Internal structure of a Neutron Star

A model of a neutron star's internal structure

Current understanding of the structure of neutron stars is defined by existing mathematical models, but it might be possible to infer through studies of neutron-star oscillations. Similar to asteroseismology for ordinary stars, the inner structure might be derived by analyzing observed frequency spectra of stellar oscillations. On the basis of current models, the matter at the surface of a neutron star is composed of ordinary atomic nuclei as well as electrons. The "atmosphere" of the star is roughly one meter thick, below which one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities of ~5 mm), because of the extreme gravitational field.

Proceeding inward, one encounters nuclei with ever increasing numbers of neutrons; such nuclei would decay quickly on Earth, but are kept stable by tremendous pressures. Proceeding deeper, one comes to a point called neutron drip where free neutrons leak out of nuclei. In this region, there are nuclei, free electrons, and free neutrons. The nuclei become smaller and smaller until the core is reached, by definition the point where they disappear altogether. The exact nature of the superdense matter in the core is still not well understood. While this theoretical substance is referred to as neutronium in science fiction and popular literature, the term "neutronium" is rarely used in scientific publications, due to ambiguity over its meaning. The term neutron-degenerate matter is sometimes used, though not universally as the term incorporates assumptions about the nature of neutron star core material. Neutron star core material could be a superfluid mixture of neutrons with a few protons and electrons, or it could incorporate high-energy particles like pions and kaons in addition to neutrons, or it could be composed of strange matter incorporating quarks heavier than up and down quarks, or it could be quark matter not bound into hadrons. (A compact star composed entirely of strange matter would be called a strange star.) However, so far, observations have neither indicated nor ruled out such exotic states of matter.

More to neutron stars and black holes

This image(Artwork) shows a massive neutron star. Casey Reed (Penn State)

January 23, 2008

Provided by Cornell University

Neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to new research developed by using the Arecibo Observatory in Arecibo, Puerto Rico.

In the cosmic continuum of dead, remnant stars, the Arecibo astronomers have increased the mass limit for when neutron stars turn into black holes.

"The matter at the center of a neutron star is highly incompressible. Our new measurements of the mass of neutron stars will help nuclear physicists understand the properties of super-dense matter," says Paulo Freire, an astronomer from the observatory. "It also means that to form a black hole, more mass is needed than previously thought. Thus, in our universe, black holes might be more rare and neutron stars slightly more abundant."

When the cores of massive stars run out of nuclear fuel, their enormous gravitation then causes their collapse then becomes a supernova. The core, typically with a mass 1.4 times larger than that of the sun is compressed into a neutron star. These extreme objects have a radius about 10 to 16 kilometers and a density on the order of a billion tons per cubic centimeter. Freire says that a neutron star is like one single, giant atomic nucleus with about 460,000 times the mass of the Earth.

Astronomers had thought the neutron stars needed a maximum mass between 1.6 and 2.5 Suns in order to collapse and become black holes. However, this new research shows that neutron stars remain neutron stars between the mass of 1.9 and up to possibly 2.7 Suns.

"The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don't know what it is made out of," says Freire. "For that reason, we have at present no idea of how large or how massive neutron stars can be."

From June 2001 to March 2007, Freire used Arecibo's "L-wide" receiver (sensitive to radio frequencies from 1100 to 1700 MHz) and the Wide-Band Arecibo Pulsar Processors, a very fast spectrometer on the Arecibo telescope, to examine a binary pulsar called M5 B, in the globular cluster M5, which is located in the constellation Serpens. Like a lighthouse emits light, a pulsar is a strongly magnetized neutron star that emits large amounts of electromagnetic radiation, usually from its magnetic pole. As in the case of a lighthouse, distant observers perceive a sequence of pulsations, which are caused by the rotation of the pulsar. In the case of M5 B, these radio pulsations arrive at the Earth every 7.95 milliseconds.

These radio pulsations were scanned by the wide-band spectrometers once every 64 microseconds for 256 spectral channels, and then recorded to a computer disk, with accurate timing information. The precise arrival time of the pulses were then used by the astronomers to accurately measure the orbital motion of M5 B about its companion. This allowed the astronomers to estimate the mass (1.9 solar masses) of the pulsar.

Astronomers spot dead star's magnetic fingerprint

photo:The neutron star 1E1207.4-5209 is seen as the bright yellow object in the centre of this image. The neutron star is one of about 50 known to emit x rays. Only a few hundred thousand years old, 1E 1207.4-5209 lies at a center of a supernova remnant about 7,000 light-years from Earth in the constellation Centaurus. ESA / CESR

July 6, 2003

Neutron stars have been at the center of an intense detective story for more than three decades, yet their exact nature remains a mystery. A group of European astronomers have for the first time directly measured the magnetic field surrounding one of these strange stellar corpses.

By measuring the effects of magnetism on emitted x rays, some of the more intimate details of neutron-star behavior and structure are revealed. Until now, astronomers have only been able to use indirect methods like theoretical models of gravitational collapse and radio observations of pulsar spin rates to estimate the strength of their magnetic fields.

Born out of titanic supernova explosions, neutron stars are the collapsed cores of massive stars. What remains in the aftermath is a super-dense, hot ball of neutrons some 30 kilometers across with a greatly amplified rotation rate and magnetic field. This combination of rapid spin and powerful magnetic field in newborn neutron stars generates energetic emissions ranging from radio waves to x rays and gamma rays.

photo:The XMM-Newton x-ray observatory was launched by the European Space Agency into Earth orbit on December 10, 1999. ESA

Using the ultra-sensitive European Photon Imaging Camera (EPIC) aboard the orbiting XMM-Newton observatory, Giovanni Bignami of the Centre d'Etude Spatiale des Rayonnements (CESR) in France and his Italian teammates analyzed hot x-ray emissions from a young, isolated neutron star. In the longest-ever XMM-Newton observation of a galactic source, Bignami and his colleagues observed 1E1207.4-5209 for 72 hours and found a set of distinct, never-before-seen absorption lines in the high-resolution x-ray spectrum. The researchers say that what sets this discovery apart is that almost all observed spectra of isolated neutron stars have appeared featureless.

Bignami believes that particles in the neutron star's magnetic field are intercepting some of the outbound x rays, imparting a very distinct spectral fingerprint that allows the team to calculate the resulting strength of the magnetic field. "These observations now allow better understanding of the 'equation of state,' or underlying physics, of neutron stars," adds Bignami.

However, the neutron star's magnetism is 30 times weaker than all previous predictions, which creates a new puzzle for astronomers. Bignami's team speculates that an orbiting disk of debris may be dragging on 1E1207.4-5209, slowing its spin rate and decreasing its electromagnetism.

With its tell-tale spectral signature indicating a magnetic field many times weaker than expected, 1E1207.4-5209 may force scientists to reshape their models of these stellar gravestones. The question now on astronomers' minds is whether this neutron star is unique or just the first of its kind identified. Bignami and his colleagues plan to target more neutron stars using XMM-Newton to answer that question.

XMM-Newton and Integral clues on magnetic powerhouses

Credits: © 2008 Sky & Telescope: Gregg Dinderman

X-ray and gamma-ray data from ESA’s XMM-Newton and Integral orbiting observatories has been used to test, for the first time, the physical processes that make magnetars, an atypical class of neutron stars, shine in X-rays.

Neutron stars are remnants of massive stars (10-50 times as massive as our Sun) that have collapsed on to themselves under their own weight. Made almost entirely of neutrons (subatomic particles with no electric charge), these stellar corpses concentrate more than the mass of our Sun within a sphere about 20 km in diameter.

They are so compact that a teaspoon of neutron star stuff would weigh about one hundred million tons. Two other physical properties characterise a neutron star: their fast rotation and strong magnetic field.

Magnetars form a class of neutron stars with ultra-strong magnetic fields. With magnetic fields a thousand times stronger than that of ordinary neutron stars, they are the strongest known magnets in the cosmos.

In comparison, one would need 10 million million commonly-used hand magnets to generate a comparable magnetic field (most media used for data storage, for example, would be erased instantly if exposed to a magnetic field a mere million million times weaker).

So far, about 15 magnetars have been found. Five of them are known as soft gamma repeaters, or SGRs, because they sporadically release large, short bursts (lasting about 0.1 s) of low energy (soft) gamma rays and hard X-rays. The rest, about 10, are associated with anomalous X-ray pulsars, or AXPs. Although SGRs and AXPs were first thought to be different objects, we now know that they share many properties and that their activity is sustained by their strong magnetic fields.

Magnetars are different from ‘ordinary’ neutron stars because their internal magnetic field is thought to be strong enough to twist the stellar crust. Like in a circuit fed by a gigantic battery, this twist produces currents in the form of electron clouds which flow around the star. These currents interact with the radiation coming from the stellar surface, producing the X-rays.

Until now, scientists could not test their predictions, because it is not possible to produce such ultra-strong magnetic fields in laboratories on Earth.

To understand this phenomenon, a team led by Dr Nanda Rea from the University of Amsterdam used XMM-Newton and Integral data to search for these dense electron clouds around all known magnetars, for the first time.

Rea’s team found evidence that large electron currents do actually exist, and were able to measure the electron density which is a thousand times stronger than in a ‘normal’ pulsar. They have also measured the typical velocity at which the electron currents flow. With it, scientists have now established a link between an observed phenomenon and an actual physical process, an important clue in the puzzle of understanding these celestial objects.

The team is now working hard to develop and test more detailed models on the same line, to fully understand the behaviour of matter under the influence of such strong magnetic fields.

Notes for editors:

The team includes Dr Silvia Zane, from University College London, Prof. Roberto Turolla from the University of Padua, Prof. Maxim Lyutikov from Purdue University, and Dr Diego Gotz from CEA-Saclay.

The results appear in ‘Resonant cyclotron scattering in magnetars’ emission’, by N. Rea, S. Zane, R. Turolla, M. Lyutikov and D. Gotz, published in the Astrophysical Journal on 20 October 2008.

The XMM-Newton science teams are based in several European and US institutes, grouped into three instrument teams and the XMM-Newton Survey Science Centre (SSC). Science operations are managed at ESA’s European Space Astronomy Centre (ESAC), at Villanueva de la Cañada near Madrid, Spain. Spacecraft operations are managed at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany.