A lighthouse that changes the color of each flash in time with its rotation: that's one way to describe what astronomers saw when they pointed the latest X-ray telescope, XRISM, at the pulsar Hercules X-1. This object is a neutron star pulling matter from a companion ordinary star. As it falls, the matter spirals into narrow columns along magnetic field lines and slams into the poles at enormous speeds — tens of thousands of kilometers per second.
In such a furnace, iron atoms caught in the flow emit a characteristic X-ray glow. Previously, this appeared as a smeared line, like a note blurred into a chord by frantic motion. Now, the Resolve instrument aboard XRISM, working like a microphone with perfect pitch, has resolved the fine structure of this signal. It turns out the line's brightness varies with the pulsar's phase: when one of the hot pillars faces us, the line flares up; when it turns away, it fades.
But the most intriguing discovery lies in the long 35-day modulation. The pulsar seems to rock slowly, and its accretion columns rock along with it. This dance, the neutron star's precession, affects how we see the iron line's pulsations. Watching this dance is like listening to the changing pitch of a passing siren: from the shift in tone, you can deduce its motion. Similarly, by tracking variations in the iron line, scientists hope to measure how the neutron star rotates inside — something that remains a mystery.
as written by the authors arXiv:2605.01146
The study of X-ray pulsar accretion columns helps us characterize accretion physics in this extreme regime of strong gravity and strong magnetic fields. Previous observations of the X-ray pulsar Hercules X-1 revealed a highly broadened Fe K emission line, associated with Doppler motions exceeding 0.1c, suggesting its origin in the accretion column. We obtained a high-spectral resolution view of the Fe K energy band of Hercules X-1 thanks to a 200 ks observation with the XRISM observatory. The XRISM/Resolve microcalorimeter spectra allow us to separate the different spectral components and accurately model them with phenomenological models. We confirm the presence of a broad line near 6.5 keV with a typical $$1\sigma$$ width of 1 keV. Performing a pulse-phase-resolved analysis, we find that the feature is strongly variable with Her X-1 pulse phase. This is consistent with the proposed origin due to collisional recombination or by reprocessing of the primary X-ray emission in the accretion column, where strong variability with pulse phase is expected due to the rotation of the columns alongside with the neutron star. Additionally, the Fe K line pulsation pattern evolves with the 35-day cycle of Hercules X-1, supporting the scenario that the neutron star and its accretion columns undergo precession, in agreement with recent polarimetric results from the IXPE observatory. We discuss the future applications of modeling of this broad line in X-ray pulsars with physical spectral models. This could be used to detect and track neutron star precession, advancing our understanding of neutron star interiors.
Accretion columns above a pulsar's magnetic poles are among the hottest places in the universe: matter there heats up to hundreds of millions of degrees, emitting X-rays.
In Robert Forward's novel Dragon's Egg, life is depicted on the surface of a neutron star, where immense gravity and magnetic fields create conditions similar to accretion columns.