PSR B1913+16

Orbital decay of PSR B1913+16.^[1] The data points indicate the observed change in the epoch of periastron with date while the parabola illustrates the theoretically expected change in epoch according to general relativity.

PSR B1913+16 (also known as PSR J1915+1606 and PSR 1913+16) is a pulsar (a radiating neutron star) which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system. PSR 1913+16 was the first binary pulsar to be discovered. In 1974 it was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst. 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."^[4]

The system is also called the Hulse–Taylor binary pulsar after its discoverers.

Discovery

Using the Arecibo 305m antenna, 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 17 times per second; thus the pulse period is 59 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 7.75 hours. They realized that such behavior is predicted if the pulsar were in a binary orbit with another star. Pulses from the companion neutron star have not been detected, but this might only be the result of an unfavorable viewing angle.

The pulses from the pulsar arrive 3 seconds earlier at some times relative to others, showing that the pulsar’s orbit is 3 light-seconds across, approximately the diameter of the Sun. Since this is a binary system, the masses of the two neutron stars can be determined, and they are each around 1–3 times the mass of the Sun. Observations have shown that the pulsar’s orbit is gradually contracting, due to the emission of energy in the form of Gravitational Waves, as predicted by Einstein’s theory of general relativity, causing the pulsar to reach periastron slightly early. Also, periastron advances 4º per year in longitude due to the gravitational field (thus the pulsar’s periastron moves as far in a day as Mercury’s moves in a century).

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 7.75 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.^[3][5]

The orbit has decayed since the binary system was initially discovered, in precise agreement with the loss of energy due to gravitational waves predicted by Einstein's general theory of relativity.^[3][5][6][7] The ratio of observed to predicted rate of orbital decay to be 0.997±0.002.^[7] The total power of the gravitational radiation (waves) emitted by this system presently, is calculated to be 7.35 × 10²⁴ watts. For comparison, this is 1.9% of the power radiated in light by our own Sun. (Another comparison is that our own Solar System radiates only about 5000 watts in gravitational waves, due to the much larger distances and orbit times, particularly between the Sun and Jupiter).

With this comparatively large energy loss due to gravitational radiation, the rate of decrease of orbital period is 76.5 microseconds per year, the rate of decrease of semimajor axis is 3.5 meters per year, and the calculated lifetime to final inspiral is 300,000,000 years.^[3][7]

• Mass of companion: 1.387 MSun
• Total mass of the system: 2.828378(7) MSun
• Orbital period: 7.751938773864 hr
• Eccentricity: 0.6171334
• Semimajor 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

In 2004, Taylor and Joel M. Weisberg published a new analysis of the experimental data to date, concluding that the 0.2% disparity between the data and the predicted results is due to poorly known galactic constants, including the Sun's distance from the galactic center, the pulsar's proper motion and its distance from Earth. While there are efforts underway for better measurement of the first two quantities, they saw "little prospect for a significant improvement in knowledge of the pulsar distance," so tighter bounds will be difficult to attain. Taylor and Weisberg also mapped the pulsar's two-dimensional beam structure using the fact that the system's precession leads to varying pulse shapes. They found that the beam shape is latitudinally elongated, and pinched longitudinally near the centre, leading to an overall figure-of-eight shape.^[1]

In popular culture

Science fiction writer Arthur C. Clarke offhandedly speculated, in his television series Mysterious World, that this pulsar was the Star of Bethlehem. He ended the 12th episode with the line, "How romantic if even now we can hear the dying voice of the star which heralded the Christian era."^{[citation needed]}

See also

• Binary pulsar
• Tests of general relativity
• Square Kilometre Array

References