Neutron stars generate the strongest magnetic fields in the universe. However, the only way we have to measure their surface magnetic field directly is to observe the cyclotron absorption lines in their X-ray energy spectra.
Recently, the Insight-HXMT team discovered a cyclotron absorption line with an energy of 146 keV in the neutron star X-ray binary Swift J0243.6+6124, which translates to a surface magnetic field of more than 1.6 billion Tesla. After direct measurement of the strongest magnetic field in the universe at about 1 billion Tesla in 2020, the world records for the highest energy cyclotron absorption line and direct measurement of the strongest magnetic field in the universe have been smashed by a large margin. The findings, which were published on June 28, 2022, in Astrophysical Journal Letters (ApJL), were obtained jointly by the Key Laboratory for Particle Astrophysics at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences and the Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, University of Tübingen (IAAT). Dr. Lingda Kong, Professor Shu Zhang, and Professor Shuangnan Zhang from IHEP are the corresponding authors of the paper. Dr. Victor Doroshenko and Professor Andrea Santangelo from the University of Tübingen significantly contributed to the discovery.
A neutron star X-ray binary system consists of a neutron star and its companion star. Under the strong gravitational force of the neutron star, the gas of the companion star falls towards the neutron star, forming an accretion disk. The plasma in the accretion disk will fall along magnetic lines to the neutron star’s surface, where powerful X-ray radiation is released. Along with the rotation of the neutron star, such emissions result in periodic X-ray pulse signals, hence the name “X-ray accretion pulsar” for these objects. Many observations have revealed that these types of objects have absorption structures in their X-ray radiation spectra, namely cyclotron absorption lines, which are thought to be caused by resonant scattering and thus absorption of X-rays by electrons moving along the strong magnetic fields. The energy of the absorption structure corresponds to the strength of the surface magnetic field of a neutron star; therefore, this phenomenon can be used to directly measure the strength of the magnetic field near the surface of the neutron star.
Ultraluminous X-ray pulsars are a class of objects whose X-ray luminosity far exceeds that of canonical X-ray accreting pulsars. They have previously been discovered in several galaxies far from the Milky Way. Astronomers have speculated that their pulsars have high magnetic field strengths even though direct measurement evidence is still lacking. Insight-HXMT made detailed and broadband observations of the outburst of Swift J0243.6+6124, the Milky Way’s first ultraluminous X-ray pulsar, and unambiguously discovered its cyclotron absorption line. This line revealed energy up to 146 keV (with detection significance of about 10 times the standard deviation), which corresponds to a surface magnetic field of more than 1.6 billion Tesla. This is not only the strongest magnetic field directly measured in the universe to date but also the first detection of an electron cyclotron absorption line in an ultraluminous X-ray source, thus providing direct measurement of the neutron star’s surface magnetic field.
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