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An international team of scientists, including astronomers from the University of Amsterdam, has succeeded in producing the first precise and dependable measurements of both a pulsar’s size and its mass, as well as the first surface map of one of these mysterious objects. Pulsar J0030+0451 turns out to be 1.3 times heavier than the sun, measure 25 kilometres in diameter and have a more complicated magnetic field than the theoretical 'bar magnet model' predicts. The researchers are publishing their findings in a series of articles in the Astrophysical Journal Letters.

Image from a computer simulation of the bizarre hotspots of pulsar J0030+0451. The hot spots do not appear to be directly opposite each other. Source: NASA

The astronomers made their observations between July 2017 and December 2018 using NICER, NASA's Neutron star Interior Composition Explorer. This x-ray instrument aboard the International Space Station is helping researchers to gain a greater understanding of the ultra-dense composition of this type of neutron star.

Pulsars are small, compact neutron stars that rotate hundreds of times a second. They are the remnants of dead heavy stars. The pulsar J0030+0451 is located 1,100 lightyears from Earth in the direction of the constellation Pisces. It rotates around its axis 205 times a second.

Observations show bizarre hotspots

For decades, astronomers have been trying to discover what pulsar magnetic fields look like. In the most common models, a pulsar is represented as a sphere containing an upright bar magnet. Field lines run between its poles. The theory is that the magnetic field is so strong that particles that happen to be in the vicinity are immediately dragged to the poles and hit them at high speeds. This leads to hot spots at the poles. After an extensive study of pulsar J0030+0451, however, it turns out that the hot spots are not directly opposite each other, which means that the magnetic field structure is much more complex than previously imagined.

Supercomputer simulations

The researchers concluded this after developing detailed models of the pulsar and its hot spots, and simulating them using Cartesius, the Dutch national supercomputer. Cartesius concluded that the data were best explained by hot spots that were spread over the pulsar. Armed with this information, the researchers were able to make the first measurement of the star’s mass and size using this new technique. Such measurements allowed them to learn about the behaviour of ultradense matter in the star’s core.

Scientific articles

The results of the research on J0030+0451 appear in a series of articles in the journal Astrophysical Journal Letters. Three articles have first authors from the University of Amsterdam. The research is co-funded by the European Research Council and the Netherlands Organisation for Scientific Research.

A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation.
T.E. Riley, A.L. Watts et al. In Astrophysical Journal Letters. https://iopscience.iop.org/article/10.3847/2041-8213/ab481c

A NICER View of PSR J0030+0451: Implications for the Dense Matter Equation of State.
G. Raaijmakers, T.E. Riley, A.L. Watts et al. In Astrophysical Journal Letters. https://iopscience.iop.org/article/10.3847/2041-8213/ab451a

A NICER View of PSR J0030+0451: Evidence for A Global-Scale Multipolar Magnetic Field.
A.V. Bilous, A.L. Watts et al. In Astrophysical Journal Letters. https://iopscience.iop.org/article/10.3847/2041-8213/ab53e7

Commentary: Focus on NICER Constraints on the Dense Matter Equation of State.
Zaven Arzoumanian & Keith C. Gendreau. In Astrophysical Journal Letters. https://iopscience.iop.org/journal/2041-8205/page/Focus_on_NICER_Constraints_on_the_Dense_Matter_Equation_of_State