Scientists from Moscow Institute for Physics and Technology, Space Research Institute of the Russian Academy of Sciences (IKI), and Pulkovo Observatory discovered a unique neutron star, the magnetic field of which is apparent only when the star is seen under a certain angle relative to the observer. Previously, all neutron stars could be grouped into two big families: the first one included objects where the magnetic field manifests itself during the whole spin cycle, and the other one included objects where the magnetic field is not measured at all. The neutron star GRO J2058+42 studied by the researchers offers an insight into the internal structure of neutron star’s magnetic field only at a certain phase of its rotational period. The work was published in the Astrophysical Journal Letters and supported by the Russian Science Foundation.
The neutron star in the GRO J2058+42 system was discovered almost quarter of a century ago with the Compton Gamma-Ray Observatory (CGRO), USA. It belongs to the class of so-called transient X-ray pulsars. This object was studied using different instruments and nothing set it apart from other objects of its class. Only recent observations with the NuSTAR space observatory that has an outstanding combination of high energy resolution (<400 eV) and extremely wide energy range (3–79 keV), enabled the scientists to detect a peculiar feature in the pulsar’s emission, potentially making it the first object of its own family.
A cyclotron absorption line was registered in the source energy spectrum that allows for estimating the magnetic field strength of the neutron star. Such an observational phenomenon (cyclotron line) is not new and is currently observed in approximately 30 X-ray pulsars. The uniqueness of the Russian scientists’ discovery is that this line manifests itself only when the neutron star is seen under a certain angle to the observer. This discovery became possible due to a detailed “tomographic” analysis of the system. X-ray spectra of the neutron star GROJ2058+42 were measured from ten different directions and only in one of them a significant depression in the emission intensity around 10 keV was found. This energy corresponds approximately to the magnetic field strength of 1012 G at the surface of the neutron star. The obtained result is especially interesting due to a simultaneous registration of higher harmonics of the cyclotron line at the same rotational phase of the neutron star (figure 1).
Neutron stars are superdense objects with a radius of about 10 km (6 miles) and a mass of 1.4–2.5 mass of the Sun. Neutron stars are born as a result of supernova explosions that can lead to such compression of the matter that electrons merge with protons and form neutrons, resulting in colossal masses in small volumes. Moreover, the magnetic field strength at the surface of the neutron star after the collapse may reach 1011–1012 G (which is tens of millions of times higher than achieved in the most powerful Earth labs). Typically, neutron stars have a dipole configuration of the magnetic field, i.e. they have two poles (similar to the Earth, which has the North and the South magnetic poles).
Some of the neutron stars may form binary systems with normal stars, capturing the matter from their normal companions and accreting it onto magnetic poles This process is somewhat similar to the Earth capturing solar wind particles, which results in a phenomenon known as aurora. If the neutron star’s rotation axis does not coincide with its magnetic axis, the observer will register a periodic signal, like one from a lighthouse, and the star appears as an X-ray pulsar.
GRO J2058+42 is a quite peculiar X-ray pulsar because its emission can be observed only during bright outbursts. Such behavior is explained by the fact that the companion star in this system belongs to the so-called class Be-stars. Such stars rotate around their axis so rapidly that an outflowing (or the so-called decretion) disc of matter forms around their equator. As the neutron star moves around a high mass normal component, the matter from such disc starts to flow to its surface, which leads to an outburst, or a quick increase of the luminosity. These are ideal moments for studying the physical properties of such objects.
Such studies are typically complicated by the fact that outbursts in most such systems are rather rare and cannot be reliably predicted. Therefore, it is important to promptly organize observations with space observatories when such events do happen. Scientists from the above-mentioned institutes were fortunate to catch the beginning of a new outburst from GRO J2058+42 and quickly organize a series of observations with the NuSTAR observatory. These observations showed that the magnetic field manifests itself only during certain phases of the neutron star rotation, which may point to its unusual configuration or peculiarities in the system’s geometry. The obtained results were so intriguing that the Russian scientists contacted their colleagues from the NuSTAR team and suggested carrying out additional observations that confirmed the initial findings.
In general, possible inhomogeneities in the magnetic field structure of neutron stars were predicted by the theoretical calculations (figure 2), but previously such inhomogeneities had been believed to form only through short outbursts, observed from magnetars. The discovery by the Russian scientists proved for the first time that the magnetic field of a neutron star has a considerably more complex structure than what had been believed earlier, and that this complex structure may retain its shape for a rather long time and be a fundamental property of an object.
Alexander Lutovinov, Professor of the Russian Academy of Sciences, Deputy Director for Research at Space Research Institute, MIPT professor, and one of the discovery authors, said, “The structure of magnetic fields of neutron stars is a fundamental issue of its formation and evolution. On the one hand, the dipole structure of the progenitor star should be preserved during the collapse, but on the other hand, even our own Sun has local magnetic field inhomogeneities that are manifested as sun spots. Similar structures were theoretically predicted for neutron stars as well. It is great to witness them in real data for the first time. The theorists will now have new factual data for their modeling, and we will have a new tool for studying parameters of neutron stars.”
 Cyclotron frequency is the frequency of a charged particle (in this case, an electron) moving and rotating in a magnetic field. Depending on external conditions, one can observe either an emission or absorption at this frequency. The latter is registered in X-ray pulsars’ spectra, which enables direct measurement of their magnetic field.
Reference: “Discovery of a Pulse-phase-transient Cyclotron Line in the X-Ray pulsar GRO J2058+42” by S. Molkov, A. Lutovinov, S. Tsygankov, I. Mereminskiy and A. Mushtukov, 18 September 2019, Astrophysical Journal Letters.