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Starts With A Bang

NASA’s NICER Mission Reveals An Unexpected Neutron Star Surprise

The NICER experiment, designed to measure neutron stars as never before, just released their first pulsar map, and it’s amazing.

After typical supernovae, remnant collapsed cores of matter are left behind.

This image of the Crab Nebula’s core, a young, massive star that’s recently died in a spectacular supernova explosion, exhibits these characteristic ripples due to the presence of a pulsing, rapidly rotating neutron star: a pulsar. At just 1,000 years old, this young pulsar, which spins 30 times per second, appears to be typical of ordinary pulsars. (NASA / ESA)

These objects — neutron stars — are approximately 90% neutrons, surrounded by shells containing charged particles.

The core of a neutron star is expected to be made of neutrons and neutral quark-gluon plasmas, with the outermost layers containing free, charged particles. The rotating star was thought to lead to a dipole magnetic field, but the true field may be even more complicated. (NASA / GSFC / NICER)

As they rapidly spin, they generate strong magnetic fields, accelerating particles and emitting electromagnetic pulses.

The Vela pulsar, like all pulsars, is an example of a neutron star corpse. The gas and matter surrounding it is quite common, and is capable of providing fuel for the pulsing behavior of these neutron stars. (NASA/CXC/PSU/G.PAVLOV ET AL.)

When a pulse intersects our line-of-sight, we detect it: this is why some neutron stars are pulsars.

In 2019, scientists were measuring the pulses coming from a neutron star and were able to measure how a white dwarf orbiting it delayed the pulses. From the observations, scientists determined it had a mass of around 2.2 solar masses: the heaviest neutron star seen thus far. (B. SAXTON, NRAO/AUI/NSF)

They are denser than atomic nuclei but cannot be too massive, otherwise they collapse into black holes.

Looking at binary sources, such as black holes and neutron stars, has revealed two populations of objects: low-mass ones below about 2.5 solar masses and high-mass ones of 5 solar masses and above. While LIGO and Virgo have detected black holes more massive than that and one instance of neutron star mergers whose post-merger product falls into the gap region, we are still not sure what persists in there otherwise. (FRANK ELAVSKY, NORTHWESTERN UNIVERSITY AND LIGO-VIRGO COLLABORATIONS)

Even with our most powerful telescopes in all wavelengths of light, neutron stars only appear as points.

VLT image of the area around the very faint neutron star RX J1856.5–3754. The blue circle, added by the author, shows the location of the neutron star. (ESO / E. SIEGEL)

NASA’s NICER mission, installed aboard the ISS in 2017, sought to change all that.

NASA’s Neutron star Interior Composition ExploreR, going by the strained acronym NICER, is installed aboard the International Space Station and provides humanity with unprecedented X-ray measurements of neutron stars. (NASA)

The low-energy X-ray observatory measures timing signals down to 300 nanoseconds and at unprecedented sensitivities.

In terms of flux, timing, and energy resolution, NASA’s NICER mission surpasses all other pre-existing observatories in its observation of pulsars in particular and neutron stars in general. (NASA / GSFC / NICER)

NICER enables measurements of neutron stars’ sizes, masses, cooling times, stabilities, and internal structures.

The two best-fit models of the map of the neutron star J0030+0451, constructed by the two independent teams who used the NICER data, show that either two or three ‘hot spots’ can be fitted to the data, but that the legacy idea of a simple, bipolar field cannot accommodate what NICER has seen. (ZAVEN ARZOUMANIAN & KEITH C. GENDREAU (NASA GODDARD SPACE FLIGHT CENTER))

For one pulsar in particular, J0030+0451, they determined its mass (1.35 Suns) and diameter (25.7 km) explicitly.

The pulsar J0030+0451, based on NICER data, is shown to have ‘hot spots’ only in its southern hemisphere, which means that a magnetic model involving only a typical magnetic dipole cannot explain what we observe. Here, a large quadrupole, from simulations, is shown to be a far superior fit to the data. (NASA’S GODDARD SPACE FLIGHT CENTER)

They detected “hot spots” on the surface and constructed the first-ever neutron star map.

A neutron star is one of the densest collections of matter in the Universe, whose strong magnetic field generates pulses by accelerating matter. The fastest-spinning neutron star we’ve ever discovered is a pulsar that revolves 766 times per second. However, now that we have a map of a pulsar from NICER, we know that this two-pole model cannot be correct; the pulsar’s magnetic field is more complex. (ESO/LUÍS CALÇADA)

They concluded that pulsar magnetic fields are more complex than typical, naive two-pole models.

At the cores of the most massive neutrons stars, the individual nuclei may break down into a quark-gluon plasma. Theorists presently argue over whether that plasma would exist, and if so, whether it would be composed of up-and-down quarks only, or whether strange quarks would be a part of that mix, too. (CXC/M. WEISS)

It’s one step closer to the ultimate goal: discovering which matter states exist in pulsar cores.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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