A radio-quiet neutron star is a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

that does not seem to emit

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...

emissions, but is still visible to Earth through

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...

at other parts of the

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

, particularly

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

Background

Most detected neutron stars are pulsars, and emit radio-frequency electromagnetic radiation. About 700 radio pulsars are listed in the Princeton catalog, and all but one emit radio waves at the 400 MHz and 1400 MHz frequencies. That exception is

Geminga Geminga is a gamma ray and x-ray pulsar source thought to be a neutron star approximately 250 parsecs (around 800 light-years) from the Sun in the constellation Gemini. Its name, attributed by its discoverer Giovanni Bignami, is both ...

, which is radio quiet at frequencies above 100 MHz, but is a strong emitter of X-rays and gamma rays. In all, ten bodies have been proposed as rotation-powered neutron stars that are not visible as radio sources, but are visible as X-ray and gamma ray sources. Indicators that they are indeed neutron stars include them having a high X-ray to lower frequencies emission ratio, a constant X-ray emission profile, and coincidence with a gamma ray source.

Theories

Quark star A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks. Background Some massive ...

s, theoretical neutron star-like objects composed of quark matter, may be radio quiet, according to some theories. More plausibly, however, radio-quiet neutron stars may simply be pulsars which do not pulse in our direction. As pulsars spin, it is theorized they emit radiation from their magnetic poles. When the magnetic poles do not lie on the axis of rotation, and cross the line of sight of the observer, one can detect radio emission emitted near the star's magnetic poles. Due to the star's rotation this radiation appears to pulse, colloquially called the "lighthouse effect". Radio-quiet neutron stars may be neutron stars whose magnetic poles do not point towards the Earth during their rotation. The group of radio-quiet neutrons stars informally known as the Magnificent Seven are thought to emit mainly

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...

. Possibly some powerful neutron star radio emissions are caused by a positron-electron jet emanating from the star blasting through outer material such as a cloud or accretion material. Note some radio quiet neutron stars listed in this article do not have accretion material.

Magnetars

Magnetar A magnetar is a type of neutron star with an extremely powerful magnetic field (∼109 to 1011 T, ∼1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.War ...

s, the most widely accepted explanation for

soft gamma repeater A soft gamma repeater (SGR) is an astronomical object which emits large bursts of gamma-rays and X-rays at irregular intervals. It is conjectured that they are a type of magnetar or, alternatively, neutron stars with fossil disks around them. Hi ...

s (SGRs) and

anomalous X-ray pulsar Anomalous X-ray pulsars (AXPs) are an observational manifestation of magnetars—young, isolated, highly magnetized neutron stars. These energetic X-ray pulsars are characterized by slow rotation periods of ~2–12 seconds and large magnetic fields ...

s (AXPs), are often characterized as being radio-quiet. However, magnetars can produce radio emissions, but the radio spectrums tend to be flat, with only intermittent broad pulses of variable length.

List of radio-quiet neutron stars

X-ray Dim Isolated Neutron Stars

Can be classified as ''XDINS'' (X-ray Dim Isolated Neutron Stars), ''XTINS'' (X-ray Thermal Isolated Neutron Stars), ''XINS'' (X-ray Isolated Neutron Stars), ''TEINS'' (Thermally Emitting Neutron Star), ''INS'' (Isolated Neutron Stars). Defined as thermally emitting neutron stars of high magnetic fields, although lower than that of

magnetar A magnetar is a type of neutron star with an extremely powerful magnetic field (∼109 to 1011 T, ∼1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.War ...

s. Identified in thermal

s, and thought to be radio-quiet. * A group of seven individual, physically similar and relatively nearby neutron stars nicknamed ''

The Magnificent Seven ''The Magnificent Seven'' is a 1960 American Western film directed by John Sturges. The screenplay by William Roberts is a remake – in an Old West–style – of Akira Kurosawa's 1954 Japanese film ''Seven Samurai'' (itself initially relea ...

'', consisting of: ** RX J185635-3754 ** RX J0720.4-3125 ** RBS1556 ** RBS1223 ** RX J0806.4-4132 ** RX J0420.0-5022 ** MS 0317.7-6647 * 1RXS J214303.7+065419/RBS 1774

Compact Central Objects in Supernova remnants

Compact Central Objects in

Supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...

s (CCOs in SNRs) are identified as being radio-quiet compact X-ray sources surrounded by supernova remnants. They have thermal emission spectra, and lower magnetic fields than XDINSs and magnetars. * RX J0822-4300 (1E 0820–4247) in the

Puppis A Puppis A (Pup A) is a supernova remnant (SNR) about 100 light-years in diameter and roughly 6500–7000 light-years distant. Its apparent angular diameter is about 1 degree. The light of the supernova explosion reached Earth approximately 3700 ye ...

supernova remnant (SNR G260.4-3.4). * 1E 1207.4-5209 in the PKS 1209-51/52 supernova remnant (SNR G296.5+10). *RXJ0007.0+7302 (in SNR G119.5+10.2, CTA1) *RXJ0201.8+6435 (in SNR G130.7+3.1, 3C58) *1E 161348–5055 (in SNR G332.4-0.4, RCW103) *RXJ2020.2+4026 (in SNR G078.2+2.1, γ–Cyg)

Other neutron stars

* IGR J11014-6103: a runaway pulsar ejected from a supernova remnant.

Notes

References

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