Remeis astronomers have devised a new method for measuring magnetic fields in neutron-star atmospheres.
Astronomers from the X-ray Group of Prof. Jörn Wilms at the Dr. Karl Remeis Observatory have uncovered a direct correlation between the shape of X-ray spectra and the magnetic field strength of neutron stars in Be-X-ray binaries. Such systems contain a highly magnetized neutron star and a Be-type star. Using archival observations from the NuSTAR telescope, the team led by PhD student Nicolas Zalot studied nine Be X-ray binary systems in very low accretion states, where their spectra display two distinct “humps.” They found that the energy at which these two components intersect increases in step with the neutron star’s magnetic field strength measured independently during outbursts. This reveals that the magnetic field leaves a measurable imprint on the overall spectral shape, not just in narrow absorption features.
To understand this behavior, the team compared the observations with detailed theoretical models of X-ray emission in magnetized neutron star atmospheres. The models show the same trend: stronger magnetic fields shift the high-energy hump to higher energies, naturally moving the intersection point between the two components. The agreement between observations and theory demonstrates that this correlation most likely arises from the underlying physics of how X-rays are produced near the neutron star surface at low accretion rates.
X-ray spectra from the paper, fitted with the “double-hump” model; Each spectrum corresponds to a different astrophysical object with a magnetic-field strength measured through the newly developed model
Read more: A simple relation: Neutron star magnetic field strength and spectral shape at low mass accretion rates
Zalot, N., Sokolova-Lapa, E., et al., 2026, A&A, in press
Nicolas Zalot
nicolas.zalot@fau.de
+49 9131 85-81021
Jörn Wilms
Joern.Wilms@sternwarte.uni-erlangen.de
+49 9131 85-81013