GR still looking good

Einstein’s theory of gravity – general relativity – has been confirmed (again!) by a newly discovered relativistic binary. The theory of general relativity, which describes how matter warps space-time and thus causes gravity, predicts that objects in orbit will produce ripples in space-time called gravitational waves.  The effect is amplified and potentially detectable when the objects are massive and in a tight orbit (meaning they are very close to one another). While gravitational waves have still not been detected, the energy carried away by gravitational waves will cause the orbit of the binary to shrink by a specific amount each year.  Such a change in a binary orbit, specifically the decline in the orbital period with time, was measured for the double pulsar system PSR B1913+16 (also known as the ‘Hulse-Taylor binary pulsar’), for which Russell Hulse and Joe Taylor  were awarded the 1993 Novel Prize in Physics.

Artist’s impression of the pulsar PSR J0348+0432 (which radio jets) and its white dwarf companion. The binary emits gravitational waves, represented by ripples in (blue) spacetime. As a result, the stars spiral inwards towards each other and the binary period reduces. (Credit: ESO/L. Calçada)

The newly discovered binary contains a pulsar, called J0348+0432, which was discovered with the Green Bank Telescope with has a rotational period of 39 ms (spinning ~25.6 times every second) and a binary period of 2.46 hours.  The optical counterpart (i.e. the binary companion) was discovered in the Sloan Digital Sky Survey archive.  Both the colour and flux of the companion suggested it was a white dwarf, and follow-up spectroscopic observations with the Apache Point Optical Telescope confirmed that the companion was indeed a low-mass white dwarf. Radial velocity observations further confirmed that the pulsar and white dwarf were gravitationally bound. The short period and the fact that one stars is a pulsar suggested that the binary could be used to test general relativity if the orbital decay (resulting from the emission of gravitational waves) could be measured accurately.

Further optical observations with the ESO Very Large Telescope and precise timing observations of the pulsar with three radio telescopes (the 305-m Arecibo telescope, the 10o-m Effelsberg telescope and the 100-m Green Bank Telescope) were used to determine the masses of both components. The team, lead by PhD student John Antoniadis from the Max Planck Institute for Radioastronomy, found that the pulsar  J0348+0432 is 2.01 ± 0.04 solar masses  and the white dwarf of 0.172 ± 0.003 solar masses. With an orbital period of just 2.46 days, such a close binary should definitely be radiating gravitational waves and losing energy. Using their radio timing observations, the team determined that the orbital period of the binary is decreasing by 8.6 microseconds per year (!), which is very close to the predictions of general relativity. The team argue that these results support general relativity, and are inconsistent with some (but not all) other alternative theories of gravity.

Artist’s impression of the PSR J0348+0432 system. The 2 solar mass compact pulsar (with beams of radio emission) produces a strong distortion of spacetime (the green mesh), while spacetime around the 0.17 solar mass white-dwarf companion is less curved. (Credit: Science, Antoniadis et al.)

Artist’s impression of the PSR J0348+0432 system. The 2 solar mass compact pulsar (with beams of radio emission) produces a strong distortion of spacetime (the green mesh), while spacetime around the 0.17 solar mass white-dwarf companion is less curved. (Credit: Science, Antoniadis et al.)

The new binary system also provides important information on the spin evolution of pulsars after mass accretion. The predicted merger time of the two stars is about 400 Myrs, after which time the system might evolve to become a single black hole, or a pulsar and an planet-mass remnant (like the “diamond planet” discovered in 2011 by Swinburne astronomers).

For more information, see:

  1. Boyles et al. (2013),”The Green Bank Telescope 350 MHz Drift-scan survey. I. Survey Observations and the Discovery of 13 Pulsars“, ApJ, 763, 80 and
  2. Lynch et al. (2013), “The Green Bank Telescope 350 MHz Drift-scan Survey II: Data Analysis and the Timing of 10 New Pulsars, Including a Relativistic Binary”, ApJ, 763, 81
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