The team behind the first Chinese X-ray astronomy satellite, Insight-HXMT, has discovered the strongest magnetic field directly measured in the universe hitherto.
It is a known fact that neutron stars generate the strongest magnetic fields in the universe. These magnetic fields, close to a neutron star’s surface, can only be measured accurately and directly by looking for cyclon resonance scattering features (CRSF). 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.
The findings were published last month in Astrophysical Journal Letters. They 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.
This isn’t Insight-HXMT’s first record.
Two years ago, the Insight-HXMT team reported the detection of a 90 keV cyclotron absorption line from a neutron star in the X-ray binary system GRO J1008-57, which corresponds to a surface magnetic field of 1 billion Tesla, which set a world record for direct measurement of the universe’s strongest magnetic field at the time. Later, another record for a cyclotron absorption line—with its highest energy around 100 keV—was detected by Insight-HXMT from another neutron star in 1A 0535+262.
Time and again, the astronomy satellite has illustrated its ability to explore the energy spectrum. And this time, it beat its previous record by 60 percent.
A neutron star X-ray binary system comprises a neutron star and its companion star. The gas of the companion star falls towards the neutron star, forming an accretion disk. In turn, the plasma in the accretion disk will fall along magnetic lines to the neutron star’s surface, releasing powerful X-ray radiation. Such emissions result in periodic X-ray pulse signals, resulting in the name “X-ray accretion pulsar.”
Prior observations have revealed that such pulsars have absorption structures in their X-ray radiation spectra, known as cyclotron absorption lines. This might be caused by resonant scattering and thus the absorption of X-rays by electrons moving along the strong magnetic fields.
This phenomenon can be used to directly measure the strength of the magnetic field near the surface of a neutron star as the energy of the absorption structure corresponds to the strength of the surface magnetic field.
Now, ultraluminous X-ray pulsars, objects whose X-ray luminosity far exceeds that of canonical X-ray accreting pulsars, have previously been discovered in several galaxies far from the Milky Way.
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 particular line revealed energy up to 146 keV, which equals a surface magnetic field of more than 1.6 billion Tesla, resulting in 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.
The direct magnetic field measurement by Insight-HXMT based on the cyclotron absorption line is an order of magnitude greater than that estimated using indirect means. This is the first solid evidence that a neutron star’s magnetic field structure is more complex and nonsymmetric than a traditional symmetric component of a neutron star’s magnetic field.