Binary pulsar

Binary pulsar

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A binary pulsar is a pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 with a binary companion
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

, often a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

 or neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

. (In at least one case, the double pulsar PSR J0737-3039
PSR J0737-3039
|- style="vertical-align: top;"| Distance | 1600 - 2000 Ly PSR J0737-3039 is currently the only known double pulsar, it consists of two neutron stars emitting electromagnetic waves in the radio wavelength in a relativistic binary system. The two Pulsars are known as PSR J0737-3039A and PSR...

, the companion star is another pulsar as well.) Binary pulsars are one of the few objects which allow physicists to test general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 in the case of a strong gravitational field. Although the binary companion to the pulsar is usually difficult or impossible to observe, the timing of the pulses from the pulsar can be measured with extraordinary accuracy by radio telescope
Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...

s. A relatively simple 10-parameter model incorporating information about the pulsar timing, the Keplerian orbits and three post-Keplerian corrections (the rate of periastron advance, a factor for gravitational redshift
Gravitational redshift
In astrophysics, gravitational redshift or Einstein shift describes light or other forms of electromagnetic radiation of certain wavelengths that originate from a source that is in a region of a stronger gravitational field that appear to be of longer wavelength, or redshifted, when seen or...

 and a rate of change of the orbital period from gravitational radiation) is sufficient to completely model the pulsar timing. Binary pulsar timing has thus indirectly confirmed the existence of gravitational radiation and verified Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's general theory of relativity in a previously unknown regime.

The first binary pulsar, PSR B1913+16 or the "Hulse-Taylor binary pulsar" was discovered in 1974 at Arecibo
Arecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

 by Joseph Hooton Taylor, Jr.
Joseph Hooton Taylor, Jr.
Joseph Hooton Taylor, Jr. is an American astrophysicist and Nobel Prize in Physics laureate for his discovery with Russell Alan Hulse of a "new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."...

 and Russell Hulse, for which they won the 1993 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

. Pulses from this system have been tracked, without glitches, to within 15 μs
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...

 since its discovery.

Binary pulsars are one of the few tools scientists have to detect evidence of gravitational waves; Einstein’s theory of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 predicts that two neutron stars would emit gravitational waves as they orbit a common center of mass, which would carry away orbital energy and cause the two stars to draw closer together. As the two stellar bodies draw closer to one another, often a pulsar will absorb matter from the other causing a violent accretion process. This interaction can heat the gas being exchanged between the bodies and produce X-ray light which can appear to pulsate, causing binary pulsars to occasionally be referred to as X-ray binaries. This flow of matter from one stellar body to another is known as an accretion disk. Millisecond pulsar
Millisecond pulsar
A millisecond pulsar is a pulsar with a rotational period in the range of about 1-10 milliseconds. Millisecond pulsars have been detected in the radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The origin of millisecond pulsars is still unknown...

s (or MSP's) create a sort of "wind", which in the case of binary pulsars can blow away the magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

 of the neutron stars and have a dramatic effect on the pulse emission.
The 1993 Nobel Prize was awarded to Joseph Taylor and Russell Hulse after they discovered two such stars. While Hulse was observing a new pulsar, named PSR B1913+16, he noticed that the frequency with which it pulsed fluctuated. It was concluded that the simplest explanation was that the pulsar was orbiting another star very closely at a high velocity. Hulse and Taylor determined that the stars were equally heavy by observing these pulse fluctuations, which led them to believe the other spacial object was also a neutron star.

The observations made of the orbital decay of this star system was a near perfect match to Einstein’s equations. Relativity predicts that over time a binary system’s orbital energy will be converted to gravitational radiation. Data collected by Taylor and his colleagues of the orbital period of PRS B1913+16 supported this relativistic prediction; they reported in 1983 that there was a difference in the observed minimum separation of the two pulsars compared to that expected if the orbital separation had remained constant. In the decade following its discovery the system’s orbital period had decreased by about 76 millionths of a second per year - this means that the pulsar was approaching its maximum separation more than a second earlier than it would have if the orbit had remained the same (Haynes 2007). Subsequent observations continue to show this decrease.

The study of binary pulsars also led to the first accurate determination of neutron star masses, using relativistic timing effects. Scientists can find the radial velocity of a pulsar as it moves through its orbit by observing the number of pulses received each second. As a pulsar is moving towards us, the pulses will be more frequent and the pulse repetition rate will be its highest. Conversely, as it moves away from us the pulses will be more spread out, and fewer will be detected in a given time period. One can think of the pulses like the ticks of a clock; changes in the ticking are indications of changes in time due to these relativistic changes. When the two bodies are in close proximity, the gravitational field is stronger, the passage of time is slowed – and the time between pulses (or ticks) is lengthened. As the pulsar clock travels more slowly through the weakest part of the field it regains time. This relativistic time delay is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around it companion in a circular orbit, and what is actually observed (Haynes 2007).

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