Neutron stars have the strongest magnetic fields in the universe, and the
only way to measure their surface magnetic field directly is to observe the
cyclotron absorption lines in their X-ray energy spectra. 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. After direct
measurement of the strongest magnetic field in the universe at about 1
billion Tesla in 2020, the world records for the highest energy cyclotron
absorption line and direct measurement of the strongest magnetic field in
the universe have been broken.
The findings, obtained jointly by the Key Laboratory for Particle
Astrophysics at the Institute of High Energy Physics (IHEP) of the Chinese
Academy of Sciences and the Institute for Astronomy and Astrophysics, Kepler
Center for Astro and Particle Physics, University of Tübingen (IAAT), were
published on June 28 in Astrophysical Journal Letters (ApJL). Dr. Kong
Lingda, Prof. Zhang Shu, and Prof. Zhang Shuangnan from IHEP are the
corresponding authors of the paper. Dr. Victor Doroshenko and Prof. Andrea
Santangelo from the University of Tübingen significantly contributed to the
discovery.
A neutron star X-ray binary system consists of a neutron star and its
companion star. Under the strong gravitational force of the neutron star,
the gas of the companion star falls towards the neutron star, forming an
accretion disk. The plasma in the accretion disk will fall along magnetic
lines to the neutron star's surface, where powerful X-ray radiation is
released. Along with the rotation of the neutron star, such emissions result
in periodic X-ray pulse signals, hence the name "X-ray accretion pulsar" for
these objects.
Many observations have revealed that these types of objects have absorption
structures in their X-ray radiation spectra, namely cyclotron absorption
lines, which are thought to be caused by resonant scattering and thus
absorption of X-rays by electrons moving along the strong magnetic fields.
The energy of the absorption structure corresponds to the strength of the
surface magnetic field of a neutron star; therefore, this phenomenon can be
used to directly measure the strength of the magnetic field near the surface
of the neutron star.
Ultraluminous X-ray pulsars are a class of objects whose X-ray luminosity
far exceeds that of canonical X-ray accreting pulsars. They have previously
been discovered in several galaxies far from the Milky Way. Astronomers have
speculated that their pulsars have high magnetic field strengths even though
direct measurement evidence is still lacking.
Insight-HXMT made detailed and broadband observations of the outburst of
Swift J0243.6+6124, the Milky Way's first ultraluminous X-ray pulsar, and
unambiguously discovered its cyclotron absorption line. This line revealed
energy up to 146 keV (with detection significance of about 10 times the
standard deviation), which corresponds 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 direct measurement of the neutron star's surface magnetic field.
It is believed that the surface magnetic fields of neutron stars 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 serves as the first concrete
evidence that a neutron star's magnetic field structure is more complex than
that of a traditional symmetric dipole field, and it also provides the first
measurement of the nonsymmetric component of a neutron star's magnetic
field.
Insight-HXMT is the first Chinese X-ray astronomy satellite. It comprises
scientific payloads including a high-energy telescope, medium-energy
telescope, low-energy telescope, and a space environment monitor.
Insight-HXMT has advantages over other X-ray satellites in terms of
broadband (1-250 keV) spectral coverage, large effective area at high
energies, high time resolution, low dead-time, and no pile-up effects for
bright sources, thus opening up a new window for observing black holes,
neutron stars with hard X-ray fast transitions, and energy spectrum studies.
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. Later, a new record for a cyclotron absorption line—with
its highest energy around 100 keV—was detected by Insight-HXMT from another
neutron star in 1A 0535+262. Insight-HXMT has demonstrated its exceptional
capacity to explore the energy spectrum by breaking its own records for
cyclotron absorption line discoveries.
Reference:
Ling-Da Kong et al, Insight-HXMT Discovery of the Highest-energy CRSF from
the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124, The
Astrophysical Journal Letters (2022).
DOI: 10.3847/2041-8213/ac7711
Tags:
Space & Astrophysics