The magnetic field close to a neutron star’s surface can only be measured directly and accurately by looking for cyclotron resonance scattering features (CRSFs). The broad energy coverage and large collection area of Insight-HXMT in the hard X-ray band allowed the detection of CRSF with the highest energy known to date.
In 2020, 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, corresponding 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. The Insight-HXMT team has recently discovered a cyclotron absorption line with an energy of 146 keV in the neutron star X-ray binary Swift J0243.6+6124, corresponding to a surface magnetic field of more than 1.6 billion Tesla.
This new measurement records the highest energy cyclotron absorption line and direct measurement of the strongest magnetic field in the universe.
The cyclotron absorption lines are thought to be caused by resonant scattering and thus the absorption of X-rays by electrons moving along the strong magnetic fields. This phenomenon can be utilized to directly assess the strength of the magnetic field close to the surface of the neutron star because the energy of the absorption structure matches the strength of the surface magnetic field of a neutron star.
The cyclotron absorption line of Swift J0243.6+6124, the Milky Way‘s first ultraluminous X-ray pulsar, was definitively identified by Insight-HXMT through extensive and broad-band studies of its outburst. With a detection significance of almost ten times the standard deviation, this line indicated energy up to 146 keV, which is equivalent 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 a direct measurement of the neutron star’s surface magnetic field.
The surface magnetic fields of neutron stars are believed to have complex structures, ranging from dipole fields very far from the neutron star to multipole fields only influencing the area close to the neutron star. However, most earlier indirect estimates of the magnetic fields of neutron stars have probed only the dipole fields.
This time, the direct magnetic field measurement by Insight-HXMT based on the cyclotron absorption line is about an order of magnitude greater than that estimated using indirect means. This is the first concrete evidence that a neutron star’s magnetic field structure is more complex than a traditional symmetric dipole field. It also provides the first measurement of the nonsymmetric component of a neutron star’s magnetic field.