Categories: neutron starPulsars

Astronomers Measure a 1-billion Tesla Magnetic Field on the Surface of a Neutron Star

We recently observed the strongest magnetic field ever recorded in the Universe. The record-breaking field was discovered at the surface of a neutron star called GRO J1008-57 with a magnetic field strength of approximately 1 BILLION Tesla. For comparison, the Earth’s magnetic field clocks in at about 1/20,000 of a Tesla – tens of trillions of times weaker than you’d experience on this neutron star…and that is a good thing for your general health and wellbeing.

Neutron stars are the “dead cores” of once massive stars which have ended their lives as supernova. These stars exhausted their supply of hydrogen fuel in their core and a power balance between the internal energy of the star surging outward, and the star’s own massive gravity crushing inward, is cataclysmically unbalanced – gravity wins. The star collapses in on itself. The outer layers fall onto the core crushing it into the densest object we know of in the Universe – a neutron star. Even atoms are crushed. Negatively charged electrons are forced into the atomic nuclei meeting their positive proton counterparts creating more neutrons. When the core can be crushed no further, the outer remaining material of the star rebounds back into space in a massive explosion – a supernova. The resulting neutron star, made of the crushed stellar core, is so dense that a single sugar-cube-sized sampling would weigh billions of tons – as much as a mountain (though if you’re “worthy” you MIGHT able to lift it since Thor’s Hammer is made of the stuff). Neutron stars are typically about 20km in diameter and can still be a million degrees Kelvin at the surface.

But if they’re “dead,” how can neutron stars be some of the most magnetic and powerful objects in the Universe?

Composite image of the maelstrom at the heart of the Crab Nebula powered by a neutron star – Chandra X-Ray Observatory

GRO J1008-57 is a spinning neutron star or “pulsar.” Pulsars were first discovered in 1967 by Jocelyn Bell through observations of a regular radio “pulse” of 1.33 seconds.  The pulses were determined not to be of human origin so the object was designated – though facetiously – LGM1 (Little Green Men 1). A spinning neutron star projects a beam of energy along its magnetic poles that sweeps across space as the star rotates – like the beams from a turning lighthouse. Depending on the orientation of the star, those beams can sweep along Earth’s field of view resulting in a “pulse” of energy with each of the star’s rotations. But why do neutron stars have incredibly powerful magnetic fields? Seems counterintuitive given that they are made of neutrally charged particles (where neutron gets its name). Well, if you were to cut away a neutron star, it is formed of several layers. A cloud of remaining electrons near the surface, further down traces of charged “impurities” of various atomic nuclei remaining after the formation of the neutron star, a crust of neutrons, and a core of a theorized frictionless neutron fluid further mixed with impurities. The combination of layers makes the star incredibly conductive. Spin a very conductive object and you create a churning flow of charged particles which generates a powerful magnetic field. Our planet’s own magnetic field is itself created by the rotation of the Earth’s nickel-iron core. However, neutron star rotations are astonishingly fast. Like a figure skater retracting their arms to spin more quickly, the “angular momentum” of the original giant star, millions of kilomtetres in radius, is preserved and transferred to an ever faster spinning compact object only 10 km wide (imagine a spinning figure skater with arms millions of kilometres long pulling them all the way to the centre of their body). The first neutron star discovered had a rotation period of 1.33s. GRO J1008-57 is 93.3s. Some rotate in mere milliseconds. So, these “dead” stars are the size of a city, denser than any material in the universe, are a million degrees, and spin at a good fraction…of the speed of light. (BTW, on the theme of “dead” stars one of the Grateful Dead members, Mickey Hart, created songs out of pulsar beats.)

5 month time-lapse of the shockwaves emanating from the Crab Nebula Pulsar as it rotates within the Crab Nebula. The Nebula is itself the remains of a supernova explosion – Chandra Observatory
The rotation of a pulsar is seen from Earth similar to how we see light from a lighthouse at night

But how can we measure the strength of a pulsar’s magnetic energy? A special technique can be used with a specific class of pulsars which GRO J1008-57 belongs to called accretion powered X-Ray pulsars.

GRO J1008-57, about 20,000 light years from Earth, is actually in a binary gravitational relationship with a living class B companion star. B’s are hefty stars, a dozen or so times the mass of our Sun and thousands of times brighter. GRO J1008-57’s super density creates a powerful gravitational pull 100 billion times more powerful than Earth’s which rips stellar material off its companion. That material falls toward the neutron star. It becomes entangled in the neutron star’s magnetic field flowing along the “lines” of that field to the north and south magnetic poles where it accumulates or accretes on the surface.

Plasma in the Sun following its own magnetic field lines – Solar Dynamics Observatory

The stellar material slams into the surface at half the speed of light releasing tremendous X-Ray energy.  These X-Rays, before radiating away from the neutron star, pass through the magnetic field at the neutron star’s surface. The magnetic field scatters some of the X-Rays leaving a gap or “absorption line” in the spectrum of the X-Rays. It’s like a fingerprint left by the magnetic field on the X-Ray energy that we can see with our telescopes. Where that absorption line appears along the X-Ray spectrum directly relates to the strength of the magnetic field at the neutron star’s surface where the stellar material is falling. The line phenomenon is known as a Cyclotron Resonance Scattering Feature.

Artist rendition of an accreting binary X-Ray pulsar ripping material off of its stellar companion – NASA

In 2017, the brightest X-Ray outburst ever observed from GRO J1008-57 was recorded by the Chinese Insight-HXMT satellite. A team of scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences and Eberhard Karls University of Tübingen, Germany analyzed the Cyclotron abortion lines in the X-Ray spectrum received. The team recently announced they had discovered lines in the spectrum corresponding to a 1-billion Tesla magnetic field – the most powerful ever recorded in the Universe. Powerful enough to literally pull atoms apart. So, if it doesn’t vaporize you with its immense heat, or obliterating gravity, your atomic structure would basically dissolve in the magnetic forces.

Jocelyn Bell in 1967 when she discovered the first Pulsar. Nerdy before it was cool
Roger W HaworthFlickr CC 2.0

At the Simon Fraser University Trottier Observatory, where I’ve done astrophotography imaging, we recently installed a spectrometer. Similar to Insight-HXMT, we observed spectra from objects in space – though in visible light rather than X-Rays. I was admittedly underwhelmed. I was used to seeing data come through the scope as these beautiful images of stars and galaxies rather than absorption lines in a spectrum. As I was interpreting the data, Dr. Howard Trottier, founder of the Observatory pointed at some lines in a spectrum and said, “that’s an accretion disk orbiting a star” and my mind exploded. Suddenly a line was a churning mass of plasma around some distant star. And that’s science!! A tiny line reveals a distant part of the Universe that we may not be able to “see” but can deduce through decades of research, and our imagination, transforming data into accretion disks, giant stars, plasma flying at near light-speeds, powerful X-Rays, and spinning stellar relics. SCIENCE!!

Optical Light Spectrum of star “HD224355” The faint gaps or lines in the spectrum represent elements in the star that absorbed some of the star’s light which tells us about the star’s composition – Trottier Observatory

More to Explore:

Strongest Magnetic Field in the Universe Detected by X-Ray Space Observatory

Insight-HXMT firm detection of the highest energy fundamental cyclotron resonance scattering feature in the spectrum of GRO J1008-57

Detection of Cyclotron Resonance Scattering Feature in High Mass X-ray Binary Pulsar SMC X-2

How are neutron stars magnetic? Ethan Siegel Starts with a Bang

Crab Nebula Pulsar Video

NASA – “Imagine the Universe” “Pulsars”

What Magnetic Fields Do to Your Brain and Body – Discovery Magazine

The Theory of Cyclotron Lines in Accreting X-Ray Pulsars – Harvard

Cyclotron Lines in Highly magnetized Neutron Stars – Cornell University

Matthew Cimone

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