thumb|right|200px|Evidence of orbital decay in PSR B1913+16. The data points indicate the observed change in the time of [[periastron with date, relative to a system not undergoing decay. The parabola illustrates the theoretically expected change according to general relativity.]]
The Hulse–Taylor pulsar (known as PSR B1913+16, PSR J1915+1606 or PSR 1913+16) is a binary star system composed of a neutron star and a pulsar which orbit around their common center of mass. It is the first binary pulsar ever discovered.
The pulsar was discovered by Russell Alan Hulse and Joseph Hooton Taylor Jr., of the University of Massachusetts Amherst in 1974. Their discovery of the system and analysis of it earned them the 1993 Nobel Prize in Physics "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."
Discovery
Using the Arecibo 305 m dish, Hulse and Taylor detected pulsed radio emissions and thus identified the source as a pulsar, a rapidly rotating, highly magnetized neutron star. The neutron star rotates on its axis 16.94 times per second; thus the pulse period is 59.03 milliseconds.
After timing the radio pulses for some time, Hulse and Taylor noticed that there was a systematic variation in the arrival time of the pulses. Sometimes, the pulses were received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.7519 hours. They realized that such behavior is predicted if the pulsar were in a binary orbit with another star, later confirmed to be another neutron star.
Star system
The pulsar and its neutron star companion both follow elliptical orbits around their common center of mass. The period of the orbital motion is 7.7519 hours, and the two neutron stars are believed to be nearly equal in mass, about 1.4 solar masses. Radio emissions have been detected from only one of the two neutron stars.
The minimum separation at periastron is about 1.1 solar radii; the maximum separation at apastron is 4.8 solar radii. The orbit is inclined at about 45 degrees with respect to the plane of the sky. The orientation of periastron changes by about 4.2 degrees per year in direction of the orbital motion (relativistic precession of periastron). In January 1975, it was oriented so that periastron occurred perpendicular to the line of sight from Earth.
Use as a test of general relativity
The orbit has decayed since the binary system was initially discovered, in precise agreement with the loss of energy due to gravitational waves described by Albert Einstein's general theory of relativity. The ratio of observed to predicted rate of orbital decay is calculated to be 0.997 ± 0.002. They name the main driver of this improvement, from 1.8σ to 1σ discrepancy, as being improved galactic constants published in 2014.
Characteristics
- Mass of companion:
- Total mass of the system:
- Orbital period: 7.751938773864 hr
- Eccentricity: 0.6171334
- Semi-major axis: 1,950,100 km
- Periastron separation: 746,600 km
- Apastron separation: 3,153,600 km
- Orbital velocity of stars at periastron (relative to center of mass): 450 km/s
- Orbital velocity of stars at apastron (relative to center of mass): 110 km/s
See also
- Tests of general relativity
- Timeline of gravitational physics and relativity