Artist’s impression of Swift J0243.6 + 6124 with its spectrum below and the Insight-HXMT used to study it. Image Credit: IHEP
The surfaces of neutron stars have magnetic fields stronger than anything else we know. All neutron stars are not the same, however, and measuring these fields can be challenging, which means we don’t know just how epic these fields can be. A new measurement breaks the previous record by sixty percent.
The Astrophysical Journal Letters reports a 1.6 billion Tesla magnetic field in the Swift J0243.6 + 6124 system, exceeding the record of 1 billion Teslas set in 2020. Rather than car sales beyond Elon Musk’s wildest dreams, the figure represents about 150 million times the strength of the magnet in the world’s most powerful MRI machine, or 300 billion times a fridge magnet if that’s your unit of choice. It sounds even more impressive as 16 trillion Gauss. This is not a case where humanity has beaten nature in the lab, or even close.
To determine this value, Ling-Da Kong of the Chinese Institute of High Energy Physics and co-authors studied the X-ray spectrum of the system looking for cyclotron absorption lines during the system’s powerful 2017 outburst, using the Insight-HXMT space telescope. The higher the energy of these lines, the stronger the field required to produce them. In this case lines were seen at up to 146 keV.
It is common for neutron stars to have strong magnetic fields. A few are so powerful they have earned the name magnetars. However, it is not always possible to measure these fields from the safety of our distance.
In the case of close binaries, including Swift J0243.6 + 6124, the neutron star’s powerful gravitational field (the strongest in the universe other than black holes) draws gas from its companion star, forming an accretion disk. Plasma in the disk is affected by both the gravitational and magnetic fields, and falls to the neutron star’s surface along the magnetic field lines, releasing X-rays in the process.
Since neutron stars rotate, these X-rays form pulses as seen from Earth. Electrons moving in the magnetic field absorb some of these X-rays, in a vastly more powerful version of the cyclotrons built on Earth both for basic research and radiotherapy. The more powerful the magnetic field, the higher energy X-rays the electrons can absorb, so the highest energy absorption line provides a measure of the field close to the star’s surface.
Ultraluminous X-ray pulsars have been observed in other galaxies, but Swift J0243.6 + 6124 is the first found in the Milky Way. Its relative closeness to Earth gave astronomers the chance to test the hypothesis that these objects’ brightness is a consequence of extreme magnetic fields. The 146 keV absorption line – the first detected from such a powerful X-ray pulsar – provided the proof astronomers were looking for. The 1.6 billion Tesla field required to make the line is also about ten times stronger than astronomers had estimated using several indirect techniques.
The same measurements show Swift J0243.6 + 6124’s magnetic field is complex and nonsymmetric, rather than a simple dipole, something suspected but unconfirmed for other neutron stars.
It’s common for stars in very tight orbits lasting only a few days to transfer material, but it’s a testimony to the power of the neutron star in Swift J0243.6 + 6124 that manages to pull material off a companion in a 28-day orbit.