Is there a significant population of neutron stars outside the galactic plane?

Is there a significant population of neutron stars outside the galactic plane?

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Pulsar kicks originating from supernovae can impart neutron stars with speeds of 100-500 km/s, often at or close to the escape velocity of the Milky Way. Even if a pulsar fails to escape the galaxy, it can be launched into unusual orbits. Assuming that the velocity distribution is isotropic, a decent fraction of such pulsars should have large orbital inclinations after the kicks, such that they rise above the galactic plane.

I have two questions regarding this:

  1. Is there a large - i.e. noticeable - population of such neutron stars, distinguishable by modern telescopes, or is the fraction of total neutron stars relatively small?
  2. Does observing a pulsar - at any wavelength - in such an orbit have any advantages over observing an identical pulsar firmly in the galactic disk? I would assume that some kicks could propel them high enough that scattering along the line of sight could be lessened, leading to a smaller dispersion measure smearing, and could also isolate the pulsars from ambient radio sources in the disk, such as a the galactic center.

I understand that "noticeable", at the least, is a bit subjective, but I can't come up with any non-arbitrary criteria to change that.

I'll partly answer your question with the following plot from the ATNF pulsar database (which is easy to play around with). The plot shows the Galactic z coordinate of the pulsars (distance from the Galactic plane in kpc) versus their estimated age (from their spin down rates, in years).

The basic picture is that young pulsars are all near the Galactic plane, because they are born from high mass stars that are born, live and die near the plane and even if the neutron star got a kick, there has been insufficient time to move far from the plane. A typical kick might be 500-1000 km/s, roughly 0.5-1 kpc in a million years.

Once pulsars reach a million years old, then indeed some reach a kpc above/below the Galactic plane and then a few even older examples reach 10 kpc. Most normal pulsars only show pulsar activity for 10-100 million years (the older ones on the plot are probably a different population of pulsars in binaries), so they aren't observed at much greater distances (though there will be old neutron stars out there).

Milky Way

The Milky Way [a] is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος (galaktikos kýklos, "milky circle"). [19] [20] [21] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. [22] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, [23] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

Milky Way
Observation data
Right ascension 17 h 45 m 40.0409 s
Declination−29° 0′ 28.118″
Distance25.6–27.1 kly (7.86–8.32 kpc) [1] [2]
TypeSb, Sbc, or SB(rs)bc [3] [4]
(barred spiral galaxy)
Mass (0.8–1.5) × 10 12 [5] [6] [7] [8] M
Number of stars 100–400 billion
SizeStellar disk: 185 ± 15 kly [9] [10]
Dark matter halo: 1.9 ± 0.4 Mly (580 ± 120 kpc) [11] [12]
Thickness of thin stellar disk≈2 kly (0.6 kpc) [13] [14]
Angular momentum≈ 1 × 10 67 J s [15]
Sun's Galactic rotation period240 Myr [16]
Spiral pattern rotation period220–360 Myr [17]
Bar pattern rotation period100–120 Myr [17]
Speed relative to CMB rest frame 552.2 ± 5.5 km/s [18]
Escape velocity at Sun's position 550 km/s [8]
Dark matter density at Sun's position 0.0088 +0.0024
−0.0018 M pc −3 or 0.35 +0.08
−0.07 GeV cm −3 [8]
See also: Galaxy, List of galaxies

The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years. Recent simulations suggest that a dark matter disk, also containing some visible stars, may extend up to a diameter of almost 2 million light-years. [11] [12] The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. [24] [25]

It is estimated to contain 100–400 billion stars [26] [27] and at least that number of planets. [28] [29] The Solar System is located at a radius of about 27,000 light-years from the Galactic Center, [2] on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, a supermassive black hole of 4.100 (± 0.034) million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) [30] [31] of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". [32] The rotational period is about 240 million years at the radius of the Sun. [16] The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang. [33]

Chapter 14: The Milky Way Galaxy

A) The main star forming regions are outside the Galactic plane.

B) Red stars are only found in the bulge.

C) Only old stars are found in the halo.

D) Most open clusters are in the halo.

A) in the disc and about one-half a galactic radius from the center

B) above the disc and about one-third of the galactic radius from the center

C) at the outer edge of the galactic bulge but in the plane of the disc

D) in the disc but at its outer edge

A) Most Cepheids have much lower masses and luminosities than RR Lyrae stars.

B) The pulsations of RR Lyrae stars are much less regular than most Cepheids.

C) Most Cepheids are giants, but RR Lyrae stars are still on the main sequence.

D) Most Cepheids have longer periods than the RR Lyrae stars.

A) Population I stars are not found in the galactic disc.

B) Population II stars are only in the galactic disc.

C) Population II stars probably are variable stars.

D) Population I stars are the brightest stars in the globular clusters.

A) the Sun's mass and its age

B) the Sun's composition and luminosity

C) the Sun's Galactic orbital period and its distance from the Crab Nebula

D) the Sun's Galactic orbital period and its distance from the Galactic Center

Pulsar Surveys

Data acquisition

With ALFA, we need about 47 pointings to cover one square degree, compared to about 330 pointings needed to cover one square degree with similar density with a single-pixel feed. Until 2009, we were using the Wideband Arecibo Pulsar Processors (WAPPs) to detect the signal from ALFA's seven beams. These cover 100 MHz of band (with dual polarization capability), initially centered at 1420 MHz and later at 1440 MHz. In 2009, the survey transitioned to new and improved back-ends, the Mock polyphase filterbank spectrometers, which are capable of covering 300 MHz (from 1225 MHz to 1525 MHz, the bandwidth covered by ALFA) for each of the seven beams (see detailed technical specifications here). This has resulted in greatly increased search sensitivity, but only for pointing where we can effectively deal with all the radio frequency interference.
From August 1 to October 8 2004, we conducted a preliminary survey that covered the two regions closest to the Galactic plane (|b| < 1 degrees) visible from Arecibo: the "Inner Galaxy" (40 < l < 75 degrees) and the "Anti-center" (170 < l < 210 degrees). Each pointing was 134 seconds for the Inner Galaxy and 67 seconds for the Anti-center. This was done in sparse mode, where we do only 1/3 of the pointings needed to cover the whole region. This preliminary survey found a total of 11 new pulsars, and detected 30 previously known pulsars. For a detailed description of this survey, and the strategy of the present survey, see Cordes et al. (2006).

The survey will cover the Galactic plane (|b| < 5 degrees) visible with the Arecibo 305-m radio telescope (35 < l < 75 degrees, see Fig. 1). Each pointing lasts about 268 seconds in the Inner Galaxy and 134 seconds in the Anti-center.

PALFA pulsar survey coverage and PRESTO data processing status as of December 2010, in Galactic coordinates (center of the Galaxy is at (0,0)). The stars indicate positions of known pulsars, grey indicates observed pointings with the WAPPs, while red indicates pointings processed by the PRESTO pipeline so far.

Modeling the spatial distribution of neutron stars in the Galaxy

In this paper we investigate the space and velocity distributions of old neutron stars (aged 10 9 to 10 10 yr) in our Galaxy. Galactic old Neutron Stars (NSs) population fills a torus-like area extending to a few tens kiloparsecs above the galactic plane. The initial velocity distribution of NSs is not well known, in this work we adopt a three component initial distribution, as given by the contribution of kick velocities, circular velocities and Maxwellian velocities. For the spatial initial distribution we use a Γ function. We then use Monte Carlo simulations to follow the evolution of the NSs under the influence of the Paczyński Galactic gravitational potential. Our calculations show that NS orbits have a very large Galactic radial expansion and that their radial distribution peak is quite close to their progenitors’ one. We also study the NS vertical distribution and find that it can well be described by a double exponential low. Finally, we investigate the correlation of the vertical and radial distribution and study the radial dependence of scale-heights.

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Trick to finding neutron stars easier on the map?

I'm on my first trip. Newly adventuring pilot. Currently some 3000ly out from sol, working my way toward the center, but going at and odd outside angle. I was hoping that would increase my odds of finding undiscovered systems, and I have actually found a lot of them. Finding my first one was exciting for me, but anyway I'm getting off track.

I did some reading and watching of videos about how to find neutron stars, as I had not seen one as of yet and I really wanted to find one. So, I set my map to show only non-sequenced stars and started zooming around the map to find one. I finally did find one about 200ly from my current location after much zooming in and out and moving the camera all about. So, that was exciting!

My question is, is there a trick to how I should use the map to better find them. I'm honestly embarrassed by how long I looked on the map to find one close to me.

Also, is it possible there just isn't a lot of neutron stars where I am? Perhaps more toward the core they will be easier to find?

Neutron Star Physics in the Square Kilometre Array Era: An Indian Perspective

It is an exceptionally opportune time for astrophysics when a number of next-generation mega-instruments are poised to observe the Universe across the entire electromagnetic spectrum with unprecedented data quality. The Square Kilometre Array (SKA) is undoubtedly one of the major components of this scenario. In particular, the SKA is expected to discover tens of thousands of new neutron stars giving a major fillip to a wide range of scientific investigations. India has a sizeable community of scientists working on different aspects of neutron star physics with immediate access to both the uGMRT (an SKA pathfinder) and the recently launched X-ray observatory Astrosat. The current interests of the community largely centre around studies of (a) the generation of neutron stars and the SNe connection, (b) the neutron star population and evolutionary pathways, (c) the evolution of neutron stars in binaries and the magnetic fields, (d) the neutron star equation of state, (e) the radio pulsar emission mechanism, and (f) the radio pulsars as probes of gravitational physics. Most of these studies are the main goals of the SKA first phase, which is likely to be operational in the next four years. This article summarizes the science goals of the Indian neutron star community in the SKA era, with significant focus on coordinated efforts among the SKA and other existing/upcoming instruments.

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The Wolf-Rayet Star Population of the Milky Way Galaxy

10−5 M⊙/yr). Ionizing radiation from the central star heats the expanding outer envelope of material, leading to recombination emission lines of helium, carbon, nitrogen, oxygen, and/or hydrogen in the WR star spectrum. This outflow of material enriches the surrounding ISM, which is further enriched when the WR star likely explodes as a type Ib or Ic supernova. WR stars are also likely progenitors for long soft gamma-ray bursts, and they are excellent tracers of the present sites of massive star formation in our Galaxy.
The current Galactic WR star catalog is very incomplete. I discuss three methods of selecting strong WR star candidates from crowded fields in the Galactic plane: image subtraction, narrowband (NB) color, and broadband (BB) color. Using these methods, an extensive near-infrared narrowband survey begun in 2005-2006, and extended by me, has yielded 28% of the known Galactic WR stars to date I add 59 new WR stars to the total in this thesis. I then compare two recent models of the Galactic population of WR stars, discuss the implications with respect to how many WR stars remain to be found, and use these results to inform an analysis of the remaining 834 strong carbon-rich WC star candidates from the survey. I also provide a listing of these 834 WC star candidates throughout our Galaxy, and map them a central result of this thesis. Finally, I present selection criteria which may be used to identify [WR] stars (central stars of planetary nebulae which display WR spectral features), and proof of concept observations which led to 7 new confirmed [WC] 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 × 10 9 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 × 10 17 kg/m 3 , a combination of strong force repulsion and neutron degeneracy pressure halts the contraction. [20] 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. [21]

As the core of a massive star is compressed during a Type II supernova or a 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 10 12 to 10 13 m/s 2 (more than 10 11 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.

Mass and temperature Edit

A neutron star has a mass of at least 1.1 solar masses ( M ). The upper limit of mass for a neutron star is called the Tolman–Oppenheimer–Volkoff limit and is generally held to be around 2.1 M , [22] [23] but a recent estimate puts the upper limit at 2.16 M . [24] The maximum observed mass of neutron stars is about 2.14 M for PSR J0740+6620 discovered in September, 2019. [25] Compact stars below the Chandrasekhar limit of 1.39 M are generally white dwarfs whereas compact stars with a mass between 1.4 M and 2.16 M are expected to be neutron stars, but 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. It is thought that beyond 2.16 M the stellar remnant will overcome the strong force repulsion and neutron degeneracy pressure so that gravitational collapse will occur to produce a black hole, but the smallest observed mass of a stellar black hole is about 5 M . [b] Between 2.16 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. [b]

The temperature inside a newly formed neutron star is from around 10 11 to 10 12 kelvins. [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 10 6 kelvins. [27] At this lower temperature, most of the light generated by a neutron star is in X-rays.

Some researchers have proposed a neutron star classification system using Roman numerals (not to be confused with the Yerkes luminosity classes for non-degenerate stars) to sort neutron stars by their mass and cooling rates: type I for neutron stars with low mass and cooling rates, type II for neutron stars with higher mass and cooling rates, and a proposed type III for neutron stars with even higher mass, approaching 2 M , and with higher cooling rates and possibly candidates for exotic stars. [28]

Density and pressure Edit

Neutron stars have overall densities of 3.7 × 10 17 to 5.9 × 10 17 kg/m 3 ( 2.6 × 10 14 to 4.1 × 10 14 times the density of the Sun), [c] which is comparable to the approximate density of an atomic nucleus of 3 × 10 17 kg/m 3 . [29] The neutron star's density varies from about 1 × 10 9 kg/m 3 in the crust—increasing with depth—to about 6 × 10 17 or 8 × 10 17 kg/m 3 (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 × 10 12 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 × 10 25 N , which is 15 times what the Moon would weigh if it were placed on the surface of the Earth. [d] The entire mass of the Earth at neutron star density would fit into a sphere of 305 m in diameter (the size of the Arecibo Telescope). The pressure increases from 3.2 × 10 31 to 1.6 × 10 34 Pa from the inner crust to the center. [30]

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 Edit

The magnetic field strength on the surface of neutron stars ranges from c. 10 4 to 10 11 tesla. [31] 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. [31]

The neutron stars known as magnetars have the strongest magnetic fields, in the range of 10 8 to 10 11 tesla, [32] and have become the widely accepted hypothesis for neutron star types soft gamma repeaters (SGRs) [33] and anomalous X-ray pulsars (AXPs). [34] The magnetic energy density of a 10 8 T field is extreme, greatly exceeding the mass-energy density of ordinary matter. [e] 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. [36]

The origins of the strong magnetic field are as yet unclear. [31] One hypothesis is that of "flux freezing", or conservation of the original magnetic flux during the formation of the neutron star. [31] 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. [31]

Gravity and equation of state Edit

The gravitational field at a neutron star's surface is about 2 × 10 11 times stronger than on Earth, at around 2.0 × 10 12 m/s 2 . [38] 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. [37] If the radius of the neutron star is 3GM/c 2 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 = mc 2 ). 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. [39] 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 the star's very rapid rotation. [40]

Neutron star relativistic equations of state describe the relation of radius vs. mass for various models. [41] 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, [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] [44] 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). [44]

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. [45] It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like helium and hydrogen. [45] If the surface temperature exceeds 10 6 kelvins (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 <10 6 kelvins). [45]

The "atmosphere" of a neutron star is hypothesized to be at most several micrometres 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. [46]

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. [47] 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.

Pulsars Edit

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. [48] These electrons are magnetically accelerated along the field lines, leading to curvature radiation, with the radiation being strongly polarized towards the plane of curvature. [48] 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. [48]

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. [49] 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. [48]

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 Edit

In addition to pulsars, non-pulsating neutron stars have also been identified, although they may have minor periodic variation in luminosity. [50] [51] 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. [50]

Spectra Edit

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. [49] Pulsars observed in X-rays are known as X-ray pulsars if accretion-powered, while those identified in visible light are known as optical pulsars. The majority of neutron stars detected, including those identified in optical, X-ray, and gamma rays, also emit radio waves [52] the Crab Pulsar produces electromagnetic emissions across the spectrum. [52] However, there exist neutron stars called radio-quiet neutron stars, with no radio emissions detected. [53]

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 Edit

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 ˙ >> (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). [48]

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. [48]

P and P-dot allow minimum magnetic fields of neutron stars to be estimated. [48] 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. [48]

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 ˙ >> (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". [48] [49] 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. [48] 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. [54]

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. [48]

Spin up Edit

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. [55] 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, [56] 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 Edit

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. [57]

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. [58]

"Anti-glitches" Edit

An "anti-glitch", a sudden small decrease in rotational speed, or spin down, of a neutron star has also been reported. [59] It occurred in the magnetar 1E 2259+586, 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. [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 about 424 light years. [61] 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 that would only be detectable through their blackbody radiation and gravitational effects on other stars.

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. [62] 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, has been observed through the emission of gravitational waves. [63] [64]

X-ray binaries Edit

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. [65]

Neutron star binary mergers and nucleosynthesis Edit

The distance between two neutron stars in a close binary system is observed to shrink as gravitational waves are emitted. [66] 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, [67] and finally confirmed by detection of gravitational wave GW170817 and short GRB 170817A by LIGO, Virgo, and 70 observatories covering the electromagnetic spectrum observing the event. [68] [69] [70] [71] 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, [72] as opposed to the supernova nucleosynthesis theory.

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.

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, [73] [f] less than two years after the discovery of the neutron by James Chadwick. [76] 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". [77] 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. [78]

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 1968, Richard V. E. Lovelace and collaborators discovered period P ≈ 33 ms of the Crab pulsar using Arecibo Observatory. [79] [80] After this discovery, scientists concluded that pulsars were rotating neutron stars. [81] Before that, many scientists believed that pulsars were pulsating white dwarfs.

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. [82] 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. [83]

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. Albert 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. [84]

In 1982, Don Backer and colleagues discovered the first millisecond pulsar, PSR B1937+21. [85] 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. [86] 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. [87] 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. [88] 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. [89]

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. [90] [91] [92] [93]

In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of neutron stars, following the detection of the neutron star merger of GW170817. [94] [95] Their measurement of the Hubble constant is 70.3 +5.3
−5.0 (km/s)/Mpc. [96]

  • Neutron star
    • Isolated neutron star (INS): [49][50][97][98] not in a binary system.
        (RPP or "radio pulsar"): [50] neutron stars that emit directed pulses of radiation towards us at regular intervals (due to their strong magnetic fields).
          (RRATs): [50] are thought to be pulsars which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars.
      • (SGR). [49] (AXP). [49]
    • X-ray dim isolated neutron stars. [50]
    • Central compact objects in supernova remnants (CCOs in SNRs): young, radio-quiet non-pulsating X-ray sources, thought to be Isolated Neutron Stars surrounded by supernova remnants. [50]
    • Low-mass X-ray binary pulsars: a class of low-mass X-ray binaries (LMXB), a pulsar with a main sequence star, white dwarf or red giant.
        (MSP) ("recycled pulsar").
        • "Spider Pulsar", a pulsar where their companion is a semi-degenerate star. [99]
          • "Black Widow" pulsar, a pulsar that falls under the "Spider Pulsar" if the companion has extremely low mass (less than 0.1 solar masses).
          • "Redback" pulsar, are if the companion is more massive.
          • Protoneutron star (PNS), theorized. [102]
              : currently a hypothetical merger of a neutron star into a red giant star. : currently a hypothetical type of neutron star composed of quark matter, or strange matter. As of 2018, there are three candidates. : currently a hypothetical type of extremely heavy neutron star, in which the quarks are converted to leptons through the electroweak force, but the gravitational collapse of the neutron star is prevented by radiation pressure. As of 2018, there is no evidence for their existence. : currently a hypothetical type of neutron star composed of preon matter. As of 2018, there is no evidence for the existence of preons.
            • – a millisecond pulsar that is very massive. (now known as PSR B1919+21) – the first recognized radio-pulsar. It's discovered by Jocelyn Bell Burnell in 1967. – the first neutron star discovered with planets (a millisecond pulsar). – source of the "Hand of God" photo shot by the Chandra X-ray Observatory. – closest neutron star. , a group of nearby, X-ray dim isolated neutron stars. – the most massive neutron star with a well-constrained mass, 2.01 ± 0.04 M . – neutron star source of infrared radiation. [104] – a millisecond pulsar with a stellar-type companion with planetary range mass (below brown dwarf). - youngest known magnetar

            Video – animation Edit

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

            6. Summary and Conclusions

            We have presented comprehensive imaging and integral field spectroscopy of the host galaxy and local environment of the first electromagnetic counterpart to a gravitational-wave source. These observations provide a unique view of the regions around this event, and its properties are consistent with those seen in the population of SGRB hosts. We find a highly inclined ionized gas disk that is kinematically decoupled from the stellar velocity field, as well as extended face-on arm/shell features in the stellar light profile. These indicate the galaxy has undergone a major merger relatively recently. We find that

            20% of the galaxy, by mass, is

            1 Gyr old, perhaps as a result of this merger, while most of the remaining mass is old. There is minimal contribution (if any) from a young stellar population ( of the mass), implying an old (Gyr) progenitor. The absence of absorption features in the counterpart spectrum and moderate extinction of the stellar population in the vicinity of the transient source offer tentative evidence that it lies on the near side of the galaxy, either by chance or due to a kick in our direction.

            Galaxy demographics and population synthesis have previously been used to argue for the origin of SGRBs in compact object mergers. Since this scenario now seems secure the direction of inference can now be reversed, and the properties and locations of SGRBs and gravitational-wave sources can be used to pinpoint the details of extreme stellar evolution that lead to the formation of compact object binaries.

            We thank the referee for a prompt and highly constructive report that improved the content and clarity of the manuscript. We also thank the editor, Fred Rasio, for helpful comments. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 099.D-0668 (A.J.L.), and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with programs GO 14771 (N.R.T.), GO 14804 (A.J.L.), and GO 14850 (E.T.). We thank the staff at ESO and STScI for their excellent support of these observations. A.J.L. acknowledges that this project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246) A.J.L., D.S., and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T., K.W., P.T.O., J.L.O., and S.R. acknowledge support from STFC. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). A.d.U.P., C.T., Z.C., and D.A.K. acknowledge support from the Spanish project AYA 2014-58381-P. Z.C. also acknowledges support from the Juan de la Cierva Incorporación fellowship IJCI-2014-21669, and D.A.K. from Juan de la Cierva Incorporación fellowship IJCI-2015-26153. M.I. was supported by the NRFK grant, No. 2017R1A3A3001362. E.T. acknowledges support from grants GO718062A and HSTG014850001A. S.R. has been supported by the Swedish Research Council (VR) under grant number 2016-03657_3, by the Swedish National Space Board under grant number Dnr. 107/16 and by the research environment grant "Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)" funded by the Swedish Research council (VR) under Dnr 2016-06012. P.A.E. acknowledges UKSA support.

            Facilities: Hubble Space Telescope - , Very Large Telescope. -

            Software: Numpy, PyRAF, astropy (Astropy Collaboration 2013), starlight (Cid Fernandes et al. 2005), Reflex (Freudling et al. 2013).