Scientists have measured the weakest magnetic field in X-ray pulsars.

Thanks to the joint work of several X-ray observatories, it was possible to find an accreting X-ray pulsar with the weakest magnetic field known to date.

Accretion is the fall of matter onto a space body under the influence of gravitational forces. Accretion is accompanied by the release of gravitational energy. The efficiency of energy release during accretion onto neutron stars is tens of times greater than in nuclear reactions. If the magnetic field of a neutron star is strong enough, then it is capable of directing flows of matter to the magnetic poles.

In order to measure the magnetic field of a neutron star, it is necessary to detect the so-called cyclotron absorption lines in the spectrum of its electromagnetic radiation. These are relatively narrow spectral features arising from the interaction of radiation with electrons moving along the lines of force of the magnetic field. The observed energies of these subtle features are proportional to the magnetic field and are harmonically distributed. This means that, in addition to the main line, its harmonics can be observed at energies that are multiples of the energy of the main line.

Until now, only one pulsar was known, in the spectrum of which more than four cyclotron lines were found. He was considered the weakest among the known.

This record was broken thanks to the joint work of scientists from the Space Research Institute of the Russian Academy of Sciences, the Moscow Institute of Physics and Technology, and their colleagues from scientific organizations in Germany and Finland. Onboard the International Space Station, it was discovered that the radiation flux is increasing in the direction of the little-studied X-ray pulsar Swift J1626.6-5156.

An analysis of the energy spectrum of Swift J1626.6-5156 revealed four harmonically distributed absorption features at energies divisible by 4.9 keV. These features were interpreted as the fundamental cyclotron line and its three higher harmonics, which corresponds to the magnetic field on the surface of the neutron star ~ 4 × 1011 Gauss.

This is several times less than typical values ​​and today is the smallest among all known X-ray pulsars. Our discovery will significantly expand knowledge about magnetic fields in neutron stars. In addition, it turned out to be very “timely” in light of the fact that the launch of the IXPE observatory (NASA, ESA) is planned for the fall of 2021, and the eXTP observatory (China, ESA) will go into space four years later.

Sergey Molkov, first author of the article, senior researcher at IKI RAS and employee of MIPT