Radio-quiet neutron stars
A radio-quiet neutron star is a neutron star that does not seem to emit radio emissions. But they are visible from the Earth through electromagnetic radiation at other parts of the spectrum, particularly x-rays and gamma rays.
Most detected neutron stars are pulsars, and emit radio 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, 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.
Quark stars, 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.
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.
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