Binary pulsar

A binary pulsar is a highly specialized astronomical system. It consists of a pulsar paired with a companion star. This companion is frequently a white dwarf or a neutron star. In rare cases, like the double pulsar PSR J0737-3039, both objects are pulsars. These systems are vital to modern physics. They allow scientists to test the theory of general relativity. This is possible because the gravitational fields near these objects are incredibly strong.

Astronomers often cannot see the companion star directly. Instead, they deduce its presence through precise pulsar timing. A pulsar emits pulses that act like the steady ticks of a cosmic clock. By using radio telescopes, scientists measure these pulses with extreme accuracy. As the pulsar orbits its companion, its velocity changes relative to Earth. This movement causes the Doppler effect. When the pulsar moves toward Earth, the pulses appear more frequent. When it moves away, the pulses appear less frequent. These fluctuations allow researchers to calculate the orbital motion and the mass of the companion.

The history of these discoveries is deeply significant. In 1974, Russell Hulse and Joseph Taylor discovered the binary pulsar PSR B1913+16 at Arecibo. While observing this pulsar, Hulse noticed the pulse rate varied regularly. This variation proved the pulsar was orbiting another object at high velocity. By analyzing these fluctuations, they determined the stars had nearly equal masses. This suggested the companion was also a neutron star. For this groundbreaking work, Hulse and Taylor won the 1993 Nobel Prize in Physics. Today, pulses from this specific system are tracked to within 15 microseconds.

Binary pulsars provide a laboratory for studying relativistic effects. When the two bodies reach their closest proximity, the gravitational field becomes extremely intense. This intensity causes the passage of time to slow down. This phenomenon is known as gravitational redshift. A similar effect called time dilation occurs due to special relativity as the pulsar moves through its orbit. This creates a relativistic time delay. This delay is the difference between a predicted circular orbit and the actual observed timing. Scientists use a 10-parameter model to account for these complex orbital corrections.

These systems also provide evidence for gravitational waves. According to Einstein's theory, two neutron stars orbiting a common center of mass emit gravitational radiation. This radiation carries energy away from the binary system. As energy is lost, the two stars draw closer together. This causes the orbital period to shorten over time. In the PSR B1913+16 system, the orbital period decreased by about 76 millionths of a second per year. This decay meant the pulsar reached its maximum separation over a second earlier than expected. Before the Advanced LIGO project in 2015, binary pulsars were the only way to detect such waves.

There are different categories of these systems, such as intermediate mass binary pulsars (IMBP). An IMBP is a system containing a pulsar and a white dwarf. These systems have longer spin periods, typically between 10 and 200 milliseconds. They also feature higher mass white dwarfs, higher magnetic field strengths, and larger orbital eccentricities than low mass systems. As of 2014, fewer than 20 IMBPs were known. One notable example is PSR J2222−0137, located about 870 light-years away. Its companion is a massive, cool white dwarf with a temperature below 3,000 K. This Earth-sized star may even be crystallized, earning it the nickname "diamond-star."

Sometimes, these stellar relationships become much more violent. If a companion star swells, it may overflow its Roche lobe. This is the boundary where a star's gravity can no longer hold its outer layers. When this happens, the star dumps gas onto the pulsar. This process can create an accretion disk around the recipient star. The heat from this gas exchange can produce X-ray light, creating an X-ray binary stage. Additionally, pulsars produce a wind of particles moving at relativistic speeds. This wind can blow away the magnetosphere of the companion star, dramatically altering its pulse emission.