Professor Andrea Santangelo of the Tübingen Institute for Astronomy and Astrophysics. “IXPE is just now launching the new observation window of X-ray polarimetry and paving the way for the next generation of X-ray polarimeters. It’s just the beginning of a great adventure,” he adds.

The Imaging X-ray Polarimetry Explorer (IXPE) was launched from Cape Canaveral earlier this year aboard a Falcon 9 rocket and is now orbiting 370 miles (600 kilometers) above the Earth’s equator. The mission is a collaboration between NASA and Agenzia Spaziale Italiana with partners and scientists in twelve countries. The space mission is controlled by Ball Aerospace, which is based in Broomfield, Colorado, in the USA.