Neutron star

A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars.[1] Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass lower than 2.16[2] solar masses.[3] They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are partially supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure. However neutron degeneracy pressure is not sufficient to hold up an object beyond 0.7M[4][5] and repulsive nuclear forces play a larger role in supporting more massive neutron stars.[6][7] If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit, it continues collapsing to form a black hole.

Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000 K.[8][9][10][11][a] They are so dense that a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion metric tons, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres).[12][13] Their magnetic fields are between 108 and 1015 (100 million to 1 quadrillion) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about 2×1011 (200 billion) times that of Earth's gravitational field.

As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. Indeed, the discovery of pulsars by Jocelyn Bell Burnell in 1967 was the first observational suggestion that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space (the so-called "lighthouse effect"). The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second[14][15] or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.24 c (i.e. nearly a quarter the speed of light).

There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions.[16] However, most are old and cold, and neutron stars can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are almost undetectable; however, since the Hubble Space Telescope detection of RX J185635−3754, a few nearby neutron stars that appear to emit only thermal radiation have been detected. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.[17]

Neutron stars in binary systems can undergo accretion which typically makes the system bright in X-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and potentially cause them to gain mass and spin-up to very fast rotation rates, forming the so-called millisecond pulsars. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. In 2017, a direct detection (GW170817) of the gravitational waves from such an event was made,[18] and gravitational waves have also been indirectly detected in a system where two neutron stars orbit each other.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.[19][20][21][22]

Neutron Star bending light simulation
Simulated view of a neutron star. Due to its strong gravity, the background is gravitationally lensed, making it appear distorted.
PIA18848-PSRB1509-58-ChandraXRay-WiseIR-20141023
Radiation from the pulsar PSR B1509-58, a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, from Chandra) and illuminates the rest of the nebula, here seen in infrared (blue and red, from WISE).

Formation

Neutronstarsimple
Simplistic representation of the formation of neutron stars.

Any main-sequence star with an initial mass of above 8 times the mass of the sun (8 M) has the potential to produce a neutron star. As the star evolves away from the main sequence, subsequent nuclear burning produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of mass from shell burning cause the core to exceed the Chandrasekhar limit. Electron-degeneracy pressure is overcome and the core collapses further, sending temperatures soaring to over 5×109 K. At these temperatures, photodisintegration (the breaking up of iron nuclei into alpha particles by high-energy gamma rays) occurs. As the temperature climbs even higher, electrons and protons combine to form neutrons via electron capture, releasing a flood of neutrinos. When densities reach nuclear density of 4×1017 kg/m3, neutron degeneracy pressure halts the contraction. The infalling outer envelope of the star is halted and flung outwards by a flux of neutrinos produced in the creation of the neutrons, becoming a supernova. The remnant left is a neutron star. If the remnant has a mass greater than about 3 M, it collapses further to become a black hole.[23]

As the core of a massive star is compressed during a Type II supernova, Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum. But, because it has only a tiny fraction of its parent's radius (and therefore its moment of inertia is sharply reduced), a neutron star is formed with very high rotation speed, and then over a very long period it slows. Neutron stars are known that have rotation periods from about 1.4 ms to 30 s. The neutron star's density also gives it very high surface gravity, with typical values ranging from 1012 to 1013 m/s2 (more than 1011 times that of Earth).[11] One measure of such immense gravity is the fact that neutron stars have an escape velocity ranging from 100,000 km/s to 150,000 km/s, that is, from a third to half the speed of light. The neutron star's gravity accelerates infalling matter to tremendous speed. The force of its impact would likely destroy the object's component atoms, rendering all the matter identical, in most respects, to the rest of the neutron star.

Properties

Mass and temperature

A neutron star has a mass of at least 1.1 and perhaps up to 3 solar masses (M).[24][25] The maximum observed mass of neutron stars is about 2.01 M. But in general, compact stars of less than 1.39 M (the Chandrasekhar limit) are white dwarfs, whereas compact stars with a mass between 1.4 M and 3 M (the Tolman–Oppenheimer–Volkoff limit) should be neutron stars (though there is an interval of a few tenths of a solar mass where the masses of low-mass neutron stars and high-mass white dwarfs can overlap). Between 3 M and 5 M, hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist. Beyond 10 M the stellar remnant will overcome the neutron degeneracy pressure and gravitational collapse will usually occur to produce a black hole, though the smallest observed mass of a stellar black hole is about 5 M.[26]

The temperature inside a newly formed neutron star is from around 1011 to 1012 kelvin.[27] However, the huge number of neutrinos it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around 106 kelvin.[27] At this lower temperature, most of the light generated by a neutron star is in X-rays.

Density and pressure

Neutron stars have overall densities of 3.7×1017 to 5.9×1017 kg/m3 (2.6×1014 to 4.1×1014 times the density of the Sun),[b] which is comparable to the approximate density of an atomic nucleus of 3×1017 kg/m3.[28] The neutron star's density varies from about 1×109 kg/m3 in the crust—increasing with depth—to about 6×1017 or 8×1017 kg/m3 (denser than an atomic nucleus) deeper inside.[27] A neutron star is so dense that one teaspoon (5 milliliters) of its material would have a mass over 5.5×1012 kg, about 900 times the mass of the Great Pyramid of Giza. In the enormous gravitational field of a neutron star, that teaspoon of material would weigh 1.1×1025 N, which is 15 times what the Moon would weigh if it were placed on the surface of the Earth.[c] The entire mass of the Earth at neutron star density would fit into a sphere of 305m in diameter (the size of the Arecibo Observatory). The pressure increases from 3.2×1031 to 1.6×1034 Pa from the inner crust to the center.[29]

The equation of state of matter at such high densities is not precisely known because of the theoretical difficulties associated with extrapolating the likely behavior of quantum chromodynamics, superconductivity, and superfluidity of matter in such states. The problem is exacerbated by the empirical difficulties of observing the characteristics of any object that is hundreds of parsecs away, or farther.

A neutron star has some of the properties of an atomic nucleus, including density (within an order of magnitude) and being composed of nucleons. In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities.

Magnetic field

The magnetic field strength on the surface of neutron stars ranges from c. 104 to 1011 tesla.[30] These are orders of magnitude higher than in any other object: for comparison, a continuous 16 T field has been achieved in the laboratory and is sufficient to levitate a living frog due to diamagnetic levitation. Variations in magnetic field strengths are most likely the main factor that allows different types of neutron stars to be distinguished by their spectra, and explains the periodicity of pulsars.[30]

The neutron stars known as magnetars have the strongest magnetic fields, in the range of 108 to 1011 tesla,[31] and have become the widely accepted hypothesis for neutron star types soft gamma repeaters (SGRs)[32] and anomalous X-ray pulsars (AXPs).[33] The magnetic energy density of a 108 T field is extreme, exceeding the mass−energy density of ordinary matter.[34] Fields of this strength are able to polarize the vacuum to the point that the vacuum becomes birefringent. Photons can merge or split in two, and virtual particle-antiparticle pairs are produced. The field changes electron energy levels and atoms are forced into thin cylinders. Unlike in an ordinary pulsar, magnetar spin-down can be directly powered by its magnetic field, and the magnetic field is strong enough to stress the crust to the point of fracture. Fractures of the crust cause starquakes, observed as extremely luminous millisecond hard gamma ray bursts. The fireball is trapped by the magnetic field, and comes in and out of view when the star rotates, which is observed as a periodic soft gamma repeater (SGR) emission with a period of 5–8 seconds and which lasts for a few minutes.[35]

The origins of the strong magnetic field are as yet unclear.[30] One hypothesis is that of "flux freezing", or conservation of the original magnetic flux during the formation of the neutron star.[30] If an object has a certain magnetic flux over its surface area, and that area shrinks to a smaller area, but the magnetic flux is conserved, then the magnetic field would correspondingly increase. Likewise, a collapsing star begins with a much larger surface area than the resulting neutron star, and conservation of magnetic flux would result in a far stronger magnetic field. However, this simple explanation does not fully explain magnetic field strengths of neutron stars.[30]

Gravity and equation of state

Neutronstar 2Rs
Gravitational light deflection at a neutron star. Due to relativistic light deflection more than half of the surface is visible (each grid patch here represents 30 degrees by 30 degrees).[36] In natural units, the mass of the depicted star is 1 and its radius 4, or twice its Schwarzschild radius.[36]

The gravitational field at a neutron star's surface is about 2×1011 times stronger than on Earth, at around 2.0×1012 m/s2.[37] Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible.[36] If the radius of the neutron star is 3GM/c2 or less, then the photons may be trapped in an orbit, thus making the whole surface of that neutron star visible from a single vantage point, along with destabilizing photon orbits at or below the 1 radius distance of the star.

A fraction of the mass of a star that collapses to form a neutron star is released in the supernova explosion from which it forms (from the law of mass–energy equivalence, E = mc2). The energy comes from the gravitational binding energy of a neutron star.

Hence, the gravitational force of a typical neutron star is huge. If an object were to fall from a height of one meter on a neutron star 12 kilometers in radius, it would reach the ground at around 1400 kilometers per second.[38] However, even before impact, the tidal force would cause spaghettification, breaking any sort of an ordinary object into a stream of material.

Because of the enormous gravity, time dilation between a neutron star and Earth is significant. For example, eight years could pass on the surface of a neutron star, yet ten years would have passed on Earth, not including the time-dilation effect of its very rapid rotation.[39]

Neutron star relativistic equations of state describe the relation of radius vs. mass for various models.[40] The most likely radii for a given neutron star mass are bracketed by models AP4 (smallest radius) and MS2 (largest radius). BE is the ratio of gravitational binding energy mass equivalent to the observed neutron star gravitational mass of "M" kilograms with radius "R" meters,[41]

      

Given current values

[42]
[42]

and star masses "M" commonly reported as multiples of one solar mass,

then the relativistic fractional binding energy of a neutron star is

A 2 M neutron star would not be more compact than 10,970 meters radius (AP4 model). Its mass fraction gravitational binding energy would then be 0.187, −18.7% (exothermic). This is not near 0.6/2 = 0.3, −30%.

The equation of state for a neutron star is not yet known. It is assumed that it differs significantly from that of a white dwarf, whose equation of state is that of a degenerate gas that can be described in close agreement with special relativity. However, with a neutron star the increased effects of general relativity can no longer be ignored. Several equations of state have been proposed (FPS, UU, APR, L, SLy, and others) and current research is still attempting to constrain the theories to make predictions of neutron star matter.[11][43] This means that the relation between density and mass is not fully known, and this causes uncertainties in radius estimates. For example, a 1.5 M neutron star could have a radius of 10.7, 11.1, 12.1 or 15.1 kilometers (for EOS FPS, UU, APR or L respectively).[43]

Structure

Neutron star cross section
Cross-section of neutron star. Densities are in terms of ρ0 the saturation nuclear matter density, where nucleons begin to touch.

Current understanding of the structure of neutron stars is defined by existing mathematical models, but it might be possible to infer some details through studies of neutron-star oscillations. Asteroseismology, a study applied to ordinary stars, can reveal the inner structure of neutron stars by analyzing observed spectra of stellar oscillations.[11]

Current models indicate that matter at the surface of a neutron star is composed of ordinary atomic nuclei crushed into a solid lattice with a sea of electrons flowing through the gaps between them. It is possible that the nuclei at the surface are iron, due to iron's high binding energy per nucleon.[44] It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like helium and hydrogen.[44] If the surface temperature exceeds 106 kelvin (as in the case of a young pulsar), the surface should be fluid instead of the solid phase that might exist in cooler neutron stars (temperature <106 kelvin).[44]

The "atmosphere" of a neutron star is hypothesized to be at most several micrometers thick, and its dynamics are fully controlled by the neutron star's magnetic field. Below the atmosphere one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities of ~5 mm), due to the extreme gravitational field.[45]

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. As this process continues at increasing depths, the neutron drip becomes overwhelming, and the concentration of free neutrons increases rapidly. In that region, there are nuclei, free electrons, and free neutrons. The nuclei become increasingly small (gravity and pressure overwhelming the strong force) until the core is reached, by definition the point where mostly neutrons exist. The expected hierarchy of phases of nuclear matter in the inner crust has been characterized as "nuclear pasta", with fewer voids and larger structures towards higher pressures.[46] The composition of the superdense matter in the core remains uncertain. One model describes the core as superfluid neutron-degenerate matter (mostly neutrons, with some protons and electrons). More exotic forms of matter are possible, including degenerate strange matter (containing strange quarks in addition to up and down quarks), matter containing high-energy pions and kaons in addition to neutrons,[11] or ultra-dense quark-degenerate matter.

Radiation

Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones.

Pulsars

Neutron stars are detected from their electromagnetic radiation. Neutron stars are usually observed to pulse radio waves and other electromagnetic radiation, and neutron stars observed with pulses are called pulsars.

Pulsars' radiation is thought to be caused by particle acceleration near their magnetic poles, which need not be aligned with the rotational axis of the neutron star. It is thought that a large electrostatic field builds up near the magnetic poles, leading to electron emission.[47] These electrons are magnetically accelerated along the field lines, leading to curvature radiation, with the radiation being strongly polarized towards the plane of curvature.[47] In addition, high energy photons can interact with lower energy photons and the magnetic field for electron−positron pair production, which through electron–positron annihilation leads to further high energy photons.[47]

The radiation emanating from the magnetic poles of neutron stars can be described as magnetospheric radiation, in reference to the magnetosphere of the neutron star.[48] It is not to be confused with magnetic dipole radiation, which is emitted because the magnetic axis is not aligned with the rotational axis, with a radiation frequency the same as the neutron star's rotational frequency.[47]

If the axis of rotation of the neutron star is different to the magnetic axis, external viewers will only see these beams of radiation whenever the magnetic axis point towards them during the neutron star rotation. Therefore, periodic pulses are observed, at the same rate as the rotation of the neutron star.

Non-pulsating neutron stars

In addition to pulsars, non-pulsating neutron stars have also been identified, although they may have minor periodic variation in luminosity.[49][50] This seems to be a characteristic of the X-ray sources known as Central Compact Objects in Supernova remnants (CCOs in SNRs), which are thought to be young, radio-quiet isolated neutron stars.[49]

Spectra

In addition to radio emissions, neutron stars have also been identified in other parts of the electromagnetic spectrum. This includes visible light, near infrared, ultraviolet, X-rays and gamma rays.[48] Pulsars observed in X-rays are known as X-ray pulsars if accretion-powered; while those identified in visible light as optical pulsars. The majority of neutron stars detected, including those identified in optical, X-ray and gamma rays, also emit radio waves;[51] the Crab Pulsar produces electromagnetic emissions across the spectrum.[51] However, there exist neutron stars called radio-quiet neutron stars, with no radio emissions detected.[52]

Rotation

Neutron stars rotate extremely rapidly after their formation due to the conservation of angular momentum; in analogy to spinning ice skaters pulling in their arms, the slow rotation of the original star's core speeds up as it shrinks. A newborn neutron star can rotate many times a second.

Spin down

PPdot2
PP-dot diagram for known rotation-powered pulsars (red), anomalous X-ray pulsars (green), high-energy emission pulsars (blue) and binary pulsars (pink)

Over time, neutron stars slow, as their rotating magnetic fields in effect radiate energy associated with the rotation; older neutron stars may take several seconds for each revolution. This is called spin down. The rate at which a neutron star slows its rotation is usually constant and very small.

The periodic time (P) is the rotational period, the time for one rotation of a neutron star. The spin-down rate, the rate of slowing of rotation, is then given the symbol (P-dot), the derivative of P with respect to time. It is defined as periodic time increase per unit time; it is a dimensionless quantity, but can be given the units of s⋅s−1 (seconds per second).[47]

The spin-down rate (P-dot) of neutron stars usually falls within the range of 10−22 to 10−9 s⋅s−1, with the shorter period (or faster rotating) observable neutron stars usually having smaller P-dot. As a neutron star ages, its rotation slows (as P increases); eventually, the rate of rotation will become too slow to power the radio-emission mechanism, and the neutron star can no longer be detected.[47]

P and P-dot allow minimum magnetic fields of neutron stars to be estimated.[47] P and P-dot can be also used to calculate the characteristic age of a pulsar, but gives an estimate which is somewhat larger than the true age when it is applied to young pulsars.[47]

P and P-dot can also be combined with neutron star's moment of inertia to estimate a quantity called spin-down luminosity, which is given the symbol (E-dot). It is not the measured luminosity, but rather the calculated loss rate of rotational energy that would manifest itself as radiation. For neutron stars where the spin-down luminosity is comparable to the actual luminosity, the neutron stars are said to be "rotation powered".[47][48] The observed luminosity of the Crab Pulsar is comparable to the spin-down luminosity, supporting the model that rotational kinetic energy powers the radiation from it.[47] With neutron stars such as magnetars, where the actual luminosity exceeds the spin-down luminosity by about a factor of one hundred, it is assumed that the luminosity is powered by magnetic dissipation, rather than being rotation powered.[53]

P and P-dot can also be plotted for neutron stars to create a PP-dot diagram. It encodes a tremendous amount of information about the pulsar population and its properties, and has been likened to the Hertzsprung–Russell diagram in its importance for neutron stars.[47]

Spin up

Neutron star rotational speeds can increase, a process known as spin up. Sometimes neutron stars absorb orbiting matter from companion stars, increasing the rotation rate and reshaping the neutron star into an oblate spheroid. This causes an increase in the rate of rotation of the neutron star of over a hundred times per second in the case of millisecond pulsars.

The most rapidly rotating neutron star currently known, PSR J1748-2446ad, rotates at 716 revolutions per second.[54] A 2007 paper reported the detection of an X-ray burst oscillation, which provides an indirect measure of spin, of 1122 Hz from the neutron star XTE J1739-285,[55] suggesting 1122 rotations a second. However, at present, this signal has only been seen once, and should be regarded as tentative until confirmed in another burst from that star.

Glitches and starquakes

2004 stellar quake full
NASA artist's conception of a "starquake", or "stellar quake".

Sometimes a neutron star will undergo a glitch, a sudden small increase of its rotational speed or spin up. Glitches are thought to be the effect of a starquake—as the rotation of the neutron star slows, its shape becomes more spherical. Due to the stiffness of the "neutron" crust, this happens as discrete events when the crust ruptures, creating a starquake similar to earthquakes. After the starquake, the star will have a smaller equatorial radius, and because angular momentum is conserved, its rotational speed has increased.

Starquakes occurring in magnetars, with a resulting glitch, is the leading hypothesis for the gamma-ray sources known as soft gamma repeaters.[56]

Recent work, however, suggests that a starquake would not release sufficient energy for a neutron star glitch; it has been suggested that glitches may instead be caused by transitions of vortices in the theoretical superfluid core of the neutron star from one metastable energy state to a lower one, thereby releasing energy that appears as an increase in the rotation rate.[57]

"Anti-glitches"

An "anti-glitch", a sudden small decrease in rotational speed, or spin down, of a neutron star has also been reported.[58] It occurred in a magnetar, that in one case produced an X-ray luminosity increase of a factor of 20, and a significant spin-down rate change. Current neutron star models do not predict this behavior. If the cause was internal, it suggests differential rotation of solid outer crust and the superfluid component of the magnetar's inner structure.[58]

Population and distances

Moving heart of the Crab Nebula
Central neutron star at the heart of the Crab Nebula.[59]

At present, there are about 2,000 known neutron stars in the Milky Way and the Magellanic Clouds, the majority of which have been detected as radio pulsars. Neutron stars are mostly concentrated along the disk of the Milky Way although the spread perpendicular to the disk is large because the supernova explosion process can impart high translational speeds (400 km/s) to the newly formed neutron star.

Some of the closest known neutron stars are RX J1856.5−3754, which is about 400 light-years from Earth, and PSR J0108−1431 at about 424 light years.[60] RX J1856.5-3754 is a member of a close group of neutron stars called The Magnificent Seven. Another nearby neutron star that was detected transiting the backdrop of the constellation Ursa Minor has been nicknamed Calvera by its Canadian and American discoverers, after the villain in the 1960 film The Magnificent Seven. This rapidly moving object was discovered using the ROSAT/Bright Source Catalog.

Neutron stars are only detectable with modern technology during the earliest stages of their lives (almost always less than 1 million years) and are vastly outnumbered by older neutron stars which would only be detectable through their blackbody radiation and gravitational effects on other stars. It is statistically probable based on known populations that there is at least 1 neutron star within 10 parsecs of the Sun, significantly closer than the current nearest known neutron star.

Binary neutron star systems

15-137-CircinusX1-XRayLightRings-NeutronStar-Chandra-20150624
Circinus X-1: X-ray light rings from a binary neutron star (24 June 2015; Chandra X-ray Observatory)

About 5% of all known neutron stars are members of a binary system. The formation and evolution of binary neutron stars can be a complex process.[61] Neutron stars have been observed in binaries with ordinary main-sequence stars, red giants, white dwarfs or other neutron stars. According to modern theories of binary evolution it is expected that neutron stars also exist in binary systems with black hole companions. The merger of binaries containing two neutron stars, or a neutron star and a black hole, are expected to be prime sources for the emission of detectable gravitational waves.

X-ray binaries

Binary systems containing neutron stars often emit X-rays, which are emitted by hot gas as it falls towards the surface of the neutron star. The source of the gas is the companion star, the outer layers of which can be stripped off by the gravitational force of the neutron star if the two stars are sufficiently close. As the neutron star accretes this gas its mass can increase; if enough mass is accreted the neutron star may collapse into a black hole.[62]

Neutron star binary mergers and nucleosynthesis

Binaries containing two neutron stars are observed to shrink as gravitational waves are emitted.[63] Ultimately the neutron stars will come into contact and coalesce. The coalescence of binary neutron stars is one of the leading models for the origin of short gamma-ray bursts. Strong evidence for this model came from the observation of a kilonova associated with the short-duration gamma-ray burst GRB 130603B,[64] and finally confirmed by detection of gravitational wave GW170817 and short GRB 170817A by LIGO, Virgo and 70 observatories covering the electromagnetic spectrum observed the event.[65][66][67][68] The light emitted in the kilonova is believed to come from the radioactive decay of material ejected in the merger of the two neutron stars. This material may be responsible for the production of many of the chemical elements beyond iron,[69] as opposed to the supernova nucleosynthesis theory.

Planets

Artist's concept of PSR B1257+12 system
An artist's conception of a pulsar planet with bright aurorae.

Neutron stars can host exoplanets. These can be original, circumbinary, captured, or the result of a second round of planet formation. Pulsars can also strip the atmosphere off from a star, leaving a planetary-mass remnant, which may be understood as a chthonian planet or a stellar object depending on interpretation. For pulsars, such pulsar planets can be detected with the pulsar timing method, which allows for high precision and detection of much smaller planets than with other methods. Two systems have been definitively confirmed. The first exoplanets ever to be detected were the three planets Draugr, Poltergeist and Phobetor around PSR B1257+12, discovered in 1992–1994. Of these, Draugr is the smallest exoplanet ever detected, at a mass of twice that of the Moon. Another system is PSR B1620−26, where a circumbinary planet orbits a neutron star-white dwarf binary system. Also, there are several unconfirmed candidates. Pulsar planets receive little visible light, but massive amounts of ionizing radiation and high-energy stellar wind, which makes them rather hostile environments.

History of discoveries

IsolatedNeutronStar
The first direct observation of a neutron star in visible light. The neutron star is RX J1856.5−3754.

At the meeting of the American Physical Society in December 1933 (the proceedings were published in January 1934), Walter Baade and Fritz Zwicky proposed the existence of neutron stars,[70][d] less than two years after the discovery of the neutron by Sir James Chadwick.[73] In seeking an explanation for the origin of a supernova, they tentatively proposed that in supernova explosions ordinary stars are turned into stars that consist of extremely closely packed neutrons that they called neutron stars. Baade and Zwicky correctly proposed at that time that the release of the gravitational binding energy of the neutron stars powers the supernova: "In the supernova process, mass in bulk is annihilated". Neutron stars were thought to be too faint to be detectable and little work was done on them until November 1967, when Franco Pacini pointed out that if the neutron stars were spinning and had large magnetic fields, then electromagnetic waves would be emitted. Unbeknown to him, radio astronomer Antony Hewish and his research assistant Jocelyn Bell at Cambridge were shortly to detect radio pulses from stars that are now believed to be highly magnetized, rapidly spinning neutron stars, known as pulsars.

In 1965, Antony Hewish and Samuel Okoye discovered "an unusual source of high radio brightness temperature in the Crab Nebula".[74] This source turned out to be the Crab Pulsar that resulted from the great supernova of 1054.

In 1967, Iosif Shklovsky examined the X-ray and optical observations of Scorpius X-1 and correctly concluded that the radiation comes from a neutron star at the stage of accretion.[75]

In 1967, Jocelyn Bell Burnell and Antony Hewish discovered regular radio pulses from PSR B1919+21. This pulsar was later interpreted as an isolated, rotating neutron star. The energy source of the pulsar is the rotational energy of the neutron star. The majority of known neutron stars (about 2000, as of 2010) have been discovered as pulsars, emitting regular radio pulses.

In 1971, Riccardo Giacconi, Herbert Gursky, Ed Kellogg, R. Levinson, E. Schreier, and H. Tananbaum discovered 4.8 second pulsations in an X-ray source in the constellation Centaurus, Cen X-3.[76] They interpreted this as resulting from a rotating hot neutron star. The energy source is gravitational and results from a rain of gas falling onto the surface of the neutron star from a companion star or the interstellar medium.

In 1974, Antony Hewish was awarded the Nobel Prize in Physics "for his decisive role in the discovery of pulsars" without Jocelyn Bell who shared in the discovery.[77]

In 1974, Joseph Taylor and Russell Hulse discovered the first binary pulsar, PSR B1913+16, which consists of two neutron stars (one seen as a pulsar) orbiting around their center of mass. Einstein's general theory of relativity predicts that massive objects in short binary orbits should emit gravitational waves, and thus that their orbit should decay with time. This was indeed observed, precisely as general relativity predicts, and in 1993, Taylor and Hulse were awarded the Nobel Prize in Physics for this discovery.[78]

In 1982, Don Backer and colleagues discovered the first millisecond pulsar, PSR B1937+21.[79] This object spins 642 times per second, a value that placed fundamental constraints on the mass and radius of neutron stars. Many millisecond pulsars were later discovered, but PSR B1937+21 remained the fastest-spinning known pulsar for 24 years, until PSR J1748-2446ad (which spins more than 700 times a second) was discovered.

In 2003, Marta Burgay and colleagues discovered the first double neutron star system where both components are detectable as pulsars, PSR J0737−3039.[80] The discovery of this system allows a total of 5 different tests of general relativity, some of these with unprecedented precision.

In 2010, Paul Demorest and colleagues measured the mass of the millisecond pulsar PSR J1614−2230 to be 1.97±0.04 M, using Shapiro delay.[81] This was substantially higher than any previously measured neutron star mass (1.67 M, see PSR J1903+0327), and places strong constraints on the interior composition of neutron stars.

In 2013, John Antoniadis and colleagues measured the mass of PSR J0348+0432 to be 2.01±0.04 M, using white dwarf spectroscopy.[82] This confirmed the existence of such massive stars using a different method. Furthermore, this allowed, for the first time, a test of general relativity using such a massive neutron star.

In August 2017, LIGO and Virgo made first detection of gravitational waves produced by colliding neutron stars.[83]

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.[19][20][21][22]

Subtypes table

Examples of neutron stars

Artist's impression of disc around a neutron star RX J0806.4-4123
Artist's impression of disc around a neutron star RX J0806.4-4123.[88]
  • RX J0806.4-4123 – neutron star source of infrared radiation.[89]
  • PSR J0108−1431 – closest neutron star
  • LGM-1 – the first recognized radio-pulsar
  • PSR B1257+12 – the first neutron star discovered with planets (a millisecond pulsar)
  • SWIFT J1756.9-2508 – a millisecond pulsar with a stellar-type companion with planetary range mass (below brown dwarf)
  • PSR B1509−58 source of the "Hand of God" photo shot by the Chandra X-ray Observatory.
  • PSR J0348+0432 – the most massive neutron star with a well-constrained mass, 2.01 ± 0.04 M.

Gallery

Video – animation

Neutron stars containing 500,000 Earth-masses in 25 km (16 mi) diameter sphere

Neutron star collision

See also

Notes

  1. ^ A neutron star's density increases as its mass increases, and its radius decreases non-linearly. (archived image: NASA mass radius graph) A newer page is here: "RXTE Discovers Kilohertz Quasiperiodic Oscillations". NASA. Retrieved 17 February 2016. (specifically the image [1])
  2. ^ 3.7×1017 kg/m3 derives from mass 2.68×1030 kg / volume of star of radius 12 km; 5.9×1017 kg/m3 derives from mass 4.2×1030 kg per volume of star radius 11.9 km
  3. ^ The average density of material in a neutron star of radius 10 km is 1.1×1012 kg/cm3. Therefore, 5 ml of such material is 5.5×1012 kg, or 5 500 000 000 metric tons. This is about 15 times the total mass of the human world population. Alternatively, 5 ml from a neutron star of radius 20 km radius (average density 8.35×1010 kg/cm3) has a mass of about 400 million metric tons, or about the mass of all humans. The gravitational field is ca. 2×1011g or ca. 2×1012 N/kg. Moon weight is calculated at 1g.
  4. ^ Even before the discovery of neutron, in 1931, neutron stars were anticipated by Lev Landau, who wrote about stars where "atomic nuclei come in close contact, forming one gigantic nucleus".[71] However, the widespread opinion that Landau predicted neutron stars proves to be wrong.[72]

References

  1. ^ Glendenning, Norman K. (2012). Compact Stars: Nuclear Physics, Particle Physics and General Relativity (illustrated ed.). Springer Science & Business Media. p. 1. ISBN 978-1-4684-0491-3.
  2. ^ Rezzolla, Luciano; Most, Elias R.; Weih, Lukas R. (2018). "Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars". The Astrophysical Journal. 852 (2): L25. arXiv:1711.00314. Bibcode:2018ApJ...852L..25R. doi:10.3847/2041-8213/aaa401.
  3. ^ Seeds, Michael; Backman, Dana (2009). Astronomy: The Solar System and Beyond (6th ed.). Cengage Learning. p. 339. ISBN 978-0-495-56203-0.
  4. ^ Tolman, R. C. (1939). "Static Solutions of Einstein's Field Equations for Spheres of Fluid". Physical Review. 55 (4): 364–373. Bibcode:1939PhRv...55..364T. doi:10.1103/PhysRev.55.364.
  5. ^ Oppenheimer, J. R.; Volkoff, G. M. (1939). "On Massive Neutron Cores". Physical Review. 55 (4): 374–381. Bibcode:1939PhRv...55..374O. doi:10.1103/PhysRev.55.374.
  6. ^ "Neutron Stars" (PDF). www.astro.princeton.edu. Retrieved 14 December 2018.
  7. ^ Douchin, F.; Haensel, P. (December 2001). "A unified equation of state of dense matter and neutron star structure". Astronomy & Astrophysics. 380 (1): 151–167. arXiv:astro-ph/0111092. Bibcode:2001A&A...380..151D. doi:10.1051/0004-6361:20011402. ISSN 0004-6361.
  8. ^ Kiziltan, Bulent (2011). Reassessing the Fundamentals: On the Evolution, Ages and Masses of Neutron Stars. Universal-Publishers. ISBN 978-1-61233-765-4.
  9. ^ Neutron star mass measurements
  10. ^ "NASA Ask an Astrophysicist: Maximum Mass of a Neutron Star".
  11. ^ a b c d e Haensel, Paweł; Potekhin, Alexander Y.; Yakovlev, Dmitry G. (2007). Neutron Stars. Springer. ISBN 978-0-387-33543-8.
  12. ^ "Tour the ASM Sky". heasarc.gsfc.nasa.gov.
  13. ^ "Density of the Earth". 2009-03-10.
  14. ^ Hessels, Jason; Ransom, Scott M.; Stairs, Ingrid H.; Freire, Paulo C. C.; et al. (2006). "A Radio Pulsar Spinning at 716 Hz". Science. 311 (5769): 1901–1904. arXiv:astro-ph/0601337. Bibcode:2006Sci...311.1901H. CiteSeerX 10.1.1.257.5174. doi:10.1126/science.1123430. PMID 16410486.
  15. ^ Naeye, Robert (2006-01-13). "Spinning Pulsar Smashes Record". Sky & Telescope. Retrieved 2008-01-18.
  16. ^ Camenzind, Max (24 February 2007). Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes. Springer Science & Business Media. p. 269. Bibcode:2007coaw.book.....C. ISBN 978-3-540-49912-1.
  17. ^ Zhang, Bing; Xu, R. X.; Qiao, G. J. (2000). "Nature and Nurture: a Model for Soft Gamma-Ray Repeaters". The Astrophysical Journal. 545 (2): 127–129. arXiv:astro-ph/0010225. Bibcode:2000ApJ...545L.127Z. doi:10.1086/317889.
  18. ^ Abbott, B. P.; Abbott, R.; Abbott, T. D.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T.; Addesso, P.; Richard; Howard; Adhikari, R. X.; Huang-Wei (2017). "Multi-messenger Observations of a Binary Neutron Star Merger". The Astrophysical Journal Letters. 848 (2): L12. arXiv:1710.05833. Bibcode:2017ApJ...848L..12A. doi:10.3847/2041-8213/aa91c9.
  19. ^ a b University of Maryland (16 October 2018). "All in the family: Kin of gravitational wave source discovered - New observations suggest that kilonovae -- immense cosmic explosions that produce silver, gold and platinum--may be more common than thought". EurekAlert!. Retrieved 17 October 2018.
  20. ^ a b Troja, E.; et al. (16 October 2018). "A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341". Nature Communications. 9 (4089 (2018)): 4089. arXiv:1806.10624. Bibcode:2018NatCo...9.4089T. doi:10.1038/s41467-018-06558-7. PMC 6191439. PMID 30327476.
  21. ^ a b Mohon, Lee (16 October 2018). "GRB 150101B: A Distant Cousin to GW170817". NASA. Retrieved 17 October 2018.
  22. ^ a b Wall, Mike (17 October 2018). "Powerful Cosmic Flash Is Likely Another Neutron-Star Merger". Space.com. Retrieved 17 October 2018.
  23. ^ Bally, John; Reipurth, Bo (2006). The Birth of Stars and Planets (illustrated ed.). Cambridge University Press. p. 207. ISBN 978-0-521-80105-8.
  24. ^ Özel, Feryal; Psaltis, Dimitrios; Narayan, Ramesh; Santos Villarreal, Antonio (September 2012). "On the Mass Distribution and Birth Masses of Neutron Stars". The Astrophysical Journal. 757 (1): 13. arXiv:1201.1006. Bibcode:2012ApJ...757...55O. doi:10.1088/0004-637X/757/1/55.
  25. ^ Chamel, N.; Haensel, Paweł; Zdunik, J. L.; Fantina, A. F. (19 November 2013). "On the Maximum Mass of Neutron Stars". International Journal of Modern Physics. 1 (28): 1330018. arXiv:1307.3995. Bibcode:2013IJMPE..2230018C. doi:10.1142/S021830131330018X.
  26. ^ [2], a 10 M star will collapse into a black hole.
  27. ^ a b c Lattimer, James M. (2015). "Introduction to neutron stars". American Institute of Physics Conference Series. AIP Conference Proceedings. 1645 (1): 61–78. Bibcode:2015AIPC.1645...61L. doi:10.1063/1.4909560. Retrieved 2007-11-11.
  28. ^ "Calculating a Neutron Star's Density". Retrieved 2006-03-11. NB 3 × 1017 kg/m3 is 3×1014 g/cm3
  29. ^ Ozel, Feryal; Freire, Paulo (2016). "Masses, Radii, and the Equation of State of Neutron Stars". Annu. Rev. Astron. Astrophys. 54 (1): 401–440. arXiv:1603.02698. Bibcode:2016ARA&A..54..401O. doi:10.1146/annurev-astro-081915-023322.
  30. ^ a b c d e Reisenegger, A. (2003). "Origin and Evolution of Neutron Star Magnetic Fields" (PDF). Universidade Federal do Rio Grande do Sul. arXiv:astro-ph/0307133. Bibcode:2003astro.ph..7133R. Retrieved 21 March 2016.
  31. ^ "McGill SGR/AXP Online Catalog". Retrieved 2 Jan 2014.
  32. ^ Kouveliotou, Chryssa; Duncan, Robert C.; Thompson, Christopher (February 2003). "Magnetars". Scientific American. Retrieved 21 March 2016.
  33. ^ Kaspi, V. M.; Gavriil, F. P. (2004). "(Anomalous) X-ray Pulsars". Nuclear Physics B: Proceedings Supplements. 132: 456–465. arXiv:astro-ph/0402176. Bibcode:2004NuPhS.132..456K. doi:10.1016/j.nuclphysbps.2004.04.080.
  34. ^ Magnetic energy density for a field B is U = B2/2μ0 per Eric Weisstein's World of Physics. Substituting B = 108 T, U = 4×1021 J/m3. Dividing by c2 one obtains the equivalent mass density of 44500 kg/m3, which exceeds the standard temperature and pressure density of all known materials, cf. 22590 kg/m3 for osmium, the densest stable element.
  35. ^ Duncan, Robert C. (March 2003). "'Magnetars', soft gamma repeaters & very strong magnetic fields". Retrieved 2018-04-17.
  36. ^ a b c Zahn, Corvin (1990-10-09). "Tempolimit Lichtgeschwindigkeit" (in German). Retrieved 2009-10-09. Durch die gravitative Lichtablenkung ist mehr als die Hälfte der Oberfläche sichtbar. Masse des Neutronensterns: 1, Radius des Neutronensterns: 4, ... dimensionslosen Einheiten (c, G = 1)
  37. ^ Green, Simon F.; Jones, Mark H.; Burnell, S. Jocelyn (2004). An Introduction to the Sun and Stars (illustrated ed.). Cambridge University Press. p. 322. ISBN 978-0-521-54622-5.
  38. ^ "Peligroso lugar para jugar tenis". Datos Freak (in Spanish). Retrieved 3 June 2016.
  39. ^ Marcia Bartusiak (2015). Black Hole: How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved. Yale University Press. p. 130. ISBN 978-0-300-21363-8.
  40. ^ Neutron Star Masses and Radii, p. 9/20, bottom
  41. ^ Hessels, Jason W. T; Ransom, Scott M; Stairs, Ingrid H; Freire, Paulo C. C; Kaspi, Victoria M; Camilo, Fernando (2001). "Neutron Star Structure and the Equation of State". The Astrophysical Journal. 550 (426): 426–442. arXiv:astro-ph/0002232. Bibcode:2001ApJ...550..426L. doi:10.1086/319702.
  42. ^ a b CODATA 2014
  43. ^ a b NASA. Neutron Star Equation of State Science Retrieved 2011-09-26 Archived February 20, 2013, at the Wayback Machine
  44. ^ a b c Beskin, V. S.; (1999); Radiopulsars, УФН. T. 169, №11, p. 1173-1174
  45. ^ neutron star
  46. ^ Pons, José A.; Viganò, Daniele; Rea, Nanda (2013). "Too much "pasta" for pulsars to spin down". Nature Physics. 9 (7): 431–434. arXiv:1304.6546. Bibcode:2013NatPh...9..431P. doi:10.1038/nphys2640.
  47. ^ a b c d e f g h i j k Condon, J. J. & Ransom, S. M. "Pulsar Properties (Essential radio Astronomy)". National Radio Astronomy Observatory. Retrieved 24 March 2016.
  48. ^ a b c d e f Pavlov, George. "X-ray Properties of Rotation Powered Pulsars and Thermally Emitting Neutron Stars" (PDF). pulsarastronomy.net. Retrieved 6 April 2016.
  49. ^ a b c d e f g De Luca, Andrea (2008). "Central Compact Objects in Supernova Remnants". AIP Conference Proceedings. 983: 311–319. arXiv:0712.2209. Bibcode:2008AIPC..983..311D. CiteSeerX 10.1.1.769.699. doi:10.1063/1.2900173.
  50. ^ Klochkov, D.; Puehlhofer, G.; Suleimanov, V.; Simon, S.; Werner, K.; Santangelo, A. (2013). "A non-pulsating neutron star in the supernova remnant HESS J1731-347 / G353.6–0.7 with a carbon atmosphere". Astronomy & Astrophysics. 556: A41. arXiv:1307.1230. Bibcode:2013A&A...556A..41K. doi:10.1051/0004-6361/201321740.
  51. ^ a b "7. Pulsars at Other Wavelengths". Frontiers of Modern Astronomy. Jodrell Bank Centre for Astrophysics. Retrieved 6 April 2016.
  52. ^ Brazier, K. T. S. & Johnston, S. (August 2013). "The implications of radio-quiet neutron stars". Monthly Notices of the Royal Astronomical Society. 305 (3): 671. arXiv:astro-ph/9803176. Bibcode:1999MNRAS.305..671B. doi:10.1046/j.1365-8711.1999.02490.x.
  53. ^ Zhang, B. "Spin-Down Power of Magnetars" (PDF). Universidade Federal do Rio Grande do Sul. Retrieved 24 March 2016.
  54. ^ Hessels, Jason W. T; Ransom, Scott M; Stairs, Ingrid H; Freire, Paulo C. C; Kaspi, Victoria M; Camilo, Fernando (2006). "A Radio Pulsar Spinning at 716 Hz". Science. 311 (5769): 1901–1904. arXiv:astro-ph/0601337. Bibcode:2006Sci...311.1901H. CiteSeerX 10.1.1.257.5174. doi:10.1126/science.1123430. PMID 16410486.
  55. ^ Kaaret, P.; Prieskorn, Z.; Zand, J. J. M. in 't; Brandt, S.; Lund, N.; Mereghetti, S.; Götz, D.; Kuulkers, E.; Tomsick, J. A. (2007). "Evidence of 1122 Hz X-Ray Burst Oscillations from the Neutron Star X-Ray Transient XTE J1739-285". The Astrophysical Journal. 657 (2): L97–L100. arXiv:astro-ph/0611716. Bibcode:2007ApJ...657L..97K. doi:10.1086/513270. ISSN 0004-637X.
  56. ^ Kouveliotou, C.; Duncan, R. C.; Thompson, C.; (February 2003); "Magnetars Magnetars", Scientific American
  57. ^ Alpar, M. Ali (1 January 1998). "Pulsars, glitches and superfluids". Physicsworld.com.
  58. ^ a b Archibald, R. F.; Kaspi, V. M.; Ng, C. Y.; Gourgouliatos, K. N.; Tsang, D.; Scholz, P.; Beardmore, A. P.; Gehrels, N.; Kennea, J. A. (2013). "An anti-glitch in a magnetar". Nature. 497 (7451): 591–593. arXiv:1305.6894. Bibcode:2013Natur.497..591A. doi:10.1038/nature12159. hdl:10722/186148. PMID 23719460.
  59. ^ "Powerful processes at work". Retrieved 15 July 2016.
  60. ^ Posselt, B.; Neuhäuser, R.; Haberl, F. (March 2009). "Searching for substellar companions of young isolated neutron stars". Astronomy and Astrophysics. 496 (2): 533–545. arXiv:0811.0398. Bibcode:2009A&A...496..533P. doi:10.1051/0004-6361/200810156.
  61. ^ Tauris & van den Heuvel; (2006); in Compact Stellar X-ray Sources, Eds. Lewin and van der Klis, Cambridge University Press http://adsabs.harvard.edu/abs/2006csxs.book..623T
  62. ^ Compact Stellar X-ray Sources (2006), Eds. Lewin and van der Klis, Cambridge University Press
  63. ^ Taylor, J. H.; Weisberg, J. M. (15 February 1982). "A new test of general relativity – Gravitational radiation and the binary pulsar PSR 1913+16". The Astrophysical Journal. 253: 908. Bibcode:1982ApJ...253..908T. doi:10.1086/159690.
  64. ^ Tanvir, N.; Levan, A. J.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. L. (2013). "A 'kilonova' associated with the short-duration gamma-ray burst GRB 130603B". Nature. 500 (7464): 547–549. arXiv:1306.4971. Bibcode:2013Natur.500..547T. doi:10.1038/nature12505. PMID 23912055.
  65. ^ Cho, Adrian (16 October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. Retrieved 16 October 2017.
  66. ^ Overbye, Dennis (16 October 2017). "LIGO Detects Fierce Collision of Neutron Stars for the First Time". The New York Times. Retrieved 16 October 2017.
  67. ^ Casttelvecchi, Davide (25 August 2017). "Rumours swell over new kind of gravitational-wave sighting". Nature News. doi:10.1038/nature.2017.22482. Retrieved 27 August 2017.
  68. ^ Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (16 October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225.
  69. ^ Urry, Meg (July 20, 2013). "Gold comes from stars". CNN.
  70. ^ Baade, Walter & Zwicky, Fritz (1934). "Remarks on Super-Novae and Cosmic Rays". Physical Review. 46 (1): 76–77. Bibcode:1934PhRv...46...76B. doi:10.1103/PhysRev.46.76.2.
  71. ^ Landau, Lev D. (1932). "On the theory of stars". Phys. Z. Sowjetunion. 1: 285–288.
  72. ^ Haensel, P; Potekhin, A. Y; Yakovlev, D. G, eds. (2007). Neutron Stars 1 : Equation of State and Structure. Astrophysics and Space Science Library. 326. Springer. Bibcode:2007ASSL..326.....H. ISBN 978-0387335438.
  73. ^ Chadwick, James (1932). "On the possible existence of a neutron". Nature. 129 (3252): 312. Bibcode:1932Natur.129Q.312C. doi:10.1038/129312a0.
  74. ^ Hewish, A. & Okoye, S. E. (1965). "Evidence of an unusual source of high radio brightness temperature in the Crab Nebula". Nature. 207 (4992): 59–60. Bibcode:1965Natur.207...59H. doi:10.1038/207059a0.
  75. ^ Shklovsky, I. S. (April 1967). "On the Nature of the Source of X-Ray Emission of SCO XR-1". Astrophysical Journal. 148 (1): L1–L4. Bibcode:1967ApJ...148L...1S. doi:10.1086/180001.
  76. ^ Ghosh, Pranab (2007). Rotation and Accretion Powered Pulsars (illustrated ed.). World Scientific. p. 8. ISBN 978-981-02-4744-7.
  77. ^ Lang, Kenneth (2007). A Companion to Astronomy and Astrophysics: Chronology and Glossary with Data Tables (illustrated ed.). Springer Science & Business Media. p. 82. ISBN 978-0-387-33367-0.
  78. ^ Haensel, Paweł; Potekhin, Alexander Y.; Yakovlev, Dmitry G. (2007). Neutron Stars 1: Equation of State and Structure (illustrated ed.). Springer Science & Business Media. p. 474. ISBN 978-0-387-47301-7.
  79. ^ Graham-Smith, Francis (2006). Pulsar Astronomy (illustrated ed.). Cambridge University Press. p. 11. ISBN 978-0-521-83954-9.
  80. ^ Ghosh, Pranab (2007). Rotation and Accretion Powered Pulsars (illustrated ed.). World Scientific. p. 281. ISBN 978-981-02-4744-7.
  81. ^ Demorest, Paul B.; Pennucci, T.; Ransom, S. M.; Roberts, M. S.; Hessels, J. W. (2010). "A two-solar-mass neutron star measured using Shapiro delay". Nature. 467 (7319): 1081–1083. arXiv:1010.5788. Bibcode:2010Natur.467.1081D. doi:10.1038/nature09466. PMID 20981094.
  82. ^ Antoniadis, John (2012). "A Massive Pulsar in a Compact Relativistic Binary". Science. 340 (6131): 1233232. arXiv:1304.6875. Bibcode:2013Sci...340..448A. CiteSeerX 10.1.1.769.4180. doi:10.1126/science.1233232. PMID 23620056.
  83. ^ Burtnyk, Kimberly M. (16 October 2017). "LIGO Detection of Colliding Neutron Stars Spawns Global Effort to Study the Rare Event". Retrieved 17 November 2017.
  84. ^ Mereghetti, Sandro (April 2010). "X-ray emission from isolated neutron stars". High-Energy Emission from Pulsars and their Systems. Astrophysics and Space Science Proceedings. 21. pp. 345–363. arXiv:1008.2891. Bibcode:2011ASSP...21..345M. doi:10.1007/978-3-642-17251-9_29. ISBN 978-3-642-17250-2.
  85. ^ Pavlov, George; Zavlin, Slava; Sanwal, Divas; Kargaltsev, Oleg; Romani, Roger. "Thermal Radiation from Isolated Neutron Stars" (PDF). SLAC National Accelerator Laboratory. Retrieved 28 April 2016.
  86. ^ Nakamura, T. (1989). "Binary Sub-Millisecond Pulsar and Rotating Core Collapse Model for SN1987A". Progress of Theoretical Physics. 81 (5): 1006–1020. Bibcode:1989PThPh..81.1006N. doi:10.1143/PTP.81.1006.
  87. ^ Thompson, Todd A.; Neutrino-Driven Protoneutron Star Winds
  88. ^ "Artist's impression of disc around a neutron star". www.spacetelescope.org. Retrieved 18 September 2018.
  89. ^ "HubbleSite: News - Hubble Uncovers Never Before Seen Features Around a Neutron Star". hubblesite.org. Retrieved 18 September 2018.

External links

Binary pulsar

A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. (In at least one case, the double pulsar PSR J0737-3039, the companion neutron star is another pulsar as well.) Binary pulsars are one of the few objects which allow physicists to test general relativity because of the strong gravitational fields in their vicinities. Although the binary companion to the pulsar is usually difficult or impossible to observe directly, its presence can be deduced from the timing of the pulses from the pulsar itself, which can be measured with extraordinary accuracy by radio telescopes.

GW170817

GW170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first GW observation which has been confirmed by non-gravitational means. Unlike the five previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal, the aftermath of this merger was also seen by 70 observatories on seven continents and in space, across the electromagnetic spectrum, marking a significant breakthrough for multi-messenger astronomy.

The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.The gravitational wave signal, designated GW170817, had a duration of approximately 100 seconds, and shows the characteristics in intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short (~ 2 seconds duration) gamma-ray burst, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.

An intense observing campaign then took place to search for the expected emission at optical wavelengths. An astronomical transient designated AT 2017gfo (originally, SSS17a) was found, 11 hours after the gravitational wave signal, in the galaxy NGC 4993 during a search of the region indicated by the GW detection. It was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was shown to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to GW170817. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.

Hulse–Taylor binary

PSR B1913+16 (also known as PSR J1915+1606, PSR 1913+16, and the Hulse–Taylor binary after its discoverers) is a pulsar (a radiating neutron star) which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system. PSR 1913+16 was the first binary pulsar to be discovered. It was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst in 1974. Their discovery of the system and analysis of it earned them the 1993 Nobel Prize in Physics "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."

Magnetar

A magnetar is a type of neutron star believed to have an extremely powerful magnetic field ( G, T). The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar had been detected on March 5, 1979. During the following decade, the magnetar hypothesis became widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

Neutron Star (short story)

"Neutron Star" is an English language science fiction short story by American writer Larry Niven. It was originally published in the October 1966 issue (Issue 107, Vol 16, No 10) of Worlds of If. It was later reprinted in the collection of the same name and Crashlander. The story is set in Niven's fictional Known Space universe. It is notable for including a neutron star before their (then hypothetical) existence was widely known."Neutron Star" is the first to feature Beowulf Shaeffer, the ex-pilot and reluctant hero of many of Niven's Known Space stories. It also marked the first appearance of the nearly indestructible General Products starship hull, as well as its creators, the Pierson's Puppeteers. The star itself, BVS-1, is featured in the novel Protector (1973), where it is named "Phssthpok's Star". A prelude to the story is also included in the novel Juggler of Worlds.

Neutron Star (short story collection)

Neutron Star is a collection of science fiction short stories by American writer Larry Niven, published in April 1968. The individual stories were published in If and Galaxy Science Fiction in 1966–1967, under Frederik Pohl as editor.

Neutron Star Collision (Love Is Forever)

"Neutron Star Collision (Love Is Forever)" is a song by the English alternative rock band Muse, featured on the soundtrack to the 2010 film The Twilight Saga: Eclipse. Recorded by the band in 2010, the song was released as the lead single from the album on 17 May 2010. The song is available to purchase as a digital download from the band's official website, among several other digital sources.The single became a top ten hit in Italy. It was also certified gold by Federation of the Italian Music Industry.

Neutron Star Interior Composition Explorer

The Neutron star Interior Composition Explorer (NICER) is a NASA Explorers program Mission of Opportunity dedicated to the study of the extraordinary gravitational, electromagnetic, and nuclear physics environments embodied by neutron stars, exploring the exotic states of matter where density and pressure are higher than in atomic nuclei. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft (0.2–12 keV) X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena, and the mechanisms that underlie the most powerful cosmic particle accelerators known. NICER will achieve these goals by deploying, following launch, an X-ray timing and spectroscopy instrument as an attached payload aboard the International Space Station (ISS). NICER was selected by NASA to proceed to formulation phase in April 2013.NICER-SEXTANT uses the same instrument to test X-ray timing for positioning and navigation, and MXS is a test of X-ray timing communication. In January 2018, X-ray navigation was demonstrated using NICER on ISS.

Neutron star merger

A neutron star merger is a type of stellar collision. It occurs in a fashion similar to the rare brand of type Ia supernovae resulting from merging white dwarfs. When two neutron stars orbit each other closely, they spiral inward as time passes due to gravitational radiation. When the two neutron stars meet, their merger leads to the formation of either a more massive neutron star, or a black hole (depending on whether the mass of the remnant exceeds the Chandrasekhar limit). The merger can also create a magnetic field that is trillions of times stronger than that of Earth in a matter of one or two milliseconds. These events are believed to create short gamma-ray bursts. The mergers are also believed to produce kilonovae, which are transient sources of fairly isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process.In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817, a gravitational wave event detected in 2017, and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.Also in October 2018, scientists presented a new third way (two earlier methods, one based on redshifts and another on the cosmic distance ladder, gave results that do not agree), using information from gravitational wave events (especially those involving the merger of neutron stars, like GW170817), of determining the Hubble Constant, essential in establishing the rate of expansion of the universe.In April 2019 the Zwicky Transient Facility was online to track neutron star events upon a gravitational wave observation.

PSR J1614−2230

PSR J1614–2230 is a neutron star in a binary system with a white dwarf. It was discovered in 2006 with the Parkes telescope in a survey of unidentified gamma ray sources in the Energetic Gamma Ray Experiment Telescope catalog. PSR J1614–2230 is a millisecond pulsar, a type of neutron star, that spins on its axis roughly 317 times per second, corresponding to a period of 3.15 milliseconds. Like all pulsars, it emits radiation in a beam, similar to a lighthouse. Emission from PSR J1614–2230 is observed as pulses at the spin period of PSR J1614–2230. The pulsed nature of its emission allows for the arrival of individual pulses to be timed. By measuring the arrival time of pulses, astronomers observed the delay of pulse arrivals from PSR J1614–2230 when it was passing behind its companion from the vantage point of Earth. By measuring this delay, known as the Shapiro delay, astronomers determined the mass of PSR J1614–2230 and its companion. The team performing the observations found that the mass of PSR J1614–2230 is 1.97 ± 0.04 M☉. This mass made PSR J1614–2230 the most massive known neutron star at the time of discovery, and rules out many neutron star equations of state that include exotic matter such as hyperons and kaon condensates.In 2013, a slightly higher neutron star mass measurement was announced for PSR J0348+0432, 2.01 ± 0.04 M☉.

This confirmed the existence of such massive neutron stars using a different measuring technique.

Pulsar

A pulsar (from pulse and -ar as in quasar) is a highly magnetized rotating neutron star that emits a beam of electromagnetic radiation. This radiation can be observed only when the beam of emission is pointing toward Earth (much like the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are believed to be one of the candidates for the source of ultra-high-energy cosmic rays (see also centrifugal mechanism of acceleration).

The periods of pulsars make them very useful tools. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. Certain types of pulsars rival atomic clocks in their accuracy in keeping time.

Pulsar kick

A pulsar kick is the name of the phenomenon that often causes a neutron star to move with a different, usually substantially greater, velocity than its progenitor star. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes. If true, this would give information about the supernova mechanism.

Quark-nova

A quark-nova is the hypothetical violent explosion resulting from the conversion of a neutron star to a quark star. Analogous to a supernova heralding the birth of a neutron star, a quark nova signals the creation of a quark star. The term quark-novae was coined in 2002 by Dr. Rachid Ouyed (currently at the University of Calgary, Canada) and Drs. J. Dey and M. Dey (Calcutta University, India).

R-process

The rapid neutron-capture process, or so-called r-process, is a set of nuclear reactions that in nuclear astrophysics is responsible for the creation of approximately half of the atomic nuclei heavier than iron; the "heavy elements". The other half are produced by the p-process and s-process. The r-process usually synthesizes all of the two most neutron-rich, stable isotopes, of each heavy element.

The heavy elements typically have six to ten stable isotopes. Chemical elements are defined by the number of protons in their atomic nucleus, e.g. all xenon atoms have 54 protons. But all elements also have neutrons in their atomic nucleus. Each isotope is characterized by the number of neutrons that it contains, e.g. xenon can have 70, 72, 74, 75, 76, 77, 78, 80, and 82 neutrons, and thus has 9 stable isotopes. The r-process contributes to the abundances of the heaviest four isotopes: 131Xe, 132Xe, 134Xe and 136Xe, and is solely responsible for the heaviest two of those. The s-process contributes to xenon's middle five isotopes: 128Xe, 129Xe, 130Xe, 131Xe, and 132Xe. The lightest two isotopes, 124Xe, and 126Xe, are produced by other processes.

The r-process can typically synthesize the heaviest four isotopes of every heavy element, and the two heaviest isotopes, which are referred to as r-only nuclei, can only be created via the r-process. The r-process abundances peak near atomic weights A = 82 (elements Se, Br and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir and Pt).

The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on 56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β- decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exist a high density of free neutrons. Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures about 1GK, in order to match the waiting points, at which no more neutrons can be captured, with the atomic numbers of the abundance peaks for r-process nuclei. This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova, as part of supernova nucleosynthesis, or decompression of neutron-star matter thrown off by a binary neutron star merger. The relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research.A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions. These led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. The r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate for their time scales.

Radio-quiet neutron star

A radio-quiet neutron star is a neutron star that does not seem to emit radio emissions, but is still visible to Earth through electromagnetic radiation at other parts of the spectrum, particularly x-rays and gamma rays.

Thorne–Żytkow object

A Thorne–Żytkow object (TŻO or TZO) is a conjectured type of star wherein a red giant or supergiant contains a neutron star at its core, formed from the collision of the giant with the neutron star. Such objects were hypothesized by Kip Thorne and Anna Żytkow in 1977. In 2014, it was discovered that the star HV 2112 was a strong candidate but this has since been called into question.

Tolman–Oppenheimer–Volkoff limit

The Tolman–Oppenheimer–Volkoff limit (or TOV limit) is an upper bound to the mass of cold, nonrotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Observations of GW170817, the first gravitational wave event due to merging neutron stars (which are thought to have collapsed into a black hole within a few seconds after merging), suggest that the limit is close to 2.17 solar masses. A neutron star in a binary pair (PSR J2215+5135) has been measured to have a mass close to or slightly above this limit, 2.27+0.17−0.15 M☉. Earlier theoretical work placed the limit at approximately 1.5 to 3.0 solar masses, corresponding to an original stellar mass of 15 to 20 solar masses. In the case of a rigidly spinning neutron star, the mass limit is thought to increase by up to 18-20%.

X-ray burster

X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray regime of the electromagnetic spectrum. These astrophysical systems are composed of an accreting compact object, and a main sequence companion 'donor' star. A compact object in an X-ray binary system consists of either a neutron star or a black hole; however, with the emission of an X-ray burst, the companion star can immediately be classified as a neutron star, since black holes do not have a surface and all of the accreting material disappears past the event horizon. The donor star's mass falls to the surface of the neutron star where the hydrogen fuses to helium which accumulates until it fuses in a burst, producing X-rays.

The mass of the donor star is used to categorize the system as either a high mass (above 10 solar masses (M☉)) or low mass (less than 1 M☉) X-ray binary, abbreviated as HMXB and LMXB, respectively. X-ray bursters differ observationally from other X-ray transient sources (such as X-ray pulsars and soft X-ray transients), showing a sharp rise time (1 – 10 seconds) followed by spectral softening (a property of cooling black bodies). Individual burst energetics are characterized by an integrated flux of 1032–33 joules, compared to the steady luminosity which is of the order 1032 joules for steady accretion onto a neutron star. As such the ratio α, of the burst flux to the persistent flux, ranges from 10 to 103 but is typically on the order of 100. The X-ray bursts emitted from most of these systems recur on timescales ranging from hours to days, although more extended recurrence times are exhibited in some systems, and weak bursts with recurrence times between 5–20 minutes have yet to be explained but are observed in some less usual cases. The abbreviation XRB can refer either the object (X-ray burster) or the associated emission (X-ray burst). There are two types of XRB's, designated I and II. Type I are far more common than type II, and have a distinctly different cause. Type I are caused by thermonuclear runaway, while type II are caused by gravitational energy release.

X-ray pulsar

X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several minutes.

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