An X-ray pulsar is confirmed to move like a pinwheel

Oct. 26 () –

The geometric parameters of a neutron star floating in a galaxy 21,000 light years away they confirm old ideas that this star precesses, or moves, like a pinwheel.

X-ray pulsars are strongly magnetized neutron stars driven by the accumulation of gas from a nearby companion star and are among the most prominent sources in the X-ray sky. The International X-ray Polarimeter Explorer (IXPE) space observatory, recently launched by NASA, which began operations in late 2021, now provides a new perspective on these objects. IXPE measures the polarization of X-rays and was used to measure the polarization of an X-ray pulsar for the first time, which allowed to restrict its basic geometry.

“Hercules X-1 was the first X-ray pulsar observed by IXPE, and a very low polarization that we observed was a big surprise and that’s something we still don’t fully understand,” he says. it’s a statement Victor Doroshenko of the University of Tübingen in Germany, lead author of the paper in Nature Astronomy.

The mean degree of polarization of about 9% measured by IXPE with very high precision turned out to be much lower than the 80% optimistically expected by some theorists.

“Such a large discrepancy implies that existing models of radiative transport in strongly magnetized plasma confined to the poles of a neutron star and our ideas about the geometry and structure of the emission region in Hercules X-1 and possibly other pulsars should be substantially revised. in light of the results of the IXPE“, adds Juri Poutanen from the University of Turku in Finland, leader of the IXPE working group studying the accumulation of neutron stars.

By observing the variations of the polarization angle during the spin phase, it was possible to measure the angle between the spin and magnetic dipole axes, an elementary piece of information for any emission model of such objects. Joint modeling of new X-ray polarimetric observations and earlier optical polarimetric measurements obtained at the Nordic Optical Telescope also unambiguously showed that the pulsar’s spin axis is misaligned with orbital angular momentum, which is a strong indication that the neutron star is in precession –movement associated with the change of direction in space–, like a pinwheel.

Neutron star free precession was previously invoked to explain the observed semi-regular variations of pulsar flow and pulse profile shape with a period of about 35 days, and it has some important consequences for our understanding of the internal structure of neutron stars. neutron stars, but so far only indirect evidence for this hypothesis is available. The definitive proof is expected to come later when IXPE observes the Hercules X-1 in another phase of the precession cycle.

“IXPE is just starting to explore the new observing window, X-ray polarimetry, and we continue with observations of objects of all kinds, so stay tuned for more amazing discoveries,” says Sergey Tsygankov of the University of Turku, one of the main authors of the publication.

IXPE launched on a Falcon 9 rocket from Cape Canaveral in December 2021 and now orbits 600 kilometers above the Earth’s surface. The mission is a collaboration between NASA and the Italian Space Agency with scientific partners and collaborators in 13 countries, including Finland.