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Mapping a cosmic pinpoint in space

The creation of a neutron star sounds like a science fiction tale

Two teams calculated the size and mass of the neutron star J0030, coming up with similar estimates. In addition, their findings about hot spots that release beams of radiation are extremely close. The hot spots are represented as white and mottling on the rest of the star is illustrative fill. The map on the left was compiled by researchers at the University of Maryland led by astronomy professor Cole Miller. The right view was created by scientists led by Thomas Riley, a doctoral student in computational astronomy, and his supervisor, Anna Watts, professor of astrophysics at the University of Amsterdam.
NASA, NICER, Goddard Space Flight Center’s Conceptual Image Lab

Editor’s note: A version of this has been previously published on the author’s website.

Think of an object 16 miles across. It is 1,100 light-years away. (One light year is about 5.8 trillion miles; you can multiply that by 1,100 — it’s too many digits for my little brain.) Now imagine that you are tasked with mapping the object’s surface. Impossible? Not to two teams of scientists who set out to map a neutron star, a pulsar with the ungainly appellation of J0030+0451.

The creation of a neutron star sounds like a science fiction tale. It begins when a star, forming out of a molecular cloud in space, happens to compress enough gas and dust to become a gigantic sphere far bigger than stars like our sun. The behemoth is so huge that it burns through the hydrogen fuel quickly to become a red giant; then, as it expands and puffs off its outer atmosphere, a red super giant. When the nuclear furnace has too little fuel left to fend off the pressures of gravity, it collapses in a supernova explosion brighter than any other star in its galaxy.

Giant stars are examples of the saying, “Live fast and die young.” Their lives are measured in millions of years, rather the billions enjoyed by ordinary stars like our sun.

If the remnants of the core are at least three times the mass of the sun, the collapse is unstoppable and results in a stellar-mass black hole. If the supermassive star is not large enough to form a black hole, the electrons and protons of the core’s remains are smashed together by gravity, turning into neutrons. The dense ball of neutrons stop the collapse, leaving a neutron star, a small, extremely compact object.

As NASA explains: “This collapse leaves behind the most dense object known — an object with the mass of a sun squished down to the size of a city. These stellar remnants measure about 20 kilometers (12.5 miles) across. One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth — about as much as a mountain.”

As an ice skater spins faster when she draws in her arms, a neutron star’s revolutions speed up drastically as it shrinks from the size of several suns to that of a city. Many revolve at speeds ranging from a revolution every few seconds to milliseconds (1/1000 of a second). The variety called pulsars are neutron stars that emit radiation, with X-rays the easiest type to detect.

According to the Smithsonian Astrophysical Observatory, when neutron stars “have associated magnetic fields, charged particles caught in them emit electromagnetic radiation in a lighthouse-like beam that can sweep past the Earth with great regularity every few seconds or less.” The beams must be aimed in our direction to be detectable, and when they are, receivers can pick up the beams’ blips whenever they are briefly aimed our way.

J0030 (its nickname) rotates 205 times a second.

NASA’s Neutron star Interior Composition Explorer (idiotically abbreviated NICER), an X-ray telescope installed on the outside of the International Space Station in 2017, made observations of pulsar J0030 from July that year to December 2018.

The NICER X-ray telescope is attached to the International Space Station in this illustration. The instrument is the boxlike device with multiple circular observation ports called X-ray concentrator optics.
NASA, NICER

NICER has “unique capabilities for timing and spectroscopy of fast X-ray brightness fluctuations,” NASA says. Among its potential uses is to determine if pulsars could be use as navigational beacons for spacecraft. Another is to learn about neutron stars.

Researchers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, noted that “NICER measures the arrival of each X-ray from a pulsar to better than a hundred nanoseconds, a precision about 20 times greater than previously available.” A nanosecond is one billionth of a second, meaning the precision of timing the pulses’ arrival is accurate to within 100 billionths of a second.

Two teams of researchers from Holland and the United States analyzed the readings independently. They were based at the University of Amsterdam and the University of Maryland, College Park. The timing figures and Einsteinian mathematics were used with computer simulations that showed hot spots where beams of radiation blast outward. The simulations included “overlapping circles of different sizes and temperatures.” Calculations were applied to the models to show resultant X-ray signals, which were checked to see which best fit the observations.

The Dutch team used a supercomputer called Cartesius that delivered its findings in less than a months, a project that would have taken 10 years on a modern desktop computer, the Space Flight Center noted. The Maryland group used a computer called Deepthought2 and ovals of different sizes and temperatures.

A Dec. 12, 2019, release by Goddard says the teams “obtained the first precise and dependable measurements of both a pulsar’s size and its mass, as well as the first-ever map of hot spots on its surface.” Their findings were strikingly similar.

Dutch team: the pulsar is about 1.3 times the sun’s mass and 15.8 miles across, with two hot spots, both in the star’s southern hemisphere. One is small and circular, the other is long and crescent-shaped.

Maryland team: the pulsar is about 1.4 times the sun’s mass and 16.2 miles across. It has two possible configurations for hot spots, both in the southern hemisphere. One configuration closely matches the Dutch findings, while the other possibility “adds a third, cooler spot slightly askew of the pulsar’s south rotational pole.”

Goddard’s release says the traditional understanding of pulsars would indicate one hot spot at each of the star’s magnetic poles. Because our view of the star looks down on its northern hemisphere, one hot spot was expected there. Instead, there are none in the north and either two or two plus one slightly cooler spot in the south.

”Previous theoretical predictions suggested that hot spot locations and shapes could vary, but the J0030 studies are the first to map these surface features,” says Goddard. “Scientists are still trying to determine why J0030’s spots are arranged and shaped as they are, but for now it’s clear that pulsar magnetic fields are more complicated than the traditional two-pole model.”

Goddard’s release says that once NICER has analyzed several pulsars, scientists will be able to understand the material at the cores of neutron stars, an environment impossible to duplicate on Earth.

Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at the-nightly-news.com and is an avid amateur astronomer. His email is joe@the-nightly-news.com.