- Astrophysicists discover a unique neutron star whose magnetic field is apparent only when viewed from a certain angle.
- It belongs to the transient X-ray pulsar named GRO J2058+42
A neutron star is formed when a giant star dies in a supernova and its core collapses, with the electrons and protons essentially melting into each other to form neutrons.
Neutrons stars are the superdense objects with a radius of about 10 kilometers and a mass of about 1.4 solar masses. The magnetic field strength on neutron stars’ surface ranges from 104 to 1011 Tesla, millions of times higher than achieved in the most powerful laboratories.
Usually, the magnetic field on neutron stars has a dipole configuration, which means they have two magnetic poles just like Earth. Until now, all neutron stars could be categorized into two groups:
- Stars in which the magnetic field is observed during the entire spin cycle.
- Stars in which the magnetic field is not detected at all.
Recently, astrophysicists from Moscow Institue of Physics and Technology and Pulkovo Observatory found a unique neutron star whose magnetic field is apparent only when viewed from a certain angle (relative to the observer).
How Did They Discover It?
The Compton Gamma Ray Observatory (CGRO) discovered a transient X-ray pulsar named GRO J2058+42 in 1995. The neutron star analyzed in this study is part of this system. Although it has already been examined using various instruments, nothing has set it apart from other neutron stars.
However, the recent observations made from the Nuclear Spectroscopic Telescope Array (NuSTAR space observatory) revealed some exciting information. The X-ray telescope operates in an extremely wide range (3 to 79 keV), which enabled astrophysics to detect an unusual feature in the pulsar’s emission.
To determine the neutron star’s magnetic field, a cyclotron absorption line (frequency of a charged particle moving a magnetic field) is registered in the source energy spectrum.
In this case, the cyclotron absorption line manifests itself when the neutron star is viewed from a specific angle. This makes it the first object of its own family.
The in-depth ‘tomographic’ investigation of the GROJ2058+42 system has made this discovery possible. Researchers measured the neutron star’s X-ray spectra from ten different angles.
Neutron star’s magnetic field in its initial phase and after its transition to the unstable phase | Courtesy of researches
They found a substantial depression in the emission intensity (about 10 keV) in only one of those ten angles. The emission intensity corresponds to the magnetic field strength of 108 at the neutron star’s surface.
Researchers also note that the GRO J2058+42’s emission can be detected only during bright outbursts, which means the system consists of Be Stars (a heterogeneous set of stars with B spectral types and emission lines).
The outbursts in such binary systems do not occur very often and are hard to predict. Researchers consider themselves lucky to catch the GRO J2058+42 outburst from the beginning.
The findings prove that the magnetic field of neutron stars are much more complex than previously estimated. This complex structure of the magnetic field can retain its shape for a long time and determine the behavior of a neutron star. Overall, scientists will now have a new tool and data for investigating parameters of neutron stars.