IXPE Measures X-ray Pulsar Hercules X-1 Polarization

By James Anderson •  Updated: 10/26/22 •  4 min read

Hercules X-1, a neutron star floating in a galaxy 21,000 light years away, has had its geometrical parameters identified by researchers from the University of Turku. The discovery supports previous theories that this star’s precession is like a whirligig.

Strongly magnetized neutron stars called X-ray pulsars are among the most well-known sources of X-ray light. They are powered by the accretion of gas from a nearby companion star.

The recently launched Imaging X-ray Polarimeter Explorer (IXPE) space observatory, which began operations at the end of 2021, offers a fresh perspective on these objects. IXPE measures X-ray polarization, and it was used to determine the polarization from an X-ray pulsar for the first time, allowing for the constraint of the pulsar’s fundamental geometry.

“Hercules X-1 was the first X-ray pulsar observed by IXPE, and a very low polarization we observed came as a big surprise and that is something we still do not fully understand,”

Victor Doroshenko of the University of Tuebingen in Germany, the study’s lead author, stated.

Polarization Lower Than Predicted

It turned out that the average degree of polarization, which was measured by IXPE with extremely high accuracy and was around 9%, was significantly lower than the theoreticians’ overly optimistic prediction of 80%.

“Such a large discrepancy implies that existing models of radiative transport in strongly magnetized plasma confined at the poles of a neutron star and our ideas regarding geometry and structure of the emission region in Hercules X-1 and possibly other pulsars will need to be substantially revised in light of IXPE results,”

said Juri Poutanen from the University of Turku in Finland.

It was possible to measure the angle between the spin and magnetic dipole axes by observing variations in the polarization angle over the spin phase. This information is essential to any modelling of emission from such objects.

Whirligig Precession

It was also possible to prove that the pulsar’s spin axis is not in line with its orbital angular momentum. This is a strong sign that the neutron star wobbles like a whirligig. This was made possible by jointly modelling new X-ray polarimetric observations and older optical polarimetric measurements obtained at the Nordic Optical Telescope.

Free precession of the neutron star has previously been put forth as a theory to account for observed semi-regular variations in pulsar flux and pulse profile shape with periods of approximately 35 days. This theory has important implications for how we think about how neutron stars are built on the inside, but there is only indirect evidence to back it up so far.

The conclusive evidence is anticipated to be discovered later, when the Imaging X-ray Polarimeter Explorer observes Hercules X-1 during a different precession cycle.

Precession is a shift in the position of a rotating body’s rotational axis. It can be described as a modification of the first Euler angle in the proper reference frame, while the rotation itself is defined by the third Euler angle. In other words, a body is said to be precessing about a second axis if its own axis of rotation is rotating around that second axis.

Precession in astronomy refers to any of a number of gradual changes in the rotational or orbital parameters of an astronomical body. The steady shift in the Earth’s axis of rotation’s orientation, also known as the precession of the equinoxes, is an example.

“IXPE is just starting to explore the new observational window, X-ray polarimetry, and we are continuing observations of objects of all kinds, so stay tuned for more surprising discoveries,”

said Sergey Tsygankov from the University of Turku, one of the lead authors.

The Imaging X-ray Polarimeter Explorer is currently orbiting 600 kilometres above the surface of the Earth after being launched on a Falcon 9 rocket from Cape Canaveral in December 2021. With partners and scientific collaborators from 13 nations, including Finland, the mission is a joint effort between NASA and the Italian Space Agency.


Doroshenko, V., Poutanen, J., Tsygankov, S.S. et al. Determination of X-ray pulsar geometry with IXPE polarimetry. Nat Astron (2022).

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