Getting a grip on a strongly magnetized neutron star’s geometry

Researchers from the College of Turku have decided the geometrical parameters of a neutron star floating within the galaxy 21,000 gentle years away. The discovering confirms outdated concepts that this star precesses like a whirligig.

X-ray pulsars are strongly magnetized neutron stars powered by accretion of gasoline from a close-by companion star and are amongst most distinguished sources within the X-ray sky. A brand new perspective on these objects is now offered by the lately launched Worldwide X-ray Polarimeter Explorer (IXPE) space observatory, which began operations on the finish of 2021. IXPE measures polarization of X-rays and was used to measure polarization from an X-ray pulsar for the primary time, which allowed to constrain its primary geometry.

“Hercules X-1 was the primary X-ray pulsar noticed by IXPE, and a really low polarization we noticed got here as a giant shock and that’s one thing we nonetheless don’t totally perceive,” says Victor Doroshenko from the College of Tuebingen in Germany, the lead creator of the Nature Astronomy paper.

The common diploma of the polarization of about 9% measured by IXPE with very excessive accuracy turned out to be a lot decrease than optimistically anticipated 80% by some theoreticians.

“Such a big discrepancy implies that current fashions of radiative transport in strongly magnetized plasma confined on the poles of a neutron star and our concepts concerning geometry and construction of the emission area in Hercules X-1 and probably different pulsars will must be considerably revised in gentle of IXPE outcomes,” provides Juri Poutanen from the College of Turku in Finland, chief of the IXPE’s working group learning accreting neutron stars.

Taking a look at variations of the polarization angle over the spin phase, it was attainable to measure the angle between the spin and magnetic dipole axes—a chunk of data elemental to any modeling of emission from such objects. Joint modeling of latest X-ray polarimetric observations and older optical polarimetric measurements obtained on the Nordic Optical Telescope allowed additionally to unambiguously present that the spin axis of the pulsar is misaligned with the orbital angular momentum, which is a robust indication that the neutron star precesses like a whirligig.

Free precession of the neutron star was beforehand invoked to clarify noticed semi-regular variations of pulsars flux and pulse profile form with interval of ~35 days, and has some essential penalties for our understanding of inner construction of neutron stars, however thus far solely oblique proof for this speculation is accessible. The final word proof is anticipated to reach later when IXPE observes Hercules X-1 at one other phase of the precession cycle.

“IXPE is simply beginning to discover the brand new observational window, X-ray polarimetry, and we’re persevering with observations of objects of every kind, so keep tuned for extra shocking discoveries,” says Sergey Tsygankov from the College of Turku, one of many main authors of the publication.

IXPE was launched on a Falcon 9 rocket from the Cape Canaveral in December 2021, and now orbits 600 kilometers above Earth’s floor. The mission is a collaboration between NASA and the Italian House Company with companions and science collaborators in 13 international locations together with Finland.